Are there any animal species other than humans & chimps that engage in tribal conflicts?

i.e., between tribes within the specie.

(For chimps, see, e.g.,

Am seeking examples to develop taxonomy of such conflicts, e.g., for resource acquisition, enslavement, etc.

In the introduction of an article by Gavrilets and Fortunato, the authors give (with references) some examples of intra-specific group conflicts:

Social organisms living in stable groups often engage in aggressive interactions with conspecifics from neighbouring groups over territory, mating opportunities and other resources… Examples include border patrols in chimpanzees, raiding parties in spider monkeys, clan wars in hyenas and between-group fights in lions, free-ranging dogs, meerkats, Capuchin monkeys, blue monkeys, black howler monkeys and ring-tail lemurs.

Whether these conflicts (or others, such as ant colony conflicts) meet your definition of "tribal conflicts" you will have to decide.

Surprisingly, bonobos do. A study from researchers from the Max-Planck Institute for Evolutionary Anthropology back in 2008 and other research found that even though bonobos prefer to solve problems using sex, bonobos fight each other and even other great apes.

The same thing applies to bees. Australia's stingless bees go to war where thousands of worker bees perish and the young from the losing side are dragged out of the nest to die. Slave-making ants raid and attack other ant species. These intertribal conflicts seem to happen in nature due to in-group favoritism that causes certain creatures to favor members of their own phenotype when it comes to companionship and territory defense.

Animals don’t have morality, people do

In his attempt to prove that beasts have morals, Dale Peterson airbrushes away all the things that make humans unique in the animal kingdom.

Helene Guldberg

On the dust jacket of Dale Peterson’s new book, The Moral Lives of Animals, Elizabeth Marshall Thomas, author of The Hidden Life of Dogs, is effusive in her praise: ‘There’s a special place in the hearts of many of us for books that express the “oneness” of life on Earth’, she says, ‘and this book tops them all’.

Yet reading The Moral Lives of Animals I was often left, like fellow reviewer Stephen Budiansky, ‘with the feeling of being stuck on a bar stool next to a bore’ – one intent on relaying to the reader, ‘utterly unremarkable facts about his two large mutts, Spike and Smoke’.

Peterson’s aim is to downplay what is unique about human morality. As Budiansky rightly points out: ‘Rather than a sophisticated system of language-based laws, philosophical arguments and abstract values that sets mankind apart, morality is, in [Peterson’s] view, a set of largely primitive psychological instincts.’ This is a definition broad enough to encompass much of the animal world.

Peterson argues that animals’ moral systems are not merely ‘analogous to our own’ – that is, superficially similar due to coincidental factors – but ‘homologous to our own’ – that is, similar due to a ‘common origin’. He asks us to view morality as a ‘moral organ’, ‘equivalent to the elephant’s nose: enormous, powerful, multifaceted’. Our ‘moral organ’ may have features that differ from that of other animals, Peterson tells us, but ultimately human morality is, like animal morality, an organ residing in the limbic system of the brain.

Petersen accuses of ‘Darwinian narcissism’ those who fail to recognise the existence of animal morality. If one defines morality as, for instance, ‘collectively shared norms’ one is guilty of ‘argument by definition’, he claims. But if anyone is guilty of ‘argument by definition’ it is Peterson himself: ‘The function of morality, or the moral organ, is to negotiate the inherent serious conflict between self and others.’ And, hey presto, there is ample evidence of other species – particularly group-dwelling species – managing potential conflicts between their members, so ‘animals have morality’.


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This is wrong. Humans and animals negotiate ‘conflict’ by fundamentally different means. Peterson is presenting us with examples not of animal morality, but of Darwinian evolution selecting behaviours that minimise conflict and strengthen social ties among group-dwelling animals. Take his examples of ‘you scratch my back and I’ll scratch yours’ in the animal kingdom. Chimpanzees, for instance, spend an inordinate amount of time grooming each other because grooming serves an important social function in maintaining group ties, and strong chimpanzee communities increase the chance of individual members surviving.

Human beings, however, negotiate conflict through socially created values and codes of conduct. We are able to behave morally because we are uniquely able to exert some self-control, reflect on our own behaviour, put ourselves in the shoes of other people and make judgements.

If one reduces everything to its simplest form then one can find parallels between humans and the rest of the animal kingdom. But this kind of philistinism does not deepen our understanding of human beings and human society or indeed of animal behaviour.

For instance, Peterson’s approach strips a concept like empathy of any deeper meaning. ‘I would prefer to consider empathy as appearing in two different but related forms, contagious and cognitive’, he writes. Contagious empathy is ‘the process in which a single bird, startled by some sudden movement, takes off in alarm and is instantly joined by the entire flock’. Cognitive empathy ‘is contagious empathy pressed through a cognitive filter: a brain or mind’. In other words, these two types of empathy are just different forms of the same thing.


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But there is a world of difference between an instinctual connection between organisms – including some of our instinctual responses, such as yawning when others yawn – and human empathy involving a Theory of Mind, that is, the ability to recognise that one’s own perspectives and beliefs can be different from someone else’s. Once children are able to think about thoughts in this way, their thinking is lifted to a different level.

Peterson, however, dismisses the ability to think about thoughts as a veneer covering basic primal urges: ‘It is extremely easy to define morality by identifying this or that manifestation of human morality that might indeed be uniquely ours – written codes, cultural elements, intellectual analysis, an elaborate conscience, a fine-tuned sense of guilt – and thereby fail to recognise morality as it appears elsewhere, in other species.’

The search for homologies – characteristics shared between species that were present in a common ancestor – is an entirely legitimate enterprise. It can shed some light on the evolutionary origins of particular physiological or behavioural traits. But Peterson takes the giant leap from stating the blatantly obvious – that many of our organs are homologous with those of other animals – to absurdly asking us to imagine morality as merely an instinctive emotional response.

He writes: ‘We tend to believe in the uniqueness of our own human organs… but the vast majority of such organs appear in similar form among many other species.’ It is true that other animals have eyes, ears, noses, hearts, brains and many other organs in common with us. It is neither novel nor contentious to point out that we are physiologically very similar to many other animals. We are the product of evolution after all.

But acknowledging our physiological similarity to many other animals does not necessarily lead to the acceptance of behavioural, cognitive, emotional or, indeed, moral continuities.

Human beings, unlike other animals, are not determined by instinctual drives. We are able to reflect on and make judgements about our own and others’ actions, and as a result we are able to make considered moral choices.

We are not born with this ability. As the developmental psychologist Jean Piaget showed, children progress from a very limited understanding of morality to a more sophisticated understanding – involving, for instance, the consideration of the motives and intentions behind particular acts. So, for pre-school children, a child who accidentally breaks several cups, when doing what he’d been asked to do by an adult, is ‘naughtier’ than one who breaks one cup while trying to steal some sweets. Young children judge actions by their outcomes or consequences rather than by their intentions. Claiming that our morality is merely based on ‘gut instincts’ ignores the transformations children go through in their moral understanding from infancy to adolescence.

While Peterson downplays human abilities, he exaggerates animal abilities. So he says monkeys and apes ‘can appreciate the connection between seeing and knowing, and thus possess an awareness of the awareness of others’. He uses an anecdote from primatologist Frans de Waal’s 1982 book Chimpanzee Politics to show that chimps are capable of deception: ‘Orr, an adolescent female […] would scream while she was having sex. During surreptitious copulation with younger males, however, her screams sometimes caught the attention of the alpha, who would do his mighty best to interrupt the couple. Eventually, Orr learned to suppress her vocalisations when mating with lower-ranking males, while she continued screaming whenever she mated with the alpha.’

But, as I argue in my book Just Another Ape?, anecdotal evidence can be highly deceptive. Even if there was consistent evidence that apes deceive their fellows, the question still remains whether they are aware of what they are doing. Deception itself does not necessarily imply intentionality. To be able to deceive intentionally, an animal would need to be able to think about the intentions, knowledge and beliefs of those they are deceiving. In other words, they would need to have a Theory of Mind.

There are many examples of deception in the wild that clearly do not involve a Theory of Mind. For instance, if threatened, the Eyed Hawk-moth flaps open its wings to reveal large eyespots. But as evolutionary psychologist Richard Byrne points out: ‘Moths [expose their “eyes”] to looming cardboard squares and to looming animals that could eat them, alike… So we have real reason to doubt that they understand about mental states of predators.’ (1)

Daniel Povinelli, who ran the Cognitive Evolution Group at the University of Louisiana at Lafayette, is adamant that no test to date has reliably demonstrated that even chimpanzees – the masters of deceit, according to Peterson – have an understanding of the mental life of others (2). For example, Povinelli tested whether chimps understood that their begging gestures would only be effective if the person they were begging from could actually see them. So in one experiment, one caretaker had a blindfold covering her mouth and the other had a blindfold covering her eyes. Povinelli found that the chimpanzees did not differentiate between the caretaker who could clearly see them (the one with the blindfold over her mouth) and the caretaker who could not see them (the one with the blindfold over her eyes) when making begging gestures (3).

So even if animals are found to deceive, that does not necessarily imply that they know that they are deceiving. The animal may just be very good at picking up useful routines that bring them food, sex or safety, without necessarily having any understanding or insight into what they are doing.

No doubt Peterson would accuse me of what he terms ‘false anthropo-exemptionalism’ – that is, ‘an exaggerated insistence on discontinuity’ between human beings and other species. His biological determinism prevents him from recognising that something new – something quite exceptional – emerged in the course of the evolution of humans.

Human beings have something that no other animal has: an ability to participate in a collective cognition. Because we, as individuals, are able to draw on the collective knowledge of humanity, in a way no animal can, our individual abilities go way beyond what evolution has endowed us with. Our species is no longer constrained by our biology.

Many scientists reject any notion that human beings have abilities that are profoundly different from other animals. To do so, they fear, will give ammunition to creationists and spiritualists. But we do not need spiritual or ‘magical’ explanations to grasp that the difference between human beings and other animals is fundamental rather than one of degrees. There are some fascinating theories put forward in the last decade that go quite far in explaining the emergence, through evolution, of uniquely powerful human abilities. We don’t know how or when, but there must have been some gene mutation or set of mutations tens of thousands of years ago that endowed us with the unique ability to participate in a collective cognition.

Michael Tomasello, co-director of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, persuasively argues in The Cultural Origins of Human Cognition that at some point after the ape and human line diverged – and possibly only a few hundred thousand years ago – the human lineage evolved a motivation to share emotional states with each other, leading to a unique ability to engage in ‘shared intentionality’. This motivation to engage with other humans emotionally is manifest in early infancy.

Tomasello writes: ‘Human understanding of others as intentional beings makes its initial appearance at around nine months of age, but its real power becomes apparent only gradually as children actively employ the cultural tools that this understanding enables them to master, most importantly language.’

A small difference in our innate abilities led to a unique connection between human minds – allowing us to learn through imitation and collaboration – leading to cumulative cultural evolution and the transformation of the human mind.

As I argue in Just Another Ape?: ‘It is this unique ability to copy complex actions and strategies (even those that the individual doing the copying would never have been able to come up with on their own), along with unique forms of cooperation and an ability to teach, that creates the uniquely powerful “ratchet effect” in human culture, whereby gains are consolidated and built on rather than having to be rediscovered.’

There are very many unanswered questions regarding how and why our human genetic make-up evolved. But even if we did have all the answers, we would not – as a result of these insights – be able to explain why we behave the way we do today, or the ethical codes by which we currently live. The evolution of the human genetic make-up is merely the precondition for the emergence of distinctly human cultural abilities. We need to look to cultural evolution, rather than genetic evolution, to explain the vast gulf that exists between the capabilities and achievements of humans and those of other animals.

Human beings are capable of making judgements about our own and other people’s behaviour, and have the capacity consciously to change the way we behave and society as whole. We are not perfect and never will be, but we are special and unique among the animal kingdom. As sociologist Frank Furedi argues in Debating Humanism: ‘Most important of all we need to understand that whatever the mistakes that we have made we can extract from them lessons that can guide us to move forward’. (4)

Helene Guldberg is managing editor of spiked. She is the author of Just Another Ape?, published by Imprint Academic (buy this book from Amazon(UK)) and Reclaiming Childhood, published by Routledge (buy this book from Amazon(UK)). Visit Helene’s website here.

To enquire about republishing spiked’s content, a right to reply or to request a correction, please contact the managing editor, Viv Regan.

Bonobos are an example.

Sex is key to the social life of the Bonobo.
They largely use sex as a means to alleviate conflicts or resolve them.

When ill feelings begin to form between Bonobos– everything stemming from territorial issues to competition for food– their first reaction is to smooth it over with sexual contact.

Bonobos seem to ascribe to the 1960s hippie credo, "make love, not war." They make a lot of love, and do so in every conceivable fashion.

Sex in bonobo society transcends reproduction, as it does in humans. It serves as a way of bonding, exchanging energy and sharing pleasure.

Bonobos are a prime example of sex for the sake of sex. Also Capuchins. Anyone who has watched the Discovery Channel enough knows about this behavior.

I am quite surprised that the lecturer would make such a blatantly wrong statement. Bonobos have their own domain that even states:

Sex permeates the fabric of bonobo society, weaving through all aspects of daily life. It serves an important function in keeping the society together, maintaining peaceful, cooperative relations. Besides heterosexual contact, both male and female bonobos engage in same-sex encounters, and even group sex occurs.

