Coming from an evolutionary approach, Is the only purpose of a scrotum to regulate the temperature of the testes?
Knowing all mammals are warm blooded, shouldn't all mammals have testes in a scrotum?
Having descended testes is a derived characteristic within mammals; monotremes and the Afrotheria (including elephants) all retain the ancestral character state (Kleisner, et al., 2010)2. Among those mammals with descended testes, these can be ascrotal or scrotal. Testicular descent is hypothesized to have only occurred once within Mammalia, with the ascrotal Laurasiatheria. Descended ascrotal testes are found in cetaceans, phocid seals, hippos, tapirs, rhinos, and some bats. Descended scrotal testes are found in horses, pigs, camels, and Carnivora.
Since basal mammals would presumably have to regulate testicular temperature just as much as derived mammals, the temperature regulation hypothesis seems to not hold up. So the real question is: why have a scrotum? One hypothesis has to do with evolution of fast locomotion (e.g., galloping).
According to Frey (1991, 40)4:
The strong flexions and extensions of the vertebral column during gallop should cause intense fluctuations of intra-abdominal pressure. Fluctuations of intra-abdominal pressure severely impede continuous flow of blood in the abdominal veins. Periodically reduced venous drainage resulting in fluctuations of intra-testicular pressure would impair the process of spermiohistogenesis, which is dependent on an absolutely constant pressure within the testis.
Chance (1996)5 suggests that the temperature hypothesis might represent a secondary adaptation:
Because in the human male, scrotal testes function optimally at temperatures below that of the body, much speculation, and a considerable amount of research, has gone into attempting to see what (metabolic) advantage might accrue from this lower temperature, without considering the possibility that this is a secondary adaptation to an enforced external position.
Kleisner, K., Ivell, R., and Flegr, J. 2010. The evolutionary history of testicular externalization and the origin of the scrotum. J Biosci 35:27-37
Frey, R. 1991. Zur Ursache des Hodenabstiegs (Descensus testiculorum) bei Säugetieren. Z Zool Sys Evolut-Forsch 29:40-65
Chance, M.R.A. 1996. Reason for externalization of the testis of mammals. J Zool 239:691-695
Elephants do not have a scrotal sac. One of the reasons may be their basal metabolsm is very low. Hence chance of an increase in body temperature is meager.Animals with higher metabolic rate will have scrotal sac.
The Evolutionary Origin of Descending Testicles
Descending testicles were likely present in the earliest mammals, then subsequently disappeared in elephants, manatees and their relatives, according to a new study.
Reader, here’s an incomplete list of things you shouldn’t try with elephants: a memory contest, jump rope and castration.
See, in addition to having uncanny recall and a firm relationship with gravity, elephants have their testicles nestled deep within their bodies, all the way up near their kidneys. That’s unusual: In most other mammals, testicles form during embryonic development near the kidneys and then descend, either to the lower abdomen or an external scrotum, by the time of a male’s birth.
Biologists have wondered about this discrepancy for decades. Did the earliest mammals retain their testicles, like elephants, or did they let their family jewels drop? A new study, published Thursday in PLOS Biology, says it was the latter.
Studying the DNA of 71 mammals, a German team concluded that testicular descent is an ancestral trait that was later lost in so-called afrotherians, a ragtag group that includes elephants, manatees and several insect-eaters that live in or originated from Africa.
In four afrotherian subgroups — manatees and dugongs, elephant shrews, golden moles and tenrecs (small insectivores that resemble hedgehogs) — the authors found nonfunctional remnants of two genes specifically involved in testicular descent.
Scientists often rely on geologic fossils to piece together evolutionary history, but this study shows that there is also a “fossil record in the genome,” said Mark Springer, a biology professor at the University of California, Riverside, who was not involved in the research.
These “molecular fossils” abound across the tree of life. “For pretty much any species, you’ll typically find on the order of a hundred or more broken genes that existed back in time and were lost,” said Michael Hiller, a senior research group leader at the Max Planck Institute of Molecular Cell Biology and Genetics in Dresden, Germany, and senior author of the new paper.
He and the study’s lead author, Virag Sharma, did not start out targeting testicles.
Over years, their group had developed a computational method to screen different genomes for broken genes with high precision. They observed vestiges of genes that were rendered useless by evolution: enamel-making genes in toothless whales, fat-digestion genes in sugar-dependent fruit bats and DNA repair genes in armadillos with armor that protects them from harmful UV radiation.
Additionally, they noticed that two genes called RXFP2 and INSL3 were inactive in several afrotherian species.
From a literature search the researchers learned that if you knock these genes out in male mice, the rodents’ testicles won’t descend. They also learned that evolutionary biologists have long debated whether this absence of testicular descent — called testicondy — is a primitive trait, or one that afrotherians uniquely evolved.
“It became clear that we’d be able to help resolve that debate,” Dr. Hiller said.
Based on the fact that genes start to rack up mutations once they lose their function, the researchers worked backward and estimated that testicondy independently arose at least four times, ranging from about 25 million years ago in cape golden moles to about 80 million years ago in cape elephant shrews.
This also meant that testicondy evolved after afrotherians split from other placental mammals, about 100 million years ago, which suggests the common ancestor of all mammals did indeed lower their testes, Dr. Hiller said.
But mysteries still remain. Not all afrotherians exhibit testicondy — aardvarks, for instance, have descending testicles. And although elephants and rock hyraxes (which resemble guinea pigs) do not have descending testicles, RXFP2 and INSL3 are still intact in both.
It may be that researchers are only “looking at part of the picture,” and that other genes and processes involved have not yet been identified, said Ross MacPhee, a mammalogy curator at the American Museum of Natural History who did not participate in the new study.
There’s also the question of why testicles plummet in the first place. Given that they hold precious, life-giving contents, why carry them in vulnerable sacks? Scientists know that optimal sperm production requires temperatures lower than that of rest of the body, but they don’t understand why.
Various hypotheses have been proposed, including the idea that dangling gonads are a way to signal virility and good health, but none are satisfactory.
The answer may lie in further study of afrotherians, particularly why and how they came to hold their testicles so close.
Males have two testicles of similar size contained within the scrotum, which is an extension of the abdominal wall. Scrotal asymmetry is not unusual: one testicle extends farther down into the scrotum than the other due to differences in the anatomy of the vasculature.
The volume of the testicle can be estimated by palpating it and comparing it to ellipsoids of known sizes. Another method is to use calipers (an orchidometer) or a ruler either on the person or on an ultrasound image to obtain the three measurements of the x, y, and z axes (length, depth and width). These measurements can then be used to calculate the volume, using the formula for the volume of an ellipsoid:
An average adult testicle measures up to 5 cm × 2 cm × 3 cm (2 in × 3 ⁄ 4 in × 1 + 1 ⁄ 4 in). The Tanner scale for the maturity of male genitals assigns a maturity stage to the calculated volume ranging from stage I, a volume of less than 1.5 cm 3 to stage V, a volume greater than 20 cm 3 . Normal volume is 15 to 25 cm 3 the average is 18 cm 3 per testis (range 12–30 cm 3 ). 
The testes are covered by a tough membranous shell called the tunica albuginea. Within the testes are very fine coiled tubes called seminiferous tubules. The tubules are lined with a layer of cells (germ cells) that develop from puberty through old age into sperm cells (also known as spermatozoa or male gametes). The developing sperm travel through the seminiferous tubules to the rete testis located in the mediastinum testis, to the efferent ducts, and then to the epididymis where newly created sperm cells mature (see spermatogenesis). The sperm move into the vas deferens, and are eventually expelled through the urethra and out of the urethral orifice through muscular contractions.
Primary cell types
- Here, germ cells develop into spermatogonia, spermatocytes, spermatids and spermatozoon through the process of spermatogenesis. The gametes contain DNA for fertilization of an ovum  – the true epithelium of the seminiferous epithelium, critical for the support of germ cell development into spermatozoa. Sertoli cells secrete inhibin.  surround the seminiferous tubules. 
- – cells localized between seminiferous tubules that produce and secrete testosterone and other androgens important for sexual development and puberty, secondary sexual characteristics like facial hair, sexual behavior and libido, supporting spermatogenesis and erectile function. Testosterone also controls testicular volume.
- Also present are:
- Immature Leydig cells
- Interstitial macrophages and epithelial cells.
Blood supply and lymphatic drainage
Blood supply and lymphatic drainage of the testes and scrotum are distinct:
- The paired testicular arteries arise directly from the abdominal aorta and descend through the inguinal canal, while the scrotum and the rest of the external genitalia is supplied by the internal pudendal artery (itself a branch of the internal iliac artery).