For a scholarly paper (with abstract quoted below), I suggest: Nonconceptive Sexual Behavior in Bonobos and Capuchins

Sexual behavior by infecundable females, and by same-sex and adult-immature dyads, occurs in wild and captive bonobos (Pan paniscus). Proposed functions of these behaviors, in social primates generally, include practice, paternity confusion, exchange, and communication as well as appeasement. We used this framework to interpret and to compare observations of sexual behavior in a captive bonobo group and a wild white-faced capuchin (Cebus capucinus) group. In both species, (a) sexual behavior was no more frequent in cycling females than in pregnant or lactating females and (b) same-sex and adult-immature dyads engaged in as much mounting or genitogenital contact as adult heterosexual dyads did. The species differed in that (a) bonobos engaged in sexual behavior 65 times as frequently as capuchins, (b) only bonobos engaged in sexual contact other than ventrodorsal mounting during focal observation, and (c) bonobo sexual contact was concentrated most heavily in socially tense situations in adult female–female dyads, whereas capuchin sexual contact was concentrated most heavily in socially tense situations in adult male–male dyads. These data and published literature indicate that (a) practice sex occurs in both species, (b) paternity confusion may be a current function of C. capucinus nonconceptive sex, (c) exchange sex remains undemonstrated in capuchins, and (d) communication sex is more important to members of the transferring sex—female bonobos and male capuchins—than to members of the philopatric sex.

I would treat ANY statement made to make humans appear truly unique in the animal kingdom with a great deal of skepticism. We are after all only animals after all (only marginally more clever than others).


Lions are known to engage in sex to create bonds and interact with each other. Lions live in a social group known as a pride that consists of 2–18 females and 1–7 males. The females found in these prides were born into the pride. The males enter the pride from other prides. The success of reproduction for each individual lion is dependent on the number of male lions found in their social group. Male lions create coalitions and search for prides to take over. Successful coalitions have usually created a strong bond with each other and will take over prides. Once winning in a competition, all current males in the pride will be kicked out and left to find another pride. While in search for another pride these males will often engage in sexual behavior with each other creating a strong bond in this new coalition created. [7] [8]

Sex is a basic form of communication in bonobos’ life. It seems to infuse everything from simple expressions of affection to the establishment of dominance. Female bonobos have been observed to engage in sexual activities to create bonds with dominant bonobos. Having created this bond with the male, they will share food with each other and not compete with each other. [1] All members of the group are potential sex partners, males participate in sexual activity with other males, as do females with other females. These bonds made between females are for protection against male bonobos. If a male bonobo attempts to harass a female bonobo, the other females will help the female defend herself because of the strong bonds they have with each other. [9]

Several species in the animal kingdom turn to sexual activity as a way to solve a disagreement. Bonobos are one species notoriously known for using sexual behavior to relieve their aggression with each other. [3] Sex is part of bonobo's daily routine and social life. Unlike other primates, bonobos substitute sex for aggression. Sexual activity in bonobos is very high, yet the rate of reproduction is the same as a chimpanzee. [1]

In a study concentrated on primate aggression, researchers wanted to observe primates in conflict. How primates coped and resolved conflicts was a main concern in this study. Researchers stated that after primates participated in a heated, physical fight both primates involved in the fight would hug and have a mouth-to-mouth kiss. This action was considered as a demonstration of affection and reconciliation. [1]

Sexual interaction has also been witnessed in female bonobos to avoid aggression. When hungry, the female bonobo will approach a male bonobo and engage in sexual activity to avoid aggression. After their quick sexual activity, the female will take a portion of the male's food. The male will not demonstrate any form of aggression towards the female. [3]

Awareness in species is difficult to determine. Learned behaviors that have been demonstrated in laboratories have provided good evidence that animals have instinct and a reward system. The behavior of laboratory animals demonstrates a mental experience wherein the animal's instincts tell it if it carries out a certain action, it will then receive what it needs. [10] For example, the lab rat will push the lever because it knows food will fall out of the hole in the wall. It does not need awareness, but it does seem to work on a reward system. The lab rat learned the action needed to be fed.

Studies of the brain have proven that pleasure and displeasure are an important component in the lives of animals. [11] It has been established that the limbic neural mechanism that generates reactions are very similar across all mammals. Many studies have concentrated on the brain reward system and how similar it is across mammals. Through extensive research, scientists have been able to conclude that the brain reward system in animals is extremely similar to that of humans. The mechanism of core pleasure reaction is significantly important for animals and humans. [11]

Case study Edit

In a case study, female Japanese macaques were studied to find evidence of possible female copulatory orgasms. Through the study the frequency of orgasms did not correlate with the age of the Japanese macaques or the rank. Researchers observed that the longer and higher number of pelvic thrusts, the longer copulation lasted. There was an orgasmic response in 80 of the 240 Japanese macaques studied. [12]

Reward system Edit

Evolutionary principles have predicted that the reward system is part of the proximate mechanism underlying the behavior. Because animals possess a brain reward system they are motivated to perform in different ways by desire and reinforced by pleasure. [10] Animals establish security of food, shelter, social contact, and mating because proximate mechanism, if they do not seek these necessities they will not survive. [13]

All vertebrates share similarities in body structure they all have a skeleton, a nervous system, a circulatory system, a digestive system and excretory system. Similar to humans, non-human animals also have a sensory system. The sensory system is responsible for the basic five senses from touch to tasting. Most of the physiological and biochemical responses found in animals are found in humans. Neurophysiologists have not found any fundamental difference between the structure and function of neurons and synapse between humans and other animals. [10]

Case study Edit

Recent studies using positron emission tomography (PET) and magnetic resonance imaging (MRI) has provided evidence proving that chemical changes that occur with emotions are similar between humans and non-human animals. In a study comparing guinea pigs and humans, it was determined that the distress experienced by offspring separation in a guinea pig and a human going through depression activates the same region of the brain. [ citation needed ] The opiate receptor was also examined, allowing observation of the pleasure stimuli. In the procedure both a human and a rat had their receptors blocked with a certain drug. Once receptors were blocked, both the rat and the human were exposed to pleasurable food, but both were disinclined to eat the food. [14]

Engagements of sexual activities during non-breeding seasons have been observed in the animal kingdom. Dolphins and Japanese macaques are two of the many species that engage in sexual activities that do not lead to fertilization. Great varieties of non-copulatory mounting are expressed in several species. Male lions engage in mounting with other male lions, especially when in search for another pride. [7] The varieties of mounting include mounting without erections, mounting with erection but no penetration, and mounting from the side.

Expressions of affection are displayed in the animal kingdom as well. Affectionate behaviors do not include penetration or genital rubbing, but are still seen as a manner of sexual behavior. An affectionate activity can be as simple as licking. [3] Male lions are known for head rubbing, bats engage in licking, and mountain sheep rub horns and faces with each other. [15] Animals have also engaged in kissing, touching of noses, mouths and muzzles have been witnessed in African elephants, walruses, and mountain zebras. [4] Primates also engage in kissing that is incredibly similar to human display of kissing. Chimpanzees have full mouth-to-mouth contact, and bonobos kiss with their mouth open and mutual tongue stimulation. [3] There are a variety of acts to show affection such as African elephants intertwining their trunks, giraffes engaging in “necking”, and Hanuman langurs cuddling with each other in a front to back sitting position.

Non-penetrative genital stimulation is very common throughout the animal kingdom. Different forms of self and partner genital stimulation have been observed in the animal kingdom. Oral sex has been observed throughout the animal kingdom, from dolphins to primates. Bonobos have been observed to transition from a simple demonstration of affection to non-penetrative genital stimulation. [1] [15] Animals perform oral sex by licking, sucking or nuzzling the genitals of their partner. [9] [15] Another form of genital stimulation is masturbation. Masturbation is widespread throughout mammals for both males and females. It is less common in birds. There are several techniques, in which animals engage in masturbation from using paws, feet, flippers, tails, and sometimes using objects like sticks, pebbles, and leaves. [9] Masturbation occurs more often in primate species with large testes relative to their body size. [16]

Anal penetration Edit

Anal penetration with the penis (both in heterosexual and male homosexual dyads) has been observed among some primate species. Male homosexual anal penetration has been recorded in Old World primate species, including gorillas, orangutans, and some members of the Macaca genus (namely, stumptail, rhesus, and Japanese macaques). [17] [18] [19] It has also been recorded in at least two New World primate species, the squirrel monkey and the spider monkey. [18] [20] Morris (1970) also described one heterosexual orangutan dyad where all penetration was anal. However, the practice might have been a consequence of homosexual rearing, as the male orangutan in this dyad had had extensive same–sex experience. [21]

A case of male homosexual anal penetration with the finger has also reported among orangutans, [22] and Bruce Bagemihl mentions it as one of the homosexual practices recorded at least once among male chimpanzees. [9]

Autoeroticism or masturbation Edit

It appears that many animals, both male and female, masturbate, both when partners are available and otherwise. [23] [24] For example, it has been observed in cats, [25] dogs, [26] [27] male Cape ground squirrels, [28] male deer, [29] [30] [31] rhinoceroses, [32] boars, [33] and male monkeys. [34] [35]

[The] behavior known within the horse breeding industry as masturbation . involves normal periodic erections and penile movements. This behavior, both from the descriptive field studies cited above and in extensive study of domestic horses, is now understood as normal, frequent behavior of male equids. [37] Attempting to inhibit or punish masturbation, for example by tying a brush to the area of the flank underside where the penis rubs into contact with the underside, which is still a common practice of horse managers regionally around the world, often leads to increased masturbation and disturbances of normal breeding behaviour. [38]

Castration does not prevent masturbation, as it is observed in geldings. [39] Masturbation is common in both mares and stallions, before and after puberty.

Sexologist Havelock Ellis in his 1927 Studies in the Psychology of Sex identified bulls, goats, sheep, camels and elephants as species known to practice autoeroticism, adding of some other species:

I am informed by a gentleman who is a recognized authority on goats, that they sometimes take the penis into the mouth and produce actual orgasm, thus practicing auto-fellatio. As regards ferrets . "if the bitch, when in heat, cannot obtain a dog [ie, male ferret] she pines and becomes ill. If a smooth pebble is introduced into the hutch, she will masturbate upon it, thus preserving her normal health for one season. But if this artificial substitute is given to her a second season, she will not, as formerly, be content with it." . Blumenbach observed a bear act somewhat similarly on seeing other bears coupling, and hyenas, according to Ploss and Bartels, have been seen practicing mutual masturbation by licking each other's genitals.

In his 1999 book, Biological exuberance, Bruce Bagemihl documents that:

Autoeroticism also occurs widely among animals, both male and female. A variety of creative techniques are used, including genital stimulation using the hand or front paw (primates, Lions), foot (Vampire Bats, primates), flipper (Walruses), or tail (Savanna Baboons), sometimes accompanied by stimulation of the nipples (Rhesus Macaques, Bonobos) auto-fellating or licking, sucking and/or nuzzling by a male of his own penis (Common Chimpanzees, Savanna Bonobos, Vervet Monkeys, Squirrel Monkeys, Thinhorn Sheep, Bharal, Aovdad, Dwarf Cavies) stimulation of the penis by flipping or rubbing it against the belly or in its own sheath (White-tailed and Mule Deer, Zebras and Takhi) spontaneous ejaculations (Mountain Sheep, Warthogs, Spotted Hyenas) and stimulation of the genitals using inanimate objects (found in several primates and cetaceans). [40]

Many birds masturbate by mounting and copulating with tufts of grass, leaves or mounds of earth, and some mammals such as primates and dolphins also rub their genitals against the ground or other surfaces to stimulate themselves. [40]

Autoeroticism in female mammals, as well as heterosexual and homosexual intercourse (especially in primates), often involves direct or indirect stimulation of the clitoris . This organ is present in the females of all mammalian species and several other animal groups. [40]

Apes and Monkeys use a variety of objects to masturbate with and even deliberately create implements for sexual stimulation . often in highly creative ways. [40]

David Linden, professor of neuroscience at Johns Hopkins University, remarks that:

. perhaps the most creative form of animal masturbation is that of the male bottlenose dolphin, which has been observed to wrap a live, wriggling eel around its penis. [41]

Among elephants, female same-sex behaviours have been documented only in captivity where they are known to masturbate one another with their trunks. [42]

Oral sex Edit

Animals of several species are documented as engaging in both autofellatio and oral sex. Although easily confused by laypeople, autofellatio and oral sex are separate, sexually oriented behaviors, distinct from non-sexual grooming or the investigation of scents.

In the greater short-nosed fruit bat, copulation by males is dorsoventral and the females lick the shaft or the base of the male's penis, but not the glans, which has already penetrated the vagina. While the females do this, the penis is not withdrawn and research has shown a positive relationship between length of the time that the penis is licked and the duration of copulation. Post copulation genital grooming has also been observed. [52]

Homosexual behaviour Edit

The presence of same-sex sexual behaviour was not scientifically reported on a large scale until recent times. Homosexual behaviour does occur in the animal kingdom outside humans, especially in social species, particularly in marine birds and mammals, monkeys, and the great apes. As of 1999, the scientific literature contained reports of homosexual behavior in at least 471 wild species. [54] Organisers of the Against Nature? exhibit stated that "homosexuality has been observed among 1,500 species, and that in 500 of those it is well documented." [55]

To turn the approach on its head: No species has been found in which homosexual behaviour has not been shown to exist, with the exception of species that never have sex at all, such as sea urchins and aphis. Moreover, a part of the animal kingdom is hermaphroditic, truly bisexual. For them, homosexuality is not an issue. [56]

Homosexual behavior exists on a spectrum, and may or may not involve penetration. Apart from sexual activity, it can refer to homosexual pair-bonding, homosexual parenting and homosexual acts of affection. Engaging in homosexual behavior may allow species to obtain benefits such as gaining practice, relieving tension, and experiencing pleasure. [3] [13] [15] Georgetown University professor Janet Mann has specifically theorised that homosexual behaviour, at least in dolphins, is an evolutionary advantage that minimizes intraspecies aggression, especially among males.