- The testis has collateral blood supply from 1. the cremasteric artery (a branch of the inferior epigastric artery, which is a branch of the external iliac artery), and 2. the artery to the ductus deferens (a branch of the inferior vesical artery, which is a branch of the internal iliac artery). Therefore, if the testicular artery is ligated, e.g., during a Fowler-Stevens orchiopexy for a high undescended testis, the testis will usually survive on these other blood supplies.
- Lymphatic drainage of the testes follows the testicular arteries back to the paraaortic lymph nodes, while lymph from the scrotum drains to the inguinal lymph nodes.
Many anatomical features of the adult testis reflect its developmental origin in the abdomen. The layers of tissue enclosing each testicle are derived from the layers of the anterior abdominal wall. Notably, the cremasteric muscle arises from the internal oblique muscle.
The blood–testis barrier
Large molecules cannot pass from the blood into the lumen of a seminiferous tubule due to the presence of tight junctions between adjacent Sertoli cells. The spermatogonia are in the basal compartment (deep to the level of the tight junctions) and the more mature forms such as primary and secondary spermatocytes and spermatids are in the adluminal compartment.
The function of the blood–testis barrier may be to prevent an auto-immune reaction. Mature sperm (and their antigens) arise long after immune tolerance is established in infancy. Therefore, since sperm are antigenically different from self tissue, a male animal can react immunologically to his own sperm. In fact, he is capable of making antibodies against them.
Injection of sperm antigens causes inflammation of the testis (auto-immune orchitis) and reduced fertility. Thus, the blood–testis barrier may reduce the likelihood that sperm proteins will induce an immune response, reducing fertility and so progeny.
Spermatogenesis is enhanced at temperatures slightly less than core body temperature.  The spermatogenesis is less efficient at lower and higher temperatures than 33 °C.  Because the testes are located outside the body, the smooth tissue of the scrotum can move them closer or further away from the body.  The temperature of the testes is maintained at 35 degrees Celsius (95 degrees Fahrenheit), i.e. two degrees below the body temperature of 37 degrees Celsius (98.6 degrees Fahrenheit). Higher temperatures affect spermatogenesis.  There are a number of mechanisms to maintain the testes at the optimum temperature. 
The cremasteric muscle is part of the spermatic cord. When this muscle contracts, the cord is shortened and the testicle is moved closer up toward the body, which provides slightly more warmth to maintain optimal testicular temperature. When cooling is required, the cremasteric muscle relaxes and the testicle is lowered away from the warm body and is able to cool. Contraction also occurs in response to stress (the testicles rise up toward the body in an effort to protect them in a fight).
The cremaster muscle can reflexively raise each testicle individually if properly triggered. This phenomenon is known as the cremasteric reflex. The testicles can also be lifted voluntarily using the pubococcygeus muscle, which partially activates related muscles.
Gene and protein expression
The human genome includes approximately 20,000 protein coding genes: 80% of these genes are expressed in adult testes.  The testes have the highest fraction of tissue type-specific genes compared to other organs and tissues:  about 1000 of them are highly specific for the testes,  and about 2,200 show an elevated pattern of expression here. A majority of these genes encode for proteins that are expressed in the seminiferous tubules and have functions related to spermatogenesis.   Sperm cells express proteins that result in the development of flagella these same proteins are expressed in the female in cells lining the fallopian tube, and cause the development of cilia. In other words, sperm cell flagella and Fallopian tube cilia are homologous structures. The testis-specific proteins that show the highest level of expression are protamines.
There are two phases in which the testes grow substantially namely in embryonic and pubertal age.
During mammalian development, the gonads are at first capable of becoming either ovaries or testes.  In humans, starting at about week 4 the gonadal rudiments are present within the intermediate mesoderm adjacent to the developing kidneys. At about week 6, sex cords develop within the forming testes. These are made up of early Sertoli cells that surround and nurture the germ cells that migrate into the gonads shortly before sex determination begins. In males, the sex-specific gene SRY that is found on the Y-chromosome initiates sex determination by downstream regulation of sex-determining factors, (such as GATA4, SOX9 and AMH), which leads to development of the male phenotype, including directing development of the early bipotential gonad down the male path of development.
Testes follow the "path of descent" from high in the posterior fetal abdomen to the inguinal ring and beyond to the inguinal canal and into the scrotum. In most cases (97% full-term, 70% preterm), both testes have descended by birth. In most other cases, only one testis fails to descend (cryptorchidism) and that will probably express itself within a year.