"Humans have created the myth that sexuality can be justified only by reproduction, which by definition limits it to hetero sex," says Michael Bronski, author of The Pleasure Principle: Culture, Backlash, and the Struggle for Gay Freedom. "But here is an animal society that uses homosexuality to improve its social life."

After studying bonobos for his book Bonobo: The Forgotten Ape, primatologist Frans de Waal, a professor of psychology at Emory University in Atlanta, says that such expressions of intimacy are consistent with the homosexual behaviour of what he terms "the erotic champions of the world". "Same-sex, opposite-sex — bonobos just love sex play," de Waal said in an interview. "They have so much sex, it gets boring."

Homosexual behaviour is found in 6–10% of rams (sheep) and associated with variations in cerebral mass distribution and chemical activity. [57]

Approximately eight percent of [male] rams exhibit sexual preferences [that is, even when given a choice] for male partners (male-oriented rams) in contrast to most rams, which prefer female partners (female-oriented rams). We identified a cell group within the medial preoptic area/anterior hypothalamus of age-matched adult sheep that was significantly larger in adult rams than in ewes .

Male bighorn sheep are divisible into two kinds: the typical males among whom homosexual behaviour, including intercourse, is common and "effeminate sheep", or "behavioural transvestites", which are not known to engage in homosexual behaviour. [58] [59]

Male-male copulation has been observed in captive penguins [60] and homosexual behaviour has been observed among bats, in particular, the fruit bat. [61]

Homosexual pair-bonding and parenting Edit

Homosexual pair-bonding can be established several ways two of the main ways are pair bonding as partners or as companions. [9] As partners, both animals will engage in sexual activities with each other. In companion bonding, sexual engagement is not necessary in the relationship. This form of homosexuality is more of a partnership and friendship they spend all their time together. More than 70 species of birds engage in one of these two bonding. [9]

Homosexual parenting (sometimes referred to as cooperative breeding) occurs in a wide variety of species in the animal kingdom. [9] Homosexual parenting can occur in different ways, one of the most common being two females (typically related) coming together to help one another raise their offspring. An example of this is in meadow vole populations. Summer is peak breeding season for meadow voles however, going into winter and spring there is a division between the male and female meadow vole populations. They prefer communal nesting (because of the thermoregulatory benefits), and therefore, in the winter and spring female meadow voles will commonly not only nest with another female, but nurse their offspring together as well. This kind of communal nursing, and same-sex social bonds, among meadow voles is actually thought to benefit the young — increasing growth and survival rates. [62]

Homosexual parenting is especially present among certain species of birds, [9] one of the most famous examples being Laysan albatross. It is fairly uncommon among different species for unrelated individuals of the same sex to raise offspring together, but female-female pairings in Laysan albatross populations are one of the exceptions. This same-sex pairing and mutual cooperation in chick-rearing often occurs in the Laysan albatross populations which have uneven sex ratios (and an overall greater surplus of females). Also, Laysan albatross are known for being monogamous, and this tendency actually allows same-sex parenting to persist. [63]

Genital-genital rubbing Edit

Genital-genital rubbing, or GG rubbing, among non-human animals is sexual activity in which one animal rubs his or her genitals against the genitals of another animal. The term GG rubbing is frequently used by primatologists to describe this type of sexual intimacy among female bonobos, and is stated to be the "bonobo's most typical sexual pattern, undocumented in any other primate". [64] [65] The term is sometimes used in reference to GG rubbing among male bonobos, under the term "penis fencing", which is the non-human form of frot that human males engage in. Such rubbing between males is thought, according to varying evolutionary theorists, to have existed before the development of hominids into humans and bonobos, and may or may not have occurred in the homosexual activity of both of these genetically related species. [66]

Genital rubbing has been observed once among male orangutans [22] and several times in a small group of lar gibbons, where two males thrust their genitals together, sometimes resulting in ejaculation in one of the partners. [67] It has been observed among bull manatees, in conjunction with "kissing", [53] and is also common among homosexually active mammals. [53]

Inter-species sex Edit

Some animals opportunistically mate with individuals of another species. This is more commonly observed in domesticated species and animals in captivity, possibly because captivity is associated with a decrease in aggression and an increase in sexual receptivity. [68] Nevertheless, animals in the wild have been observed to attempt sexual activity with other species. [69] It is mostly documented among species that belong to the same genus, but sometimes occurs between species of distant taxa. [70] Alfred Kinsey cites reports of sexual activity involving a female eland with an ostrich, a male dog with a chicken, a male monkey with a snake, and a female chimpanzee with a cat. [71]

A 2008 review of the literature found 44 species pairs that had been observed attempting interspecies mating, and 46 species pairs that had completed interspecies matings, not counting cases that had resulted in hybridization. Most were known from laboratory experiments, but field observations had also been made. [70] It may result in fitness loss because of the waste of time, energy, and nutrients. [70]

Male sea otters have been observed forcibly copulating with seals, [72] [73] and male seals have been observed forcibly copulating with penguins. [74] Inter-species sexual behavior has also been observed in sea lions. [75] Male grasshoppers of the species Tetrix ceperoi often mount other species of either sex and even flies, but are normally repelled by the larger females. [70] Males of the spider mite species Panonychus citri copulate with female Panonychus mori mites almost as often as with their own species, even though it does not result in reproduction. [70]

The Japanese macaque has been observed attempting to mate with the sika deer. [76]

Sex involving juveniles Edit

Male stoats (Mustela erminea) will sometimes mate with infant females of their species. [77] This is a natural part of their reproductive biology – they have a delayed gestation period, so these females give birth the following year when they are fully grown.

In one reported observation, a male spotted hyena attempted to mate with a female hyena, but she succeeded in driving him off. He eventually turned to her ten-month-old cub, repeatedly mounting and ejaculating on it. The cub sometimes ignored this and sometimes struggled "slightly as if in play". The mother did not intervene. [78] [79]

It appears to be common in the Adélie penguin. [80]

Among insects, there have been reports of immature females being forcibly copulated with. [81]

Juvenile male chimpanzees have been recorded mounting and copulating with immature chimps. Infants in bonobo societies are often involved in sexual behaviour. [82] Immature male bonobos have been recorded initiating genital play with both adolescent and mature female bonobos. Copulation-like contact between immature bonobo males and mature female bonobos increases with age and continues until the male bonobo has reached juvenile age. In contrast, adult gorillas do not show any sexual interest in juvenile or infant members of their species. Primates regularly have sex in full view of infants, juveniles and younger members of their species. [83]

Necrophilia Edit

Necrophilia describes when an animal engages in a sexual act with a dead animal. It has been observed in mammals, birds, reptiles and frogs. [5] It sometimes occurs in the Adélie penguin. [80] Homosexual necrophilia has been reported between two male mallard ducks. One duck was believed to be pursuing another duck with the goal of rape (a common aspect of duck sexual behaviour) when the second duck collided with a window and died immediately. The observer, Kees Moeliker, suggested that "when one died the other one just went for it and didn't get any negative feedback—well, didn't get any feedback." [85] The case study earned Moeliker an Ig Nobel Prize in biology, awarded for research that cannot or should not be reproduced. [86]

Helping in humans and other animals: a fruitful interdisciplinary dialogue

Humans are arguably unique in the extent and scale of cooperation with unrelated individuals. While pairwise interactions among non-relatives occur in some non-human species, there is scant evidence of the large-scale, often unconditional prosociality that characterizes human social behaviour. Consequently, one may ask whether research on cooperation in humans can offer general insights to researchers working on similar questions in non-human species, and whether research on humans should be published in biology journals. We contend that the answer to both of these questions is yes. Most importantly, social behaviour in humans and other species operates under the same evolutionary framework. Moreover, we highlight how an open dialogue between different fields can inspire studies on humans and non-human species, leading to novel approaches and insights. Biology journals should encourage these discussions rather than drawing artificial boundaries between disciplines. Shared current and future challenges are to study helping in ecologically relevant contexts in order to correctly interpret how payoff matrices translate into inclusive fitness, and to integrate mechanisms into the hitherto largely functional theory. We can and should study human cooperation within a comparative framework in order to gain a full understanding of the evolution of helping.

1. Introduction

Helping behaviours that increase the direct fitness of recipients underpin several major evolutionary transitions [1]. Acts in which helpers provide any resource (e.g. food, time) are interesting because evolutionary theory strongly emphasizes the importance of competition and selfish behaviour. Humans are adept at helping each other. From a quantitative perspective, this trait is not unique in the animal kingdom arguably, hymenopterans and other eusocial species are even more helpful within their colonies. However, helping by the latter is explained by biological altruism based on kin selection [2,3], while humans also cooperate with unrelated individuals for direct fitness benefits on a scale that is unmatched by any other species. Importantly, the criteria for cooperating are highly flexible: the same individual may cooperate with friends, colleagues, supporters of the same football club, political affiliates, compatriots or even international alliances. Help can be provided in different currencies (e.g. time/money/physical effort) and is also often provided in situations where it is unclear how return benefits may be accrued, from letting a car out at a busy junction to donating to victims of natural disasters in far-away countries.

The frequency and scale of human helping could depend on several factors that appear to be unique to humans: our capacity for spoken and written language, the use of tags to identify groups, societal-level norms and institutions that both prescribe cooperation and punish defection, various media channels that allow for large-scale communication and coordination, and banks to transfer money—a unique non-perishable resource—anywhere. One might therefore wonder to what extent research on human cooperation yields idiosyncratic explanations, rendering comparisons with other species useless. One may also ask whether research on human cooperation is suitable for publication in biological journals like Proceedings of the Royal Society B. Here, we address this question. We first summarize briefly the enormous impact that theoretical concepts and empirical studies of human cooperation have had on research in other species. We then highlight topics of interdisciplinary interest and shared future challenges. It should become clear that we favour an open-minded and inclusive approach, where humans are just another species that can be studied under the general framework of evolutionary theory. While human cooperation might be more peculiar than cooperation in many non-human species, each species would appear unique if every detail was taken into consideration. Therefore, a distinction between disciplines based on study organisms only hinders progress.

2. Theory on human helping as inspiration for biological research

Theoretical approaches to understand helping in humans predate evolutionary concepts of helping. The principal tools used by biologists were developed by economists in the form of game theory—a framework to understand how humans should make decisions in strategic interactions [4]. A ‘game’ is a formal mathematical model of an interaction, defining the payoffs to all players. A key insight is that players' payoffs are affected by their own decisions and also by those of their partner(s). Thus, the dominant strategy depends on the strategy that is used by the partner(s). Economists assume that payoffs translate into utility and that players maximize utility. Stylized economic games were developed to study optimal decision rules. In their simplest form, these games consist of two players who can each choose between two actions, for example to cooperate or to defect. Games can be one-shot or repeated over a number of rounds. The resulting payoffs of action combinations can be captured by a 2 × 2 matrix. The matrices for well-known games [5], like the prisoner's dilemma game, the prisoner's delight game and the snowdrift game (also called hawk–dove game) are summarized in figure 1. These games were subsequently adopted by evolutionary biologists to explore when helping behaviour could be evolutionarily stable [6] in a population. Under this evolutionary approach, strategies are inherited traits that specify behaviours [7]. Rather than utility, evolutionary biologists assume that payoffs translate into fitness, with the accompanying assumption that strategies which, on average, increase fitness will be under positive selection.

Figure 1. Three stylized economic games that differ with respect to the payoff matrix. In the prisoner's dilemma, not helping yields a higher payoff in each interaction no matter how the partner behaves, which makes helping an investment that needs to yield future benefits. Thus, iterated interactions are required for conditional helping to evolve. In the prisoner's delight, helping yields a higher payoff no matter how the partner behaves, which makes helping a self-serving action, even in a single-round game. In the snowdrift game, the best choice depends on the partner's action: help if the partner does not help and do not help if the partner helps. Under these circumstances helping is under negative frequency-dependent selection in a single-round game.

A common goal is to understand why individuals should provide help to others. Economists, never considering the genetic structure of human populations, focused on how helping may increase on average the direct fitness of that actor. This form of helping has been termed ‘mutual benefits’ [8] or ‘cooperation’ [9]. We will use the latter term in this paper, and restrict the term ‘mutualism’ to describe mutual helping between species [10]. Economists demonstrated that cooperative solutions are possible, when the number of rounds times the benefits of mutual cooperation outweigh the cost of cooperating (folk theorem [5,11]). Evolutionary theorists subsequently rediscovered this principle verbally [12] and then mathematically, albeit with limited generality [13]. Economists also showed how supply and demand determine exchange rates [14], an insight that was then incorporated into biological market theory [15,16]. Similarly, the idea that reputational effects in a communication network may affect animal behaviour [17] was foreshadowed by concepts explicitly developed by economists to understand human cooperation [18,19].

Evolutionary biology provided a major conceptual insight thanks to Hamilton's kin selection theory [2,3]. Helping may be altruistic in biological terms, by reducing the actor's lifetime reproductive success, and yet still be positively selected if helper and recipient are related (specifically, when rBC > 0, where r = relatedness between actor and beneficiary, B = fitness benefit conferred on beneficiary and C = personal fitness cost incurred by actor [2,3]). Thus, the one-sided borrowing from biologists eventually became a fruitful dialogue, not least because cooperation and biological altruism may act simultaneously to promote selection on helping, including in humans (e.g. [20–22]). Indeed, game-theoretic approaches have become increasingly prominent in the attempt to understand the evolution of helping behaviour [7,23]. Importantly, the logic underpinning game-theoretical models of behaviour reflects general principles in evolutionary theory and may hence be applied to any species, including humans.