The testes grow in response to the start of spermatogenesis. Size depends on lytic function, sperm production (amount of spermatogenesis present in testis), interstitial fluid, and Sertoli cell fluid production. After puberty, the volume of the testes can be increased by over 500% as compared to the pre-pubertal size. [ citation needed ] Testicles are fully descended before one reaches puberty.
Protection and injury
- The testicles are well known to be very sensitive to impact and injury. The pain involved travels up from each testicle into the abdominal cavity, via the spermatic plexus, which is the primary nerve of each testicle. This will cause pain in the hip and the back. The pain usually goes away in a few minutes. is a medical emergency. is a medical emergency caused by blunt force impact, sharp edge, or piercing impact to one or both testicles, which can lead to necrosis of the testis in as little as 30 minutes. 
- Penetrating injuries to the scrotum may cause castration, or physical separation or destruction of the testes, possibly along with part or all of the penis, which results in total sterility if the testicles are not reattached quickly.
- Some jockstraps are designed to provide support to the testicles. 
Diseases and conditions
- and other neoplasms – To improve the chances of catching possible cases of testicular cancer or other health issues early, regular testicular self-examination is recommended. , swollen vein(s) from the testes, usually affecting the left side,  the testis usually being normal , swelling around testes caused by accumulation of clear liquid within a membranous sac, the testis usually being normal , a retention cyst of a tubule of the rete testis or the head of the epididymis distended with barely watery fluid that contains spermatozoa can also affect the size and function of the testis.
- Certain inherited conditions involving mutations in key developmental genes also impair testicular descent, resulting in abdominal or inguinal testes which remain nonfunctional and may become cancerous. Other genetic conditions can result in the loss of the Wolffian ducts and allow for the persistence of Müllerian ducts. Both excess and deficient levels of estrogens can disrupt spermatogenesis and cause infertility.  is a deformity in which the testicle is not attached to the scrotal walls, and can rotate freely on the spermatic cord within the tunica vaginalis. It is the most common underlying cause of testicular torsion. inflammation of the testicles , a painful inflammation of the epididymis or epididymides frequently caused by bacterial infection but sometimes of unknown origin. , the absence of one or both testicles. or "undescended testicles", when the testicle does not descend into the scrotum of the infant boy.
Testicular enlargement is an unspecific sign of various testicular diseases, and can be defined as a testicular size of more than 5 cm (long axis) x 3 cm (short axis). 
Blue balls is a slang term for a temporary fluid congestion in the testicles and prostate region caused by prolonged sexual arousal.
Testicular prostheses are available to mimic the appearance and feel of one or both testicles, when absent as from injury or as treatment in association to gender dysphoria. There have also been some instances of their implantation in dogs. 
Effects of exogenous hormones
To some extent, it is possible to change testicular size. Short of direct injury or subjecting them to adverse conditions, e.g., higher temperature than they are normally accustomed to, they can be shrunk by competing against their intrinsic hormonal function through the use of externally administered steroidal hormones. Steroids taken for muscle enhancement (especially anabolic steroids) often have the undesired side effect of testicular shrinkage.
Similarly, stimulation of testicular functions via gonadotropic-like hormones may enlarge their size. Testes may shrink or atrophy during hormone replacement therapy or through chemical castration.
In all cases, the loss in testes volume corresponds with a loss of spermatogenesis.
Testicles of a male calf or other livestock are cooked and eaten in a dish sometimes called Rocky Mountain oysters. 
As early as 330 BC, Aristotle prescribed the ligation (tying off) of the left testicle in men wishing to have boys.  In the Middle Ages, men who wanted a boy sometimes had their left testicle removed. This was because people believed that the right testicle made "boy" sperm and the left made "girl" sperm. 
One theory about the etymology of the word testis is based on Roman law. The original Latin word testis, "witness", was used in the firmly established legal principle "Testis unus, testis nullus" (one witness [equals] no witness), meaning that testimony by any one person in court was to be disregarded unless corroborated by the testimony of at least another. This led to the common practice of producing two witnesses, bribed to testify the same way in cases of lawsuits with ulterior motives. Since such "witnesses" always came in pairs, the meaning was accordingly extended, often in the diminutive (testiculus, testiculi). [ citation needed ]
Another theory says that testis is influenced by a loan translation, from Greek parastatēs "defender (in law), supporter" that is "two glands side by side". 
In slang, the testes are usually referred to as "balls" as a reference to blue balls. Frequently, "nuts" (sometimes intentionally misspelled as "nutz") are also a slang term for the testes due to the geometric resemblance., as evidenced by the various usages of the term "Deez Nuts", which include a satirical political candidate in 2016.
In sharks, the testicle on the right side is usually larger, and in many bird and mammal species, the left may be the larger. The primitive jawless fish have only a single testis, located in the midline of the body, although even this forms from the fusion of paired structures in the embryo. 
In seasonal breeders, the weight of the testes often increases during the breeding season.  The testicles of a dromedary camel are 7–10 cm (2.8–3.9 in) long, 4.5 cm (1.8 in) deep and 5 cm (2.0 in) in width. The right testicle is often smaller than the left. 
The basal condition for mammals is to have internal testes.  The testes of monotremes,   xenarthrans,  and elephants  remain within the abdomen. There are also some marsupials with external testes    and Boreoeutherian mammals with internal testes, such as the rhinoceros.  Cetaceans such as whales and dolphins also have internal testes.   As external testes would increase drag in the water they have internal testes which are kept cool by special circulatory systems that cool the arterial blood going to the testes by placing the arteries near veins bringing cooled venous blood from the skin.   In odobenids and phocids, the location of the testes is para-abdominal, though otariids have scrotal testes. 
Boreoeutherian land mammals, the large group of mammals that includes humans, have externalized testes.  Their testes function best at temperatures lower than their core body temperature. Their testes are located outside of the body, suspended by the spermatic cord within the scrotum.
There are several hypotheses why most boreotherian mammals have external testes which operate best at a temperature that is slightly less than the core body temperature, e.g. that it is stuck with enzymes evolved in a colder temperature due to external testes evolving for different reasons, that the lower temperature of the testes simply is more efficient for sperm production.
1) More efficient. The classic hypothesis is that cooler temperature of the testes allows for more efficient fertile spermatogenesis. In other words, there are no possible enzymes operating at normal core body temperature that are as efficient as the ones evolved, at least none appearing in our evolution so far.
The early mammals had lower body temperatures and thus their testes worked efficiently within their body. However it is argued that boreotherian mammals have higher body temperatures than the other mammals and had to develop external testes to keep them cool. It is argued that those mammals with internal testes, such as the monotremes, armadillos, sloths, elephants, and rhinoceroses, have a lower core body temperatures than those mammals with external testes. [ citation needed ]
However, the question remains why birds despite having very high core body temperatures have internal testes and did not evolve external testes.  It was once theorized that birds used their air sacs to cool the testes internally, but later studies revealed that birds' testes are able to function at core body temperature. 
Some mammals which have seasonal breeding cycles keep their testes internal until the breeding season at which point their testes descend and increase in size and become external. 
2) Irreversible adaptation to sperm competition. It has been suggested that the ancestor of the boreoeutherian mammals was a small mammal that required very large testes (perhaps rather like those of a hamster) for sperm competition and thus had to place its testes outside the body.  This led to enzymes involved in spermatogenesis, spermatogenic DNA polymerase beta and recombinase activities evolving a unique temperature optimum, slightly less than core body temperature. When the boreoeutherian mammals then diversified into forms that were larger and/or did not require intense sperm competition they still produced enzymes that operated best at cooler temperatures and had to keep their testes outside the body. This position is made less parsimonious by the fact that the kangaroo, a non-boreoeutherian mammal, has external testicles. The ancestors of kangaroos might, separately from boreotherian mammals, have also been subject to heavy sperm competition and thus developed external testes, however, kangaroo external testes are suggestive of a possible adaptive function for external testes in large animals.
3) Protection from abdominal cavity pressure changes. One argument for the evolution of external testes is that it protects the testes from abdominal cavity pressure changes caused by jumping and galloping. 
4) Protection against DNA damage. Mild, transient scrotal heat stress causes DNA damage, reduced fertility and abnormal embryonic development in mice.  DNA strand breaks were found in spermatocytes recovered from testicles subjected to 40 °C or 42 °C for 30 minutes.  These findings suggest that the external location of the testicles provides the adaptive benefit of protecting spermatogenic cells from heat-induced DNA damage that could otherwise lead to infertility and germline mutation.
The relative size of testes is often influenced by mating systems.  Testicular size as a proportion of body weight varies widely. In the mammalian kingdom, there is a tendency for testicular size to correspond with multiple mates (e.g., harems, polygamy). Production of testicular output sperm and spermatic fluid is also larger in polygamous animals, possibly a spermatogenic competition for survival. The testes of the right whale are likely to be the largest of any animal, each weighing around 500 kg (1,100 lb). 