3. Empirical research on human helping as inspiration for animal research

We focus on supposed examples of cooperation based on investments. We define an investment as a behaviour that reduces the current payoff of the actor and increases the current payoff of the recipient. Cooperation based on investment appears to be vulnerable to cheaters who do not invest but receive investment from others. A vast theoretical literature has shown that higher-level selection processes (kin/group selection, interdependencies between individuals) may select against cheating. These processes have been relatively neglected in empirical studies, partly because of the difficulty of quantifying them. More ecologically motivated future research may hence reveal that some apparent investments are actually self-serving forms of helping [24]. As with theoretical concepts, empirical research on human helping has had a serious head start over similar research on non-human animals. It is impossible to summarize the existing literature on human helping adequately here. Though most of this research focuses on understanding human social behaviour only, the data and conclusions nevertheless provide inspiration to researchers studying non-human animals, who might look for similar behaviours in their own study systems.

In this context, it is important to distinguish ultimate from proximate questions [25]. Ultimate questions address the adaptive value of helping, which is rather simple: helpful strategies can only be under positive selection if they provide lifetime fitness benefits (+/+) to all participants, the exception being biological altruism (−/+) based on kin selection. Thus, from an ultimate perspective, there is no a priori reason to demarcate research aimed at understanding the evolution of costly social behaviour in humans from similar research on other species. In contrast, the proximate mechanisms underlying social decision-making can be highly diverse: genetic predispositions, physiological states and cognitive mechanisms may all interact to produce social behaviour, and humans might often use idiosyncratic proximate mechanisms to achieve cooperation. Examples of these include mentalizing, fairness preferences, cultural norms, shared intentionality, and the ability to communicate intentions using gestures (such as pointing) and language. These abilities may not be unique to humans, but they are unarguably more pronounced in humans than in any other species. Moreover, variation in proximate mechanisms can affect the means by which cooperation is achieved—and sometimes even the possibility to achieve it [26]. We therefore discuss research on ultimate and proximate explanations for costly social strategies separately.

(a) Ultimate explanations

Humans appear to be an excellent model species to test the predictions of evolutionary game theory. Experimenters can construct precise material payoffs for any possible combination of individual decisions, decide how many rounds are played with whom, and how much information subjects obtain. To understand the adaptive significance of costly helping behaviour, many studies on humans have identified partner control mechanisms—responses to being cheated that reduce the cheater's payoff [27]. These include tit-for-tat-like reciprocity, punishment, reputation effects, partner choice and (relatedly) ostracism (e.g. [28–31]).

After an initial focus on tit-for-tat-like reciprocity (reviewed in [32]), biologists also searched for examples of these same partner control mechanisms in non-human species. Marine cleaning mutualism involving the cleaner wrasse Labroides dimidiatus provided experimental support for all these control mechanisms. Cleaners remove ectoparasites from visiting ‘client’ reef fishes [33]. Nevertheless, conflict arises because cleaners prefer to eat client mucus, which constitutes cheating. Therefore, clients have to make cleaners feed against their preference to receive a good service [34]. Partner control mechanisms become visible when clients respond to cleaners taking a bite of mucus (which correlates with clients visibly jolting in response to cleaner mouth contact). As summarized in [34], client species with access to a single cleaning station punish cleaners through aggressive chases, while clients with access to several cleaning stations terminate the interaction and visit another cleaner for their next inspection. In addition, clients arriving at a cleaning station extract information from any ongoing interaction and invite for inspection only if the cleaner behaves cooperatively. Thus, the cleaner's reputation depends on their behaviour, and they behave more cooperatively if they are observed. Finally, the larger cleaner males may also punish their female partner for cheating a jointly inspected client, a simple form of third party punishment [35] that is fine-tuned to the stakes (i.e. the quality of the client as a food source) [36].

Research on cleaning mutualism was partly inspired by classic studies on the effects of punishment and reputation on human cooperation, which highlighted that the possibility of being punished or being chosen for interactions by observers, respectively, could both promote cooperation at higher levels than when these incentives were absent (e.g. [28,30]). Meanwhile, results from the cleaner fish mutualism have, in turn, inspired subsequent studies on partner choice and asymmetric punishment in humans, for example leading us to investigate whether punishment or partner choice is a more effective incentive to cooperate when both incentives are co-present [37], and to explore whether power asymmetries increase the efficacy of punishment as a cooperation-enforcing mechanism in two-player games [38].

Unlike most non-human species, humans regularly cooperate in large groups of unrelated individuals. Economists and social scientists have therefore pioneered the study of cooperation in groups. The payoffs can be captured using public goods games, where benefits are assumed to be either a linear or a sigmoid function of investments (figure 2). Under the former assumption, the interaction is an n-player prisoner's dilemma and investments therefore risk being biologically altruistic. Where benefits are a nonlinear function of investments, then the interaction is an n-player snowdrift game (a volunteer's dilemma) and contributions are negatively frequency dependent (figure 2) [39,40]. Again, claims about human uniqueness with respect to n-player cooperation have inspired biologists interested in a comparative approach to find suitable non-human model systems in which to apply the human literature on public goods games. Importantly, the most suitable species will not necessarily be the species that are phylogenetically most related to humans, but those that routinely interact in n-player social dilemmas (with non-relatives)—such that n-player social dilemmas constitute an ecologically valid scenario. To this end, species that regularly engage in inter-group conflict may provide a promising arena. Humans promote cooperation in larger groups by providing incentives: rewarding contributors to the public good and punishing so-called free-riders [28,41]. Similarly, female vervet monkeys use these same incentives to increase male participation in inter-group conflicts [42].

Figure 2. Public goods games. (a) Contribution to a public good creates a surplus. In an n-player prisoner's dilemma game the created value is a linear function of the amount contributed, while in a n-player snowdrift game it is nonlinear (a step function in the figure). The created value is then shared equally among players irrespective of initial contributions. (b) Case examples for the payoffs of a focal player depending on whether she contributes or defects and what her three partners are doing. In the n-player prisoner's dilemma, it is assumed that contributing costs 1 unit and generates a value of 2 units. In the n-player snowdrift it is assumed that contributing costs 1 unit and that 2 contributions are needed to produce a public good of 8 units.

One of the key difficulties in identifying n-player public goods games outside of humans is to obtain informed estimates of both the precise payoff matrices and the fitness consequences. In some cases, in contrast to the frameworks described above, individual actions appear to be self-serving and to provide public goods only as a by-product (e.g. punishment of scale-eating sabre-tooth blennies by their victims [43] group hunting of multiple non-shared prey [44]). In contrast, many terrestrial group hunting examples involve the killing of a single large prey, where individual payoffs depend crucially on how the prey is shared rather than on the increased hunting success [45]. In such cases, payoffs are affected by ownership, contribution to the hunt, sex and/or position in the hierarchy (e.g. [46–49]), variables that are not typically considered in standard public goods games (but see [50]). Many examples of n-player public goods have been described in microbes, where the production of extracellular molecules constitutes an investment that can provide benefits to non-producers (reviewed in [51]). Since increased production typically yields diminishing benefits, many of these examples yield fitness consequences that correspond to the volunteer's dilemma payoff matrix [40]. These various case studies highlight an important issue: despite the continued focus on n-player prisoner's dilemma payoffs in human laboratory studies, many public goods in humans might also better approximate the nonlinear payoffs of snowdrift/volunteer's dilemma games [40,52]. A key priority for future research on humans is therefore to evaluate the payoffs of real-world interactions and design experiments to capture these in the laboratory.

(b) Proximate explanations

Research on the cognitive mechanisms underpinning human helping might initially appear of little value for understanding helping in other species. This is because humans have a cognitive toolbox that is unmatched by any other species (though there is considerable debate regarding the extent to which differences are qualitative or only quantitative [53]). Many of these cognitive tools are tightly linked to/enhanced by human language, which is in itself arguably the most important tool. Spoken and written language does not only allow for basic communication about behaviour it also facilitates negotiation, coordination, the expression of some emotions and the establishment of shared intentionality. Language is also the basis for some forms of teaching [54] and the establishment of shared cultural norms. Culture in turn provides a variety of cues that can be used to generate cooperation even between strangers. It seems highly likely that there is a tight link between our cognitive abilities and our ability to cooperate, though it remains unclear whether ecological pressures to cooperate selected for our cognitive abilities or whether these abilities created opportunities for extreme cooperation. Comparative research that evaluates what cognitive processes are used by humans and other species during social interactions might help address this question.

Claims about uniquely human cognition inspired research on animal cooperation that challenged these claims. For example, it has been proposed that humans achieve high levels of cooperation because they have a unique sense of fairness (‘inequity aversion’) and thus split payoffs according to individual contributions [55]. A large body of research has shown that rudimentary forms of disadvantageous inequity aversion—aversion against receiving less than the interaction partner(s)—may be present in some non-human species (reviewed in [56] but see [57]). In contrast, evidence for advantageous inequity aversion—aversion against receiving more than the interaction partner(s)—is currently lacking in non-human species and is apparently not even ubiquitous in humans [58].

In contrast to cognition, endocrinological research offers straightforward opportunities for a comparative approach, as humans are just standard mammals when it comes to hormones, neurohormones or neurotransmitters. Nevertheless, social scientists have often taken the lead in exploring the effect of these substances on helping behaviour. Research on the effects of oxytocin provides a case example. Oxytocin facilitates bonding between mammalian mothers and their offspring [59]. Research on humans revealed that this function may have been co-opted for creating bonds between unrelated individuals: increased oxytocin increases trust, without increasing risky behaviour overall, and increases within-group cooperation and between-group competition [60]. Oxytocin also mediates helping between unrelated individuals in several non-human mammals, including chimpanzees [61], dogs [62], vampire bats [63] and voles [64]. We note however that the robustness of various findings—in particular those based on exogenous application of oxytocin—is strongly debated (see [65]), and the jury is still out on how central-nervous and peripheral oxytocin mediates social behaviour in humans and other species. These concerns notwithstanding, research on endocrinological mechanisms underpinning social behaviour has been and will remain an interdisciplinary project.

4. Towards a more ecologically valid approach to helping in humans and other species

We have repeatedly emphasized how influential research on human helping was for biological research on other species. We believe that there is great potential for ever closer exchange of ideas and methods. Most importantly, biology has a long history of thinking about the problem of laboratory artefacts. While this does not mean that all biological research on helping is ecologically relevant [66], we will now highlight various important concerns about research on human helping from an ecological perspective. We postulate that further progress will depend on empirical data informing models rather than on experiments fitted to the assumptions of models.

First, we note that most experimental research taking an evolutionary approach to understand human cooperation precludes key features of the human cognitive toolbox, such as language, shared intentionality and shared group identity. This is because empiricists typically develop experiments in accordance with evolutionary game theory, which focuses on strategies rather than underlying mechanisms [67]. For instance, as theoretical models do not incorporate communication, subjects are typically prevented from talking to each other in experiments. Also, high levels of cooperation in humans typically occur between friends, colleagues or culturally created in-groups, while experiments often follow model assumptions and hence involve anonymous interactions between strangers. Thus, many experimental studies on humans are designed to test the predictions of general evolutionary game theory models rather than designed to explain how humans achieve extremely high levels of cooperation. As a consequence, we propose that typical economic experiments only yield baseline levels of human cooperation and that such levels may also be observed in various other species [68]. Of course, cooperation could similarly decrease under different conditions. A recent study [69] found that human cooperation increased under conditions in which subjects could talk to each other, in particular when in-group identity was triggered. We expect that the larger the group size and/or the incentive to cheat and/or the challenge to coordinate, the more important the human cognitive toolbox becomes to achieve high levels of cooperation. Studies that have allowed for communication during experiments have shown that communication can increase cooperation, either via gossip to deter cheating [70] or by allowing subjects to more efficiently coordinate actions [71]. Varying relationship quality between subjects will also be likely to yield additional insights, both in humans and other species.

An important goal for a biological approach to cooperation is to determine how the findings of abstract laboratory experiments apply in the real world [24,51,65,66]. Economic games that are typically used to study human behaviour are theory-driven but highly artificial. These abstract games can allow us to identify with a high degree of control how the various pillars that structure social interactions (e.g. anonymity, punishment, partner choice) directionally affect behaviour under the assumption that ceteris paribus these general effects apply in all settings [72]. Simple abstract games also allow a method for studying and quantifying variation in helping behaviour within and across populations (e.g. [73–75]). It is also likely that excluding more human-specific features like language has encouraged the interdisciplinary dialogue as both biologists and social scientists could use similar paradigms. Nevertheless, more effort should now be aimed at identifying if and how the findings from laboratory studies translate into real-world behaviours. Failing to do so runs the risk that empirical findings serve only to test the predictions of game-theoretical models and have little real-world relevance. We illustrate this point by summarizing discussion first on the meaning of payoff matrices in general, and then on indirect reciprocity as one specific example.