Among the Hominidae, gorillas have little female promiscuity and sperm competition and the testes are small compared to body weight (0.03%). Chimpanzees have high promiscuity and large testes compared to body weight (0.3%). Human testicular size falls between these extremes (0.08%). 
Testis weight also varies in seasonal breeders like red foxes,  golden jackals  and coyotes. 
Under a tough membranous shell called the tunica albuginea, the testis of amniotes, as well as some teleost fish, contains very fine coiled tubes called seminiferous tubules.
Amphibians and most fish do not possess seminiferous tubules. Instead, the sperm are produced in spherical structures called sperm ampullae. These are seasonal structures, releasing their contents during the breeding season, and then being reabsorbed by the body. Before the next breeding season, new sperm ampullae begin to form and ripen. The ampullae are otherwise essentially identical to the seminiferous tubules in higher vertebrates, including the same range of cell types. 
The Scrotum Is Nuts
Courtesy of Gijs Joost Brouwer
Soccer fans call it brave goalkeeping, the act of springing into a star shape in front of an attacker who is about to kick the ball as hard as possible toward the goal. As I shuffled from the field, bent forward, eyes watering, waiting for the excruciating whack of pain in my crotch to metamorphose into a gut-wrenching ache, I thought only stupid goalkeeping. But after the fourth customary slap on the back from a teammate chortling, “Hope you never wanted kids, pal,” I thought only stupid, stupid testicles.
Natural selection has sculpted the mammalian forelimb into horses’ front legs, dolphins’ fins, bats’ wings, and my soccer ball-catching hands. Why, on the path from the primordial soup to us curious hairless apes, did evolution house the essential male reproductive organs in an exposed sac? It’s like a bank deciding against a vault and keeping its money in a tent on the sidewalk.
Some of you may be thinking that there is a simple answer: temperature. This arrangement evolved to keep them cool. I thought so, too, and assumed that a quick glimpse at the scientific literature would reveal the biological reasons and I’d move on. But what I found was that the small band of scientists who have dedicated their professional time to pondering the scrotum’s existence are starkly divided over this so-called cooling hypothesis.
Reams of data show that scrotal sperm factories, including our own, work best a few degrees below core body temperature. The problem is, this doesn’t prove cooling was the reason that testicles originally descended. It’s a straight-up chicken-and-egg situation—did testicles leave the kitchen because they couldn’t stand the heat, or do they work best in the cold because they had to leave the body?
Vital organs that work optimally at 98.5 degrees Fahrenheit get bony protection: My brain and liver are shielded by skull and ribs, and my girlfriend’s ovaries are defended by her pelvis. Forgoing skeletal protection is dangerous. Each year, thousands of men go to the hospital with ruptured testes or torsions caused by having this essential organ suspended chandelierlike on a flexible twine of tubes and cords. But having exposed testicles as an adult is not even the most dangerous aspect of our reproductive organs’ arrangement.
The developmental journey to the scrotum is treacherous. At eight weeks of development, a human fetus has two unisex structures that will become either testicles or ovaries. In girls, they don’t stray far from this starting point up by the kidneys. But in boys, the nascent gonads make a seven-week voyage across the abdomen on a pulley system of muscles and ligaments. They then sit for a few weeks before coordinated waves of muscular contractions force them out through the inguinal canal.
The complexity of this journey means that it frequently goes wrong. About 3 percent of male infants are born with undescended testicles, and although often this eventually self-corrects, it persists in 1 percent of 1-year-old boys and typically leads to infertility.
Excavating the inguinal canal also introduces a significant weakness in the abdominal wall, a passage through which internal organs can slip. In the United States, more than 600,000 surgeries are performed annually to repair inguinal hernias—the vast majority of them in men.
This increased risk of hernias and sterilizing mishaps seems hardly in keeping with the idea of evolution as survival of the fittest. Natural selection’s tagline reflects the importance of attributes that help keep creatures alive—not dying being an essential part of evolutionary success. How can a trait such as scrotality (to use the scientific term for possessing a scrotum), with all the obvious handicaps it confers, fit into this framework? Its story is certainly going to be less straightforward than the evolution of a cheetah’s leg muscles. Most investigators have tended to think that the advantages of this curious anatomical arrangement must come in the shape of improved fertility. But this is far from proven.
When considering any evolved characteristic, good first questions are who has it and who had it first. In birds, reptiles, fish, and amphibians, male gonads are internal. The scrotum is a curiosity unique to mammals. A recent testicle’s-eye view of the mammalian family tree revealed that the monumental descent occurred pretty early in mammalian evolution. And what’s more, the scrotum was so important that it evolved twice.
The first mammals lived about 220 million years ago. The most primitive living mammals are the duck-billed platypus and its ilk—creatures with key mammalian features such as warm blood, fur, and lactation (the platypus kind of sweats milk rather than having tidy nipples), although they still lay eggs like the ancestors they share with reptiles. Platypus testicles, and almost certainly those of all early mammals, sit right where they start life, safely tucked by the kidneys.
About 70 million years later, marsupials evolved, and it is on this branch of the family tree that we find the first owner of a scrotum. Nearly all marsupials today have scrotums, and so logically the common ancestor of kangaroos, koalas, and Tasmanian devils had the first. Marsupials evolved their scrotum independently from us placental mammals, which is known thanks to a host of technical reasons, the most convincing of which is that it’s back-to-front. Marsupials’ testicles hang in front of their penises.
Fifty million years after the marsupial split is the major fork in the mammalian tree, scrotally speaking. Take a left and you will encounter elephants, mammoths, aardvarks, manatees, and groups of African shrew- and mole-like creatures. But you will never see a scrotum—all of these placental animals, like platypuses, retain their gonads close to their kidneys.
However, take a right, to the human side of the tree, at this 100 million-year-old juncture and you’ll find descended testicles everywhere. Whatever they’re for, scrotums bounce along between the hind limbs of cats, dogs, horses, bears, camels, sheep, and pigs. And, of course, we and all our primate brethren have them. This means that at the base of this branch is the second mammal to independently concoct scrotality—the one to whom we owe thanks for our dangling parts being, surely correctly, behind the penis.
Between these branches, however, is where it gets interesting, for there are numerous groups, our descended but ascrotal cousins, whose testes drop down away from the kidneys but don’t exit the abdomen. Almost certainly, these animals evolved from ancestors whose testes were external, which means at some point they backtracked on scrotality, evolving anew gonads inside the abdomen. They are a ragtag bunch including hedgehogs, moles, rhinos and tapirs, hippopotamuses, dolphins and whales, some seals and walruses, and scaly anteaters.
For mammals that returned to the water, tucking everything back up inside seems only sensible a dangling scrotum isn’t hydrodynamic and would be an easy snack for fish attacking from below. I say snack, but the world record-holders, right whales, have testicles that tip the scales at more than 1,000 pounds apiece. The trickier question, which may well be essential for understanding its function, is why did the scrotal sac lose its magic for terrestrial hedgehogs, rhinos, and scaly anteaters?
The scientific search to explain the scrotum’s raison d’être began in England in the 1890s at Cambridge University. Joseph Griffiths, using terriers as his unfortunate subjects, pushed their testicles back into their abdomens and sutured them there. As little as a week later, he found that the testes had degenerated, the tubules where sperm production occurs had constricted, and sperm were virtually absent. He put this down to the higher temperature of the abdomen, and the cooling hypothesis was born.
In the 1920s, a time when Darwin’s ideas were rapidly spreading, Carl Moore at the University of Chicago argued that after mammals had transitioned from cold- to warm-blooded, keeping the body in the mid-to-high 90 degrees must have severely hampered sperm production, and the first males to cool things off with a scrotum became the more successful breeders.
Heat disrupts sperm production so effectively that biology textbooks and medical tracts alike give cooling as the reason for the scrotum. The problem is many biologists who seriously think about animal evolution are unhappy with this. Opponents say that testicles function optimally at cooler temperatures because they evolved this trait after their exile.
If mammals became warm-blooded 220 million or so years ago, it would mean mammals carried their gonads internally for more than 100 million years before the scrotum made its bow. The two events were hardly tightly coupled.
The hypothesis’ biggest problem, though, is all the sacless branches on the family tree. Regardless of their testicular arrangements, all mammals have elevated core temperatures. If numerous mammals lack a scrotum, there is nothing fundamentally incompatible with making sperm at high temperatures. Elephants have a higher core temperature than gorillas and most marsupials. And beyond mammals it gets worse: Birds, the only other warm-blooded animals, have internal testes despite having core temperatures that in some species run to 108 degrees.
Any argument for why cooling would be better for sperm has to say exactly why. The idea that a little less heat might keep sperm DNA from mutating has been proposed, and recently it’s been suggested that keeping sperm cool may allow the warmth of a vagina to act as an extra activating signal. But these ideas still fail to surmount the main objections to the cooling hypothesis.