(a) Payoff matrices

It is unclear to what extent the payoffs used in standard laboratory games approximate the payoffs of interactions that occur in the real world. The assumption that payoffs correlate positively with individual fitness holds in populations that are well mixed both with respect to genetic structure and with respect to potential interaction partners. The situation changes when limited migration and overlapping generations lead to kin structure and the potential for biological altruism, and when populations are structured into demes (groups) that compete with each other through contest or scramble competition. In those cases, material payoffs often provide a poor correlate of fitness. Instead, interacting individuals might become interdependent [2,3,76,77]. Interdependence has been proposed to be key to the evolution of extreme cooperation in humans [78]. Importantly, letting two highly interdependent individuals play a one-shot game with a prisoner's dilemma payoff matrix leads to confusion because the players' best option with respect to fitness is to either fully cooperate or to cooperate at least with some probability [24]. This is because interdependency can alter the fitness consequences of a prisoner's dilemma payoff matrix in such a way that fitness can be described by a prisoner's delight game (where cooperating yields higher payoffs/fitness via by-product benefits to the partner) or by a snowdrift game (where cooperating is under negative frequency dependent selection figure 1). For example, zebra finches, a species with obligate bi-parental care, fail to cooperate with strangers in an experiment that uses an iterated prisoner's dilemma payoff matrix, but they show rather unconditional cooperation when paired with their social partner [79], perhaps due to interdependence between social partners [31]. A major question arising from the interdependence hypothesis to explain human uniqueness in levels of cooperation [78] is hence whether human interdependence is (or was) much more pronounced than in any other species, or whether some unique cognitive tools allowed humans to create extreme mutually beneficial interdependencies between unrelated individuals.

(b) Indirect reciprocity

Indirect reciprocity also offers a cautionary tale on the importance of ecological validity. Indirect reciprocity occurs when an investment to help a recipient yields return benefits by an investment of a third party rather than by the initial recipient. Typically, indirect reciprocity therefore involves the existence of a reputation or an image score, and assessment rules determine how different actions affect reputation. A first detailed analysis of stable decision rules was provided by Kandori [19] and extended by Ohtsuki & Iwasa [80]. However, there is mixed evidence regarding whether people actually use these stable rules to judge the actions of others. Early evidence indicated that these second-order judgement rules were too cognitively complex to be used [81], while more recent evidence has indicated that reputation assessments can be predicated on second-order information regarding the context of helpful [82] or punitive [83] behaviour. Perhaps a more fundamental concern with the importance of indirect reciprocity as a general mechanism for supporting cooperation is the lack of real-world evidence that people behave in this way (but see [84]). One key paper that claims to have demonstrated indirect reciprocity in the real world [85] instead simply demonstrates that individuals show concern for reputation, which is not the same thing, as the crucial component—individuals with good reputation receive voluntary rewards from others—is missing.

One other major mechanism by which concern for reputation could yield downstream benefits is via partner choice. There is ample real-world evidence—including from non-human species—that partner choice is an important force underpinning cooperation, and the pressure to be chosen as a partner can lead to strategic [86] (and even competitive [29,87]) investments in reputation. Laboratory studies demonstrating indirect reciprocity may therefore be tapping into psychological mechanisms aimed at striking up mutually cooperative relationships with partners that have a good reputation, even though this is not possible in most laboratory studies of indirect reciprocity. Under the logic of error management [88], one could further predict that the high payoffs of striking up just one mutually productive relationship by ‘rewarding’ a helpful individual could sustain several small investments in rewards that do not ultimately lead to a relationship (cf. [89]). Error-management strategies could therefore result in behaviours that had the appearance of ‘rewarding’ helpful individuals in one-shot encounters, but would actually function to establish productive relationships. Experiments investigating the adaptive significance of acquiring a good reputation under real-world settings are now crucial to determine the relative importance of indirect reciprocity and reputation-based partner choice as mechanisms supporting cooperation.

This discussion highlights a larger issue of experimenter demand [90] in laboratory studies of human behaviour—changes in behaviour that occur because of what the subject believes to be appropriate in that context, rather than due to intrinsic motives or preferences. Most laboratory studies of indirect reciprocity have limited the behavioural options available to players. Thus, although indirect reciprocity is observed in laboratory experiments, we cannot rule out that these behaviours result from the expression of emotions whose only possible outlet in the context of the experiment is to reward helpful others. These emotions might well produce alternative behaviours in real-world scenarios that are nevertheless prevented by the rather impoverished selection available in the laboratory. Attempts to approximate reality by giving players more options in empirical games can affect the expression of behaviour (e.g. [36,83]). We suggest that the next wave of human evolutionary behavioural sciences ought to fully embrace these complexities in order to understand how behaviour in artificial laboratory settings relates to that in the real world.

5. General conclusion and outlook

We certainly support the idea that studies on human helping behaviour are relevant for biological research. In the tradition of Darwin [91], the highest relevance is achieved by studies that take an explicit evolutionary approach and refer at least to some extent to empirical and/or theoretical studies on other species. This view is also reflected in papers published recently in Proceedings of the Royal Society B (see electronic supplementary material).

Potentially, a unifying framework could be developed by studying how individuals decide whether to help, to cheat, to punish or to switch partners. This issue of decision-making links function and mechanisms. There is a clear need to study these processes [92] because humans and other animals do not use the simple strategies investigated in game-theoretic models (e.g. [93–95]). To determine why not, we should study social cognition—the mechanisms by which animals acquire, process, store and act on information from other individuals [96]—in its broadest biological sense. Perception of relevant stimuli can fundamentally affect decision-making. For example, it has been proposed that humans and other animals use heuristics or rules of thumb [97] to reach decisions quickly by ignoring a portion of the available information [98]. These processes are probably routed in well-established universal learning mechanisms, such as learning based on positive or negative reinforcement [99]. Excitingly, even in humans, reinforcement learning may explain various deviations as well as conformity with payoff-maximising behaviour [100]: for example, if behavioural option A yields a small gain in most trials, positive reinforcement may cause subjects to prefer this option over a more profitable option B that yields a high reward in few trials.

Recent theoretical studies have started to explicitly model reinforcement learning over the lifetime of individuals and selection on specific reinforcement learning parameters (i.e. the change in the probability of repeating a behaviour after receiving a reward) to study the consequences on social behaviour [101,102]. The models show that selection acting on reinforcement learning can yield cooperative solutions in an iterated prisoner's dilemma as well as consistent co-operators and defectors within pairs playing a repeated snowdrift game. What is still missing in the models is an integration of perceptual aspects. Early ethologists pointed out that learning needs to be studied within evolutionary history (i.e. within the ecology of a species). This is because evolution may shape the perception of species such that certain stimuli are more likely than others to elicit learning through positive or negative reinforcement. To give a concrete example, cleaner wrasse need to give priority to visitor clients over resident clients, as the former would otherwise swim off and visit another cleaner [34]. Species can be identified by their colour patterns and body shape, while the food (various species of ectoparasites) is highly overlapping between residents and visitors. As a consequence, cleaners can readily learn to preferentially approach an ephemeral food plate that differs from a permanent food plate only with respect to colour and patterns, a task that is extremely difficult for primates as well as rats and pigeons [103–105]. However, if the food items are coloured differently, or if food is hidden under cups of different colours, capuchin monkeys readily learn to prefer the ephemeral food source [106]. Taken together, the studies show that performance in the same biological market task varies according to a species's ability to perceive the relevant stimulus. Perception, strength of perceived reinforcement on actions and memory capacities (declarative, episodic or simply emotional) will all contribute to variation in cooperation within and between species.

In conclusion, we affirm that humans are just another species to test evolutionary theory. Research on human cooperation that takes a clear ecological or evolutionary perspective is as biologically relevant as research on any other species. Although helping has long been considered as an evolutionary puzzle that needs to reconciled with evolutionary theory and its emphasis on egoism, we believe that this puzzle has already been solved in the sense that there are many concepts that provide conditions under which biological altruism and cooperation can be favoured by selection. What is currently lacking is a general framework that can explain variation in helping tendencies within and between species, with human cooperation being the single most idiosyncratic data point. The current puzzle is thus why human cooperation is so unique on a quantitative level—and, moreover, why we also observe such striking variation in cooperation among different human individuals, groups and societies. We have argued that to solve the puzzle, we need to be more explicit about the links between cooperation and ecology and between cooperation and cognition (see also [107]). Both issues warrant a comparative approach, making research on human cooperation an interdisciplinary project of high biological relevance.

4. American Pikas

About the size and shape of a hamster, the American pika typically lives at high elevations where cool, moist conditions prevail. Research by U.S. Geological Survey has found that pika populations are now disappearing from numerous areas that span from the Sierra Nevadas to the Rocky Mountains. Populations within some areas are migrating to higher elevations likely to avoid reduced snowpacks and warmer summer temperatures. Unfortunately, pikas are strongly tied to rocky-talus habitat that is limited and patchily distributed. This gives them few options as temperatures continue to rise. Photo by Jon LeVasseur (

The Human Evolution Blog

Is Violence What Made Humans Smarter than Other Animals?

It is readily apparent that humans are, by far, the most intelligent species on the planet. How this came to be, however, is anything but clear.

Our substantial cognitive abilities are made possible by our enormous brains. When it comes to brain size (relative to body size), humans have the largest brains of any vertebrate. Even further, the neurons of our brain are more interconnected than those of other animals. The real difference in our brains is not that we have more gray matter (cell bodies), but more white matter (axons, which connect neurons to each other).

This begs the question, why haven’t other animal lineages steadily become more intelligent as well? Why do humans stand alone?

For example, the goblin shark has been swimming the oceans for over 100 million years in more or less its current form. In all that time, they haven’t evolved larger or smarter brains. Why not? Assuming that intelligence and other advanced cognitive abilities surely bring a substantial survival advantage, one would think that the goblin shark would have gradually evolved to become more and more intelligent. But there is no evidence that this has happened. Why not?

The answer is that the evolution of a big and interconnected brain is a very improbable event. Big brains are costly in a variety of ways, which brings a whole host of drawbacks.

The cost of a big brain

For example, the human brain consumes a whopping 20% of the body’s energy resources. The constantly pumping human heart, by comparison, consumes only around 5% of the body’s energy. Feeding our big brains placed severe dietary demands on early humans. This is not a minor consideration. As I’ve written previously, most animal species are teetering on the brink of starvation pretty much all of the time. The human brain needs 3-4 times more calories (as a function of total body needs) than the brains of other mammals. This is a great cost.

Anatomical constraints are another possible reason why more animals haven’t evolved very large brains. Evolution can only work with the available anatomy. Brains are confined to the cranium. One can’t grow a larger brain without also growing a larger skull. This means that we would need simultaneous random mutations to facilitate growth of both the cranium and the brain. This would be a quite rare event.

Moreover, having a larger skull comes at great cost as well, at least for mammals. For primates especially, the size of the birth canal places strict limits on how large the head can be at birth. The price that humans pay for having such a large head is considerable. Despite human infants being roughly the same size as chimpanzee infants and having roughly the same gestational interval (8-9 months), maternal mortality is much higher in humans than it is chimps.

Before the dawn of modern medicine, mothers died in childbirth at disturbingly high rates. Even today, the maternal mortality rate is 2-3% in countries like Somalia and Afghanistan. Given the high birth rate in these same countries, the cumulative lifetime risk of dying in childbirth is about 1 in 12. This means that everyone in Somalia knows many women that died in childbirth. While Somalia is underdeveloped to be sure, they still have at least some modern advantages, such as soap, sterile bandages, and at least some access to clean or boiled water. How much higher would the maternal mortality rates have been in the pre-historical era, let alone the distant ancestral environment?

In contrast, maternal mortality in chimps, orangutans, and gorillas is vanishingly small. It’s simply unheard of for an ape mother to die giving birth. This brings the cost of a big skull into focus.

If mutations leading to bigger brains and bigger skulls bring higher rates of maternal and/or infant death, they are unlikely to be favored by natural selection. The question is not why haven’t more animals evolved big brains. The question is how did we evolve them despite the disadvantages?

Cooperative competition

The big conundrum here is that the “ancestral environment” in which humans evolved is not at all unique. We diverged from chimpanzees six or seven million years ago but our lifestyle was not very different from theirs for most of that time. We lived in the same climate, ate similar foods, and formed small family-based groups of around the same size. What was different about how we lived that favored intelligence?

Professor Sergey Gavrilets of the University of Tennessee recently sought to answer this question by considering what kind of evolutionary forces would have favored the development of big brains despite their many drawbacks.

One feature stuck out: competitive cooperation. This might seem like an oxymoron, but what is meant by this is the banding together of one group cooperatively and cohesively in order to compete with a different group. It is within-group cooperation and between-groups competition.

Humans engage in between-group competition perhaps more than any other species. It’s easy to find examples of this from the modern world: states, nations, cultures, and religions show great cohesiveness internally, but fierce competition with each other. The same is true for universities, clubs, unions, and professions. We’re always banding together (cooperating) in order to compete.

Think about team sports. From the World Series to neighborhood pick-up games, team sports involve the close cooperation of some individuals pitted fiercely against others. Players get traded teams get shuffled and yet, the cohesion is easily recreated when the next game begins. Humans seem especially built for forming bonds of cooperation in order to fight battles of competition.

This feature of human behavior seems to be unique to our species. Many other species cooperate, of course, and every species competes in one way or another. But humans are oddly flexible regarding with whom we’ll cooperate and with whom we’ll compete. Also, humans take both cooperation and competition to the extreme.

Altruism, kin selection, and group selection

One of the biggest conundrums in evolutionary biology has been the evolution of altruism – helping others at a cost to yourself. How could behaviors ever emerge and persist if they incurred a cost to one’s self? Natural selection would quickly eliminate such tendencies because the selfless never prosper and pass on their genes.

The first attempt to explain altruism is called the theory of group selection, which states that features can be favored by evolution due to their benefits for a group, rather than individuals. This idea initially failed to gather strong support from biologists because it failed in most experiments and computer simulations. Selfishness always seems to win over altruism long before altruism could spread through a population and exert its group benefits. In the natural world, thieves, cheats, and jerks seem to do well.

Group selection, however, has seen a recent resurgence because of the increasing discovery of the phenomenon of reciprocal altruism. Reciprocal altruism is the idea that some animals will help other animals in need, even at a cost to themselves, but this generosity is expected to be returned when the tables are turned. Most famously documented in vampire bats, reciprocal altruism is being discovered in hundreds of species, most especially mammals.

In addition, the power of kin selection has never been in dispute and seems to have played a larger role in primate and human evolution than previously appreciated. Kin selection is the notion that genes and behaviors can be favored by evolution not only because they help one’s self, but because they help one’s close family members. Because close family members have a high likelihood of sharing genes, helping one’s relatives is very nearly like helping one’s self, at least in terms of aiding the success of one’s genes.

The social dynamics of wolf packs are an extreme case of kin selection. A typical wolf pack includes an alpha male and an alpha female together with several of their siblings and of course the children. Only the alphas breed. The other adults forego reproduction and instead assist the pack and promote the survival of their nieces and nephews in various ways. They don’t need to breed because the majority of their genes are present in their nieces and nephews and will get passed on that way.

What does this have to do with human intelligence?

Biologists and anthropologists now see a way to explain why humans evolved to be so social, so altruistic, and yet, so fiercely competitive at the same time, and intelligence is key to the equation.

For reciprocal altruism to work, animals must have good enough memories to remember who is naughty and who is nice. Otherwise, some individuals could simply mooch off of everyone else’s generosity and never give back in return. These cheaters would prosper from the efforts of others and the whole system of reciprocity would fall apart. Because of this, reciprocal altruism and good memories reinforce each other and could be increasingly favored over time.

Furthermore, kin selection requires that animals recognize who is family and who is not. Wolf packs accomplish this by strictly maintaining packs that are small, tight-knit, and family-based. This has drawbacks because it leads to a great deal of inbreeding.

In primates, however, the social groups are more dynamic. In chimpanzees, members can switch from one band to another. In gorillas, young male interlopers periodically challenge an alpha male (the silverback) for the right to dominate a harem. The idea of “family” changes over time.

It is in this environment that humans evolved. Small bands of mostly related individuals competing against other such bands. However, the membership of the band was subject to change. For example, when two bands clashed, the triumphant band could subsume the surviving members of the vanquished band.

For this system to work, humans had to have an adaptable and expandable sense of “family.” Family isn’t just genetic relatives, but anyone with whom we closely associate.

With this in mind, we have a stage that was perfectly set for intelligence to be favored in the ancestral lineage leading to humans. Early hominids were living in small family-based bands. Reciprocal altruism was common, as was kin selection, close social interactions, and dominance hierarchies, all of which require good memories and social instincts.

Add to this, that hominids were beginning to walk upright, thus freeing their forelimbs to carry and utilize hand tools, even while on the move. Armed with tools in hand, close relationships with pack mates, and good memories, these hominids naturally turned to organized hunting. Meat provides much more calories for the increasingly demanding hominid brain than does the vegetarian diet of the other apes.

The behavior of organized group hunting would strongly favor mental abilities such as recall of past events, pattern recognition, prediction of future events, calculation, and communication. The hunt made us smart.

From hunting to violence

It probably wasn’t long after the evolution of group-organized hunting that our forebears turned to intra-species violence. The process of hunting down big game in the African savannah is not that different from tribal warfare. It involves methodical reconnaissance, careful stalking, and communicating with bandmates in order to coordinate an attack. One must observe, draw conclusions, and formulate intricate plans. This is where the social features of hominids may have led to the strong directional evolution of increasing intelligence.

Our ability to expand our sense of self to include family and other members of our band promoted our cooperative instincts. Doing battle with our tribes promoted our competitive side. In both cases, contemplation of past and future events would be strongly favored. To win a fight with a neighboring tribe, it’s not enough to overpower them you must also outsmart them.

But you must outsmart them collectively. A truly “smart group” doesn’t just have smart members they work together smartly. Social cohesion and intelligence would thus reinforce each other in a case of runaway co-evolution. Indeed, the brain size of our lineage nearly tripled over the last 2.5 million years.

In the ancestral environment that shaped the human form and our behaviors, organized hunting and violent conflict may have been key phenomena in promoting both our intelligence and our social dynamics. The prosocial and antisocial sides of human nature – our good side and dark side – may have been shaped through that conflict. Warfare isn’t just about aggression against enemies it’s about cooperating well with your allies.

We humans are uniquely able to identify with and feel connected to our fellow humans. We are also uniquely able to de-humanize and do harm to them. We all have a Dr. Jekyl and Mr. Hyde within us. Thus, the trick to peace is viewing our fellow humans as part of “us” instead of “them.”

This historical anthropologist wants to upend the conventional wisdom about human nature and violence

War and all of its brutality is attention-grabbing and memorable. Recollections of war and conquests tend to stick around and take up the spotlight in historical records. However, a war-centered narrative paints an incomplete picture of human history—and human nature. While there is a popular opinion in the anthropological community that war is an evolutionary, inborn tendency of humans, there is also pushback to that theory. There is a growing argument for a human history that predates war altogether and further points out that war is not innate to human nature, but instead, is a social and cultural development that begins at certain points around the globe.

However, once war takes place, it tends to spread, explains historical anthropologist R. Brian Ferguson, who has spent more than 40 years researching the origins of war. Ferguson, a professor of anthropology at Rutgers University, notes that war is not the same thing as interpersonal violence or homicide. War implies organized, armed conflict and killing sanctioned by society and carried out by members of one group against members of another group. Ferguson argues that current evidence suggests that war was not always present but began as a result of societal changes—with evidence of war's origins appearing at widely varying timestamps in different locations around the world. He estimates that the earliest signs of war appear between 10,000 B.C., or 12,000 years ago.

"But in some areas of the world you don't see any signs of war develop until much more recently," he says, noting that in both the U.S. Southwest and Great Plains there is no evidence of war until around 2,000 years ago.

Ferguson wrote an article in the Scientific American in 2018 titled, "War Is Not Part of Human Nature," in which he details his take on war. In the article, he summarizes the viewpoints of two anthropological camps, dubbed hawks and doves by late anthropologist Keith Otterbein. The hawks argue that war is an evolved predisposition in humans dating back to when they had a common ancestor with chimpanzees. Doves, meanwhile, argue that war has only emerged in recent millennia, motivated by changing social conditions. In the article Ferguson writes:

Ferguson has studied the anthropological and archeological records throughout ancient, and sometimes into more modern, human history. He says there is a lack of evidence of war or large-scale violence, in many places around the world throughout various periods of history. He has spent four decades researching and historically contextualizing the various origin points of war around the world. He has also contextualized incidents of group violence in humanity's closest ape cousins, chimpanzees. He argues that war is not innate, evolutionary nor inevitable behavior for humans.

Ferguson spoke with Local Peace Economy correspondent April M. Short about his findings and theories surrounding war and human history.

April M. Short: The big questions are: have humans always gone to war, or is there a point of origin for war? And, is war innate to the human species (or maybe just men)? Is there an evolved predisposition to war or is it a social, learned behavior that emerged with particular organizations in societies?

Brian Ferguson: There is a great deal of interest regarding this in anthropology in particular, and in archeology, and political science as well. It's been a very active field and they are many different issues that are involved [here] that are connected to each other.

To mention one issue about whether war has always been with us, there is the related question of how war was affected by the expansion of colonial systems. In particular, related to Western Europe, but other [colonial systems] as well. I maintain that colonial expansion generally led to more intensive warfare than a lot of the fighting that we've seen around the world in the past few hundred years, from the Age of Exploration onward. This is not a reflection of human nature but a reflection of circumstances, or the contextual situation.

But, even before the beginnings of colonialism, war was quite common around the world. War leaves a number of different signs, which is indicative of violence in the archeological records, the most important of which are skeletal trauma and settlement data of different sorts. There are other indicators as well, but if you have a lot of information on those two things, then if war is present, it will show up.

AMS: Another, related question is whether there is evidence of a clear starting point for war?

BF: Everybody wants to know when war began. It's difficult to give an answer that will satisfy people because you have to ask where you're talking about. Evidence for war appears at different times in different locations. And, once war began, sometimes it went away for a while, though that was not the case most times. Oftentimes war would spread, and it would change over time as political systems changed. It's a very complicated field.

But the question people really want to know the answer to is [whether] war [is] human nature? And in one sense, the answer is definitely yes, because humans make war, we're capable of making war, it's one of the things humans do. But I think the more meaningful question that people are trying to get at is: is there something that has evolved in human beings, or maybe just in men, that makes them inclined to try to kill—or at least to act with extreme fear to—people outside their own group. Is it a natural human tendency or predisposition to kill outsiders? That is what has been argued by a lot of people. [cognitive psychologist and science author] Steven Pinker is one, there are many others.

Other people have argued something a little different than that, which is: maybe there isn't any inborn tendency to want to kill outsiders, but war will happen naturally unless you have some kind of system in place to stop it. That's sort of what Thomas Hobbes was talking about in Leviathan, right? He didn't know about genes and this was before [Charles Darwin's theory of evolution]. He wasn't saying people had an "evolved" predisposition to kill outsiders. He just said that people pursuing their own interests, without some kind of larger civil society, will naturally turn to violence to further their own interests, and that will lead to war. And what that means is war is a natural condition of human society. So, is [war] part of human nature or is it the nature of humans in society?

The bottom line is, in one view, humans have always made war since they've been humans. But what I have been arguing for some time now is that if you look around the world, in the archeological records, the earlier remains don't have evidence of war.

Now, when we go very far back—say 30,000 years or more—there is almost nothing to indicate the humans were even there. Maybe you have a stone tool or something, but you can't say based on evidence whether there was war or not. But, when you come closer to the present and you look at the material evidence, you do not find evidence of war for some time.

What you find is a global pattern. At different times in different places around the world, if you go from the earliest archeological evidence [and move] forward, there will come a time when evidence of war will start to appear. Those changes occur without a dramatic increase in archeological recovery. It's not like we're starting to get good [evidence in] archeology, [or] good data, and only now are we starting to see [signs of] war. We had all of it but there weren't any signs of war. Then signs of war started to appear.

A colleague of mine, Doug Fry, works in this area and has been making a bigger point about this, and it's a very good point. We've been accumulating a number of cases from the archeologists who work in particular areas, and archeologists themselves aren't interested in the question of when war began, they're just digging their own digs. They're generally not interested in making global comparisons like I am. But we find that when archeologists provide summaries of the evidence of interpersonal violence of a deadly nature, more and more of them are showing that war has a starting point.

AMS: You mentioned this is the pattern everywhere you look, is it the global pattern?

BF: In the Americas alone, which I've been working on lately, [the pattern of evidence of war emerging during a given time in the records] includes the Andean region, it includes the Oaxaca region in Mexico, it includes the Pacific Northwest coast of Canada, Northwest Alaska, the Eastern Woodlands, the Great Plains. I'm not sure whether you can say the same for Western California, because Western California is unusual for having a lot of violence that goes back very far, so I'm not sure whether you can say there's clearly a time before you have evidence of war there. But it's the case in all these other places. I also looked at the patterns in Europe and the Near East where you see the same thing: you don't have any evidence of war, and then war shows up.

One more note on this: it's often said that an absence of evidence is not evidence of absence, so if you don't find evidence of war, that doesn't mean war [didn't happen] there. For any particular case, any particular [archaeological] dig, that is absolutely true. But if you are talking about a larger region with multiple excavations, that is not a scientific statement, because it cannot be challenged, it cannot be falsified. If you're saying: "Even if you don't find evidence of war, war probably [still happened] there," how do you disprove that? But if I'm saying that in these different areas you're not going to find any evidence of war before certain periods of time, because no war took place there, that's easy to disprove. You just find the evidence.

It's a little tiring to me to have the phrase repeated, "just because you don't find the evidence doesn't mean it isn't there," because the pattern of seeing [war] start-up is so clear in so many places. It's time to consider the possibility that, really, war wasn't there at all before a certain point.

AMS: Why do you think the popular theory has been that war is innate to humans, or we've always had war?

BF: That's a great question, and it's a difficult question to answer. If I'm talking about whether there are signs of war in Europe in a particular year, I can talk about that in terms of evidence. But when you get to the question of why people tend [to lean] toward either the theory that "people are innately belligerent" or "people are innately good," (which is often suggested to be the Rousseau versus Hobbes point of view), some of it is individual variation in opinions. But I also think when you look at the prevalence of these ideas, they're time specific.

Back in the late 19th-century when Darwin's work was new, there was a real emphasis on this struggle for survival. There was a racial part to it too, which was the idea that some races are superior to others, and the struggle and fight [between the races leads to] the superior ones conquering the inferior ones. That whole Social Darwinist ideology was very common, and it fed into other theories back then, which were a bleak view of humanity. Freud was very bleak. Early psychologists were very bleak and would talk about humans having instincts, and one of the big instincts was the instinct of pugnacity. Pugnacity is a word we don't use much anymore, but pugnacity was said to be the instinct in which people just wanted to fight. So, if you wanted to know why wars exist, it was because we had the instinct for pugnacity.

World War I provoked a kind of revulsion against war. There was a change in how people looked at things. There was a 1915 study that was really revelatory, titled, "The Material Culture and Social Institutions of the Simpler Peoples: An Essay in Correlation." It looked at a number of different societies around the world (in what today would be a very crude method). It said that the simplest societies may have some war, but they had less war than more developed societies. It began to seem like war wasn't part of human nature, it was part of developing larger-scale, hierarchical societies. It came with that political evolution.

Time went on and in the 1960s there developed a very strong intellectual argument for war being innate. There were several writers who were key in [the development of] this [argument]. One was an Austrian ethologist (ethologists are people who study animal behavior) named Konrad Lorenz. He was on the German side during World War II. He was of the view that if you play a martial tune, men will drop everything and go off to war. He wrote the book On Aggression that was very influential.

Then there was Raymond Dart, an Australian paleobiologist (though they didn't use that word at the time) who found early skulls and remains, and was convinced that in every skull he found he saw evidence of a violent death and cannibalism. Dart's work was picked up by a very gifted writer, Robert Ardrey, who wrote several books, including African Genesis and The Territorial Imperative, which were part of his Nature of a Man series. That was the basis for Stanley Kubrick's movie "2001: A Space Odyssey." If you've ever seen the beginning of that film, these proto-apes had something changed in their minds by black obelisks from outer space, and they start killing each other, and that's the beginning of human creativity. That's what Ardrey basically believed to be the truth about humans, and he popularized it.

And then, there was the famous book, Lord of the Flies by William Golding. Golding came up with this idea that people were just real pieces of work. All of these concepts were part of the popular culture in the 1960s, and it was very influential. It became the accepted wisdom that that's the way people are.

The Vietnam War made a big difference. Anthropologists had not really been interested much in the study of war before Vietnam. The Vietnam War went on for a long time, and demonstrations against it were very big on college campuses, which is where anthropologists are. I was a draft-age student back then and that's really when the anthropology of war as a field first developed. It grew from there and different perspectives developed. Some of them held that war has always been with us, some said it was a biological instinct, some argued that war was a cultural product, and a relatively late development. Margaret Mead [cultural anthropologist] was one of those, who said "Warfare is Only an Invention, Not a Biological Necessity." And I think she was right. Since then, this argument has continued on in a more scholarly way, with people producing evidence. Now we've been doing that for a couple of decades and we've got a lot of evidence.

AMS: You mention in your Scientific American article that the people who argue that war is innate often use the example of chimpanzees being warlike. They point to the common ancestor shared between chimpanzees and humans to argue humans are innately warlike. You have spent two decades analyzing all of the recorded incidents of violence relating to chimps, and you have written a book on the topic, which is soon to be published. In your book, you theorize that chimpanzees are not, in fact, warlike but that their incidents of violence can be attributed to cultural and social contexts, largely involving human interference. Can you share a bit about your work on chimpanzees?

BF: I'm not a primatologist. I've never worked with chimpanzees. I'm a historical researcher, so I read the observations by other scholars, and I contextualize those observations. I did that with war, and I've done it with chimpanzees.

Back in 1996, a book came out called Demonic Males: Apes and the Origins of Human Violence. It painted a really grim view of human nature, as evolved to kill strangers. And the argument was that chimpanzees do this… not because they're hungry or they're in some kind of immediate contest over resources. It's just: they're programmed biologically, by evolution, to do it. And the argument was that, then, so are humans because chimpanzees and humans got it from their common ancestors anywhere from six to 13 million years ago.

I started [going through all the literature] in the late 1990s, and now the book is finished. I've called it Chimpanzees,"War,"and History. And you'll note I put quotes around "war." For the book, I went through every site [where chimpanzee group violence took place]. What I found was that while people would say their [warlike] behavior of looking for outsiders or strangers and killing them is normal chimpanzee behavior, it's really rare. If you talk about a war as being sequential killings of members of another group, then there are only two chimpanzee wars that take place in a span of about nine years. I mention this in the Scientific American article:

The gist of my argument is that evidence shows deadly intergroup violence is not a normal, evolved behavior pattern of chimpanzees, but a situational response to a local history of human disturbance. That is what the book demonstrates.

AMS: I've read that bonobos share just as much DNA with humans as chimpanzees and are not warlike or violent—in fact, they're practically nonviolent. Do you look at bonobos in your book?

BF: Yes. My book has 10 parts and part eight [is about] bonobos. Bonobos are a fascinating comparison. They're as related to humans as chimpanzees are. We have, however, never seen a bonobo kill another bonobo (although one killing of an infant is suspected, but very possibly didn't happen). Another thing that's different about bonobos is that they have on occasion accepted outside adult males into their groups. Now, chimpanzees have accepted adolescent males to their groups, and they've also temporarily tolerated stranger adult males in their groups, so it's kind of a fine point, but it is a qualitative distinction between [chimps and bonobos].

The saying was chimps are from Mars and bonobos are from Venus. Chimpanzees are partial to violence, aggressive and totally male-dominated and bonobos are, as the story goes, female-dominated and not as hostile, not as aggressive… I wouldn't say bonobos are matriarchal, instead, I would say their society is gender-balanced—which is very different from chimpanzees.

And this takes us back to the question of inborn predispositions because if chimpanzees are born to kill, and if the bonobos don't kill, is that because somehow [bonobos] evolved out of the killing mode? Are they biologically evolved so that they don't kill?

Other than the two extreme behaviors I mentioned, accepting outside males into their groups and killing, almost everything a chimpanzee has been seen doing, a bonobo has been seen doing. There's a lot of overlap in what they do. It's kind of a difference in frequencies rather than cut and dry differences.

… Bonobos don't have the things that I think make chimpanzees fight, which is a scarcity of resources connected to human impact. Bonobos haven't had that. And at the same time, they have something that goes against fighting, which is a social organization that's very different from chimpanzees. I don't think this is a result of instincts or inborn predisposition.

I spend a lot of time in the book laying out the fact that a young male chimpanzee grows up in an adult world where males dominate females, and females don't spend a lot of time with other females. Males spend a lot of time hanging out with other males, so they've got a sort of boy club there, and this leads them to engage in status competition that's male-on-male. Very often a group of two or three males together will kind of rise in the social hierarchy by hanging together and attacking any other males as a duo or trio, and that's how they beat the alpha. And [being an] alpha has a lot of advantages.

For chimpanzees but not bonobos, the second hypothesis in my book is that the unusually aggressive, high-status males may, in some circumstances, engage in what I call 'display killing' of helpless individuals, even infants within their own group, in order to intimidate status adversaries.

But bonobos have a tendency of females to bond (which may have to do with the genito-genital rubbing that females engage in, although that's not entirely clear), and they will attack a male who is too aggressive. If a male wants to rise up in the status hierarchy of bonobos [they need to be less aggressive]… because the society structure is [based on] a bisexual ladder. For a male to rise in the status hierarchy, what they do is they stick close to their mothers. The best ally for a bonobo male in getting access to feeding, getting access to mating and going up in the status hierarchy means being close to a high-status female. The status game is played with mothers, not brothers. That's how a bonobo male takes care of his own business. It means that they're attached to females and very often not attached at all to other males.

AMS: For me, just as a layperson coming into this, learning that we are just as related to bonobos as chimps undermines the idea that human warlike tendency is due to the common ancestor with chimpanzees. It's interesting to consider how much social structures may be influencing behaviors, for humans as well as other apes.

BF: It's a big area of research now, and field research has changed for a number of different reasons. One thing that's happened in primate field research, and in laboratories too, is that work in non-intrusive studies that look at hormone levels and genetics has expanded. [Researchers] can get their samples by placing tarps under trees and waiting for chimpanzees to pee in the morning. And then they can collect data on the hormone levels and genes.

There is interest right now in the biology of these primates, and the argument in biology has been that chimpanzees and bonobos really are biologically different— genetically, hormonally and behaviorally. It's a really interesting area that I find complicated because of the nature of these biological studies and the nature-nurture interaction. The idea that biology and environment combine and influence the development of any organism and these changes may be epigenetic and may have to do with the birth environment. The main action of epigenetics, [the study of heritable changes in gene expression] is based on what happens in early life, though epigenetics works throughout life and may be transmitted through generations, too.

The way I put my argument at one point [in my book] is: what if they were switched at birth? If an infant chimpanzee was put in with bonobos and vice versa, what would they grow up to be like? Would a chimpanzee raised among bonobos grow up to act like a chimpanzee with all the aggressive notions, male bonding and all that stuff? I argue that they would follow the local customs [of the bonobos], they would do what they saw others around them doing. Then along came epigenetics, and as it was applied to chimpanzees, it seemed to fit perfectly that the early childhood and the social experience of a chimpanzee and a bonobo at birth is very different.

AMS: To bring it back full circle to humans, how do you argue this idea of nature vs. nurture, epigenetics and socialization, might come into play anthropologically, and in relation to war?

BF: The implication, or lesson here, for humans is that humans are flexible. I think chimpanzees are very flexible, I don't think that they have innate patterns to do things like fight with each other. I think it's acquired in chimpanzees and bonobos. And I think that that goes for human beings too. And humans go a lot farther than that in the complexity of culture.

A lot of people will say that chimpanzees and bonobos also have cultures, they will use the word culture for these great apes. I think what chimpanzees and bonobos have is clearly learned traditions. They learn things to do, things that others in their group do. I don't think that's the same thing as culture, because culture involves a symbolic and linguistic medium to exist. And that culture exists in our thoughts and our language and our speech. That's how you learn it. That's how you communicate it. That's how it's passed on.

Human culture has cumulative development—and it needs language and symbols for this. You learn what one generation did, then you can do something on top of that. Everything we have in this world goes back to thousands and thousands of innovations, all of which have been based on the innovations that came before. Chimpanzees do not have cumulative innovations.

For war, I think the difference plays out in that humans do not have inborn predispositions. Some anthropologists will argue that humans have an inborn predisposition to not kill other human beings, that they're born against doing that, and they have to unlearn that [in order to be violent]. That's an optimistic way of thinking about human beings, and it certainly goes against the idea that people are natural-born killers. I hope it's true, but I'm not convinced. I think that could just as easily be a result of the way that we're socialized in our own societies.

What I'm saying is that, at a minimum, we don't have a predisposition either way. We're certainly not predisposed to kill. We're not predisposed to be xenophobic. Ethnocentric is a little different because ethnocentric simply means at the basic level, that the way you were brought up is the way you think things should be done. Every culture teaches every new infant. Everybody thinks: "My way is the right way to do things." But going beyond that, to the concepts that other people are inferior, or dangerous enough to be killed—that's certainly not part of human nature. When we look at tribal people, when the Europeans first showed up, the initial response typically was to look at these strange people with curiosity. It's not a natural reaction of fear, not this kind of tribal hostility that everybody always talks about, which is a lot of bunk.

The lesson is that humans have a great deal of plasticity. And we can be molded in different ways. We can be molded to be Nazis, or we can be molded to be passivists. Thinking that it is something that comes from the genes, that it's evolved and that's the way we are, is not going to help you understand what's going on, and it's going to confuse you.

At the end of my book, I summarize all the work I've done over the years on war. For the past few years, I've been talking about human nature and war. Before that, the big question for me was not, "Is it human nature to make war?" but, "How do you explain the wars that actually happened in tribal societies, and in modern society?" The book isn't just about debunking theories about chimpanzees, it's about: If you have this idea of culture that I just described, it leads you to ask a lot of other questions that are a lot more interesting, and probably more meaningful in terms of understanding why real wars happened and why people really get killed.

There's an article I wrote in 2006 called Tribal, Ethnic and Global Wars, where I summarize my approach to wars that are going on around the world, based on what I know about tribal warfare. In it, I try to show how it is that wars have happened, and the relationship between practical self-interest and the symbolic values people have in a society. That, to me, is where the action is, and it explains what the cause of war is: it's practical, and it's also symbolic.

AMS: In this current moment in human history, where we have much more globalized and ongoing warfare than our ancient ancestors—and a more globalized world culture in general, is there hope for a future that's not so war-inclined?

BF: Is there hope? Yes, absolutely. If you look at the long history of the world as I do as an anthropologist, you see that we've gone from having thousands of independent societies on this planet, which at first I don't think were making war. Over time war developed in more places around the world and spread. Since then, over time we've had a consolidation of societies. There are fewer independent societies in the world today—and you've got to be independent to go off and make war. I've been using Europe as an example now for over 20 years. You would have never expected Europe to come together into the community that it is now [looking at where it] was heading toward [in the past]. The war between Germany and France and England and other parts of Europe was world history for quite a long time. Europe is just one thread, but it's a strong example of how things have changed.

I wrote an article in 1988 called How Can Anthropologists Promote Peace?, and one of the things I said was that as an anthropologist, you can say that there are other possible worlds out there. The things that we can't imagine to be possible now could become true. And in this article that came out in '88, I said that one thing we can say with certainty is that at some point the militarized East-West frontier in Europe will cease to exist. It was hard to imagine that happening then. But the next year [after the article], it went away. So, we don't know. There's no general direction toward peace, but I think an important part of it is for people to mobilize themselves, for people to promote peace, for peace to be of value.

It's important for people to see that a world without war is a realistic possibility. Maybe not now, but a world without war is something we can aspire to realistically, and work toward. If you think that's something that can never happen, well that fatalism is one of the main props that is keeping war going. It's good to break out of that mindset.

April M. Short is an editor, journalist and documentary editor and producer. She is a writing fellow at Local Peace Economy, a project of the Independent Media Institute. Previously, she served as a managing editor at AlterNet as well as an award-winning senior staff writer for Santa Cruz, California's weekly newspaper. Her work has been published with the San Francisco Chronicle, In These Times, Salon and many others.

This article was produced by Local Peace Economy, a project of the Independent Media Institute.

Chimps Are Naturally Violent, Study Suggests

For years, anthropologists have watched wild chimpanzees "go ape" and attack each other in coordinated assaults. But until now, scientists were unsure whether interactions with humans had brought on this violent behavior or if it was part of the apes' basic nature.

A new, 54-year study suggests this coordinated aggression is innate to chimpanzees, and is not linked to human interference.

"Violence is a natural part of life for chimpanzees," Michael Wilson, the study's lead researcher and an associate professor of anthropology at the University of Minnesota in Minneapolis, told Live Science in an email. "They don't need to be fed bananas to kill each other." [Image Gallery: Lethal Aggression in Wild Chimpanzees]

As one of humanity's closest living relatives, chimps can shed light on the evolution of people, such as when humans adopted warlike behaviors, Wilson said.

"Studies of chimpanzee violence have been especially influential in how people think about the origins of human warfare," Wilson explained. "Some people have argued that human warfare is a recent cultural invention, the result of some other recent development such as the origin of agriculture."

But observations of chimpanzees by legendary primatologist Jane Goodall and other researchers challenged the idea that warfare is a modern human development. After all, humans and chimpanzees are the only two species in the world known to attack each other in organized onslaughts. Perhaps this behavior originated with a common ancestor some 5 to 7 million years ago, Wilson said.

Yet other scientists counter that human intrusions are to blame for the chimps' coordinated, lethal aggression. As populations in Africa grow, people are infringing on chimpanzee habitats. Loggers cut down forests farmers clear land for crops, and hunters kill chimps for food.

"People have argued that these increasing human impacts could also be putting more pressure on chimpanzee populations, leading to more chimpanzee violence," Wilson said.

He and his colleagues collaborated with researchers who are studying chimpanzees and bonobos, another ape that shares a common ancestor with humans. In all, the scientists collected data on 18 chimpanzee groups and four bonobo groups living in Africa.

The chimpanzees exhibited 152 killings, including 58 that the scientists observed, 41 that were inferred and 53 suspected killings in 15 communities, the researchers said. The bonobos had one suspected killing, the researchers said. The different acts of violence did not depend on human impacts, Wilson said.

Instead, attacks were more common at sites with many males and high population densities. Also, chimpanzees in East Africa killed more frequently than did chimps in West Africa, the study found.

Unsurprisingly, the bonobos showed little violence. "We didn't find any definite cases of killing by bonobos, though there was one case of a male bonobo who was severely attacked by members of his own group and never seen again," Wilson said.

Into the woods

Many of the researchers, including Dave Morgan, a research fellow with the Lester E. Fisher Center for the Study and Conservation of Apes at Lincoln Park Zoo in Chicago, have followed the chimpanzees in the study for years. When Morgan first arrived, in 1999, the chimpanzees were not afraid of humans, suggesting that this was the animals' first encounter with people, he said.

Chimpanzees can live in groups made up of as many as 150 individuals, but group size varies, Wilson said. Some study sites had about 55 chimpanzees living together, he said. [Grooming Gallery: Chimps Get Social]

"This is a very important study, because it compiles evidence from many sites over many years, and shows that the occurrence of lethal aggression in chimpanzees is not related to the level of human disturbance," Joan Silk, a professor in the school of Human Evolution and Social Change at Arizona State University, who was not involved in the study, told Live Science in an email.

Because chimps and bonobos do not have the same levels of coordinated lethal aggression, it's impossible to say how the common ancestor acted, Silk said. "But we can learn something about circumstances that may favor the evolution of this type of aggression, such as opportunities to encounter members of neighboring groups when they are on their own," she said.

Wilson and his colleagues followed the chimps and noted the apes' daily activities, such as mating, feeding, grooming, resting and fighting. During the 14 years it spent following the apes, Wilson's team saw two killings &mdash one when a neighboring community killed an infant, and another when a male chimpanzee consumed an infant.

But chimps, an endangered species, are not always warlike, he said.

"Overall, aggression makes [up] a small percentage of their daily lives," Wilson said, adding that, "our behavior affects them, but it's not affecting them as people have suggested in the past, resulting in aggression."

Biologist E.O. Wilson on Why Humans, Like Ants, Need a Tribe

Have you ever wondered why, in the ongoing presidential campaign, we so strongly hear the pipes calling us to arms? Why the religious among us bristle at any challenge to the creation story they believe? Or even why team sports evoke such intense loyalty, joy, and despair?

The answer is that everyone, no exception, must have a tribe, an alliance with which to jockey for power and territory, to demonize the enemy, to organize rallies and raise flags.

And so it has ever been. In ancient history and prehistory, tribes gave visceral comfort and pride from familiar fellowship, and a way to defend the group enthusiastically against rival groups. It gave people a name in addition to their own and social meaning in a chaotic world. It made the environment less disorienting and dangerous. Human nature has not changed. Modern groups are psychologically equivalent to the tribes of ancient history. As such, these groups are directly descended from the bands of primitive humans and prehumans.

The drive to join is deeply ingrained, a result of a complicated evolution that has led our species to a condition that biologists call eusociality. "Eu-," of course, is a prefix meaning pleasant or good: euphony is something that sounds wonderful eugenics is the attempt to improve the gene pool. And the eusocial group contains multiple generations whose members perform altruistic acts, sometimes against their own personal interests, to benefit their group. Eusociality is an outgrowth of a new way of understanding evolution, which blends traditionally popular individual selection (based on individuals competing against each other) with group selection (based on competition among groups). Individual selection tends to favor selfish behavior. Group selection favors altruistic behavior and is responsible for the origin of the most advanced level of social behavior, that attained by ants, bees, termites&mdashand humans.

Among eusocial insects, the impulse to support the group at the expense of the individual is largely instinctual. But to play the game the human way required a complicated mix of closely calibrated altruism, cooperation, competition, domination, reciprocity, defection, and deceit. Humans had to feel empathy for others, to measure the emotions of friend and enemy alike, to judge the intentions of all of them, and to plan a strategy for personal social interactions.

As a result, the human brain became simultaneously highly intelligent and intensely social. It had to build mental scenarios of personal relationships rapidly, both short term and long term. Its memories had to travel far into the past to summon old scenarios and far into the future to imagine the consequences of every relationship. Ruling on the alternative plans of action were the amygdala and other emotion-controlling centers of the brain and autonomic nervous system. Thus was born the human condition, selfish at one time, selfless at another, and the two impulses often conflicted.

Today, the social world of each modern human is not a single tribe but rather a system of interlocking tribes, among which it is often difficult to find a single compass. People savor the company of like-minded friends, and they yearn to be in one of the best&mdasha combat Marine regiment, perhaps, an elite college, the executive committee of a company, a religious sect, a fraternity, a garden club&mdashany collectivity that can be compared favorably with other, competing groups of the same category.

Their thirst for group membership and superiority of their group can be satisfied even with symbolic victory by their warriors in clashes on ritualized battlefields: that is, in sports. Like the cheerful and well-dressed citizens of Washington, D.C., who came out to witness the First Battle of Bull Run during the Civil War, they anticipate the experience with relish. The fans are lifted by seeing the uniforms, symbols, and battle gear of the team, the championship cups and banners on display, the dancing seminude maidens appropriately called cheerleaders. When the Boston Celtics defeated the Los Angeles Lakers for the National Basketball Association championship on a June night in 1984, the mantra was "Celts Supreme!" The social psychologist Roger Brown, who witnessed the aftermath, commented, "The fans burst out of the Garden and nearby bars, practically break dancing in the air, stogies lit, arms uplifted, voices screaming. The hood of a car was flattened, about thirty people jubilantly piled aboard, and the driver&mdasha fan&mdashsmiled happily . It did not seem to me that those fans were just sympathizing or empathizing with their team. They personally were flying high. On that night each fan's self-esteem felt supreme a social identity did a lot for many personal identities."

Experiments conducted over many years by social psychologists have revealed how swiftly and decisively people divide into groups and then discriminate in favor of the one to which they belong. Even when the experimenters created the groups arbitrarily, prejudice quickly established itself. Whether groups played for pennies or were divided by their preference for some abstract painter over another, the participants always ranked the out-group below the in-group. They judged their "opponents" to be less likable, less fair, less trustworthy, less competent. The prejudices asserted themselves even when the subjects were told the in-groups and out-groups had been chosen arbitrarily.

The tendency to form groups, and then to favor in-group members, has the earmarks of instinct. That may not be intuitive: some could argue that in-group bias is conditioned, not instinctual, that we affiliate with family members and play with neighboring children because we're taught to. But the ease with which we fall into those affiliations points to the likelihood that we are already inclined that way&mdashwhat psychologists call "prepared learning," the inborn propensity to learn something swiftly and decisively. And indeed, cognitive psychologists have found that newborn infants are most sensitive to the first sounds they hear, to their mother's face, and to the sounds of their native language. Later they look preferentially at persons who previously spoke their native language within their hearing. Similarly, preschool children tend to select native-language speakers as friends.

The elementary drive to form and take deep pleasure from in-group membership easily translates at a higher level into tribalism. People are prone to ethnocentrism. It is an uncomfortable fact that even when given a guilt-free choice, individuals prefer the company of others of the same race, nation, clan, and religion. They trust them more, relax with them better in business and social events, and prefer them more often than not as marriage partners. They are quicker to anger at evidence that an out-group is behaving unfairly or receiving undeserved rewards. And they grow hostile to any out-group encroaching upon the territory or resources of their in-group.

When in experiments black and white Americans were flashed pictures of the other race, their amygdalas, the brain's center of fear and anger, were activated so quickly and subtly that the centers of the brain were unaware of the response. The subject, in effect, could not help himself. When, on the other hand, appropriate contexts were added&mdashsay, the approaching African-American was a doctor and the white his patient&mdashtwo other sites of the brain integrated with the higher learning centers, the cingulate cortex and the dorsolateral preferential cortex, lit up, silencing input through the amygdala. Thus different parts of the brain have evolved by group selection to create groupishness, as well as to mediate this hardwired propensity.

When the amygdala rules the action, however, there is little or no guilt in the pleasure experienced from watching violent sporting events and war films in which the story unwinds to a satisfying destruction of the enemy. The horrors make the fascination. War is the strong life it is life in extremis.

Literature and history are strewn with accounts of what happens at the extreme, as in the following from Judges 12: 5&ndash6 in the Old Testament: the Gileadites captured the fords of the Jordan leading to Ephraim, and whenever a survivor of Ephraim said, "Let me go over," the men of Gilead asked him, "Are you an Ephraimite?" If he replied, "No," they said, "All right, say 'Shibboleth.'?" If he said "Sibboleth," because he could not pronounce the word correctly, they seized him and killed him at the fords of the Jordan. Forty-two thousand Ephraimites were killed at that time.

Research has shown that tribal aggressiveness goes well back beyond Neolithic times. And there is a good chance that it could be a much older heritage, dating beyond the split 6 million years ago between the lines leading to modern chimpanzees and to humans, respectively.

The patterns of collective violence in which young chimp males engage are remarkably similar to those of young human males. Aside from constantly vying for status, both for themselves and for their gangs, they tend to avoid open mass confrontations with rival troops, instead relying on surprise attacks. The purpose of raids made by the male gangs on neighboring communities is evidently to kill or drive out its members and acquire new territory. The entirety of such conquest under fully natural conditions has been witnessed by John Mitani and his collaborators in Uganda's Kibale National Park. The chimp war, conducted over 10 years, was eerily humanlike. Every 10 to 14 days, patrols of up to 20 males penetrated enemy territory, moving quietly in single file, scanning the terrain from ground to the treetops, and halting cautiously at every surrounding noise. If they encountered a force larger than their own, the invaders broke rank and ran back to their own territory. When they encountered a lone male, however, they pummeled and bit him to death. When a female was encountered, they usually let her go. (This latter tolerance was not a display of gallantry. If she carried an infant, they took it from her and killed and ate it.) Finally, after such constant pressure for so long, the invading gangs simply annexed the enemy territory, adding 22 percent to the land owned by their own community.

Our bloody nature, it can now be argued in the context of modern biology, is ingrained because group-versus-group was a principal driving force that made us what we are. In prehistory, group selection lifted the hominids to heights of solidarity, to genius, to enterprise. And to fear. Each tribe knew with justification that if it was not armed and ready, its very existence was imperiled. Throughout history, the escalation of a large part of technology has had combat as its central purpose. Today, public support is best fired up by appeal to the emotions of deadly combat, over which the amygdala is grandmaster. We find ourselves in the battle to stem an oil spill, the fight to tame inflation, the war against cancer. Wherever there is an enemy, animate or inanimate, there must be a victory.

Any excuse for a real war will do, so long as it is seen as necessary to protect the tribe. The remembrance of past horrors has no effect. It should not be thought that war, often accompanied by genocide, is a cultural artifact of a few societies. Nor has it been an aberration of history, a result of the growing pains of our species' maturation. Wars and genocide have been universal and eternal, respecting no particular time or culture. Overall, big wars have been replaced around the world by small wars of the kind and magnitude more typical of hunter-gatherer and primitively agricultural societies. Civilized societies have tried to eliminate torture, execution, and the murder of civilians, but those fighting little wars do not comply.

Civilization appears to be the ultimate redeeming product of competition between groups. Because of it, we struggle on behalf of good and against evil, and reward generosity, compassion, and altruism while punishing or downplaying selfishness. But if group conflict created the best in us, it also created the deadliest. As humans, this is our greatest, and worst, genetic inheritance.