Michael Bedford of Cornell Medical College is no fan of the cooling hypothesis applied to testicles, but he does wonder whether having a cooled epididymis, the tube where sperm sit after leaving their testicular birthplace, might be important. (Sperm are impotent on exiting the testes and need a few final modifications while in the epididymis.) Bedford has noted that some animals with abdominal testes have extended their epididymis to just below the skin, and that some furry scrotums have a bald patch for heat loss directly above this storage tube. But if having a cool epididymis is the main goal, why throw the testicles out with it?
Another proposal for how the scrotum generates better sperm is that the scrotal sac serves as a school of hard knocks. Scott Freeman of the University of Washington hypothesized that the scrotum’s poor blood supply keeps the testicles in an oxygen-starved environment and so toughens up the sperm. Deprived of oxygen, sperm might react like “muscle cells to aerobic training,” increasing the number and size of mitochondria they contain and therefore becoming better prepared for the herculean task of ascending a cervix, uterus, and fallopian tube.
The main problem with the training hypothesis is that it was primarily concerned with the testicles’ lousy blood supply rather than their expulsion¾surely it would have been easier to evolve poor gonadal vasculature while keeping them in the body?
The alternative to scrotums benefiting sperm is that in some other way, despite their fragility, they actually benefit their owner. Such a notion was first presented in 1952 by a Swiss zoologist named Adolf Portmann after he’d presented the first major attack on the cooling hypothesis. What he proposed instead was the display hypothesis. Portmann argued that by placing the gonads on the outside, the male was giving a clear indication of his “reproductive pole,” a sexual signal important in intergender communication. Portmann’s best evidence was a few Old World monkeys who have brightly colored scrota.
This theory is not widely accepted because such conspicuous displays are rare (many scrotums are barely visible) and bright coloration seems to have evolved long after the original scrotum. Some have suggested it’s not surprising that in its 100 million-year existence, the scrotum has been co-opted as a sexual attractant by a handful of groups.
I was just about to discard the display hypothesis when two things happened. First, a colleague returned from her honeymoon in Tanzania excitedly showing anyone who’d look photos of a scrotum. The scrotum belonged, don’t worry, to one of Portmann’s Old World monkeys, a vervet monkey, and it was screamingly, beguilingly bright blue.*
OK, it’s just one monkey, I thought, but then I met Richard Dawkins. I had three minutes with the esteemed evolutionary biologist at a book signing, so I asked him for his opinions on the scrotum. After expressing severe doubt about the cooling hypothesis, he said he wondered whether it might have something to do with evolutionary biology’s handicap principle.
Handicap theory posits that if a female had to choose between two suitors who had beaten out all other competitors, but one had done so with a hand tied behind his back, she’d go for him because he’s obviously tougher still. It is controversial, but it does offer explanations for a number of problematic biological phenomena, such as male birds’ colorful plumage and songs that should attract predators. If the handicap theory is right, the scrotum exists to let its possessor say, “I’m so able to look after myself, I can keep these on the outside!”
In the mid-1990s, Michael Chance, a professor of animal behavior at the U.K.’s University of Birmingham, came across a newspaper story about the Oxford-Cambridge University boat race that piqued his interest in testicles. He learned that after the race, the rowers’ urine contained fluid from their prostates.
The oarsmen’s exertions, the cyclic abdominal straining, had deposited prostatic fluid in their urethras because there are no sphincters in the reproductive tract. Without such valves, squeezing of any of the sacs and tubes that make up this system is liable to empty it, or at least rearrange its contents. In 1996, in what has become known as the galloping hypothesis, Chance argued that externalization of the testes was necessary when mammals started to move in ways that sharply increased abdominal pressure.
A survey of how mammals move reveals a good deal of variety. And when Chance listed animals with internal testicles, he didn’t find many gallopers. The elephants, aardvarks, and their cousins on the undescended branch of the mammalian tree don’t bound or jump around. On the other side, the creatures such as moles and hedgehogs that reabsorbed their sexual cargo seem to have evolved away from internally disruptive types of movement. Among mammals that have returned to the sea, the few that have retained scrotums are the only ones who breed on land, such as elephant seals, who fight vigorously to defend their territory during rutting season.
One might argue that evolution could surely have thrown in a sphincter or two, or some internal shielding, but besides the possibility that the mechanics of ejaculation would struggle with such things, another argument supports Chance’s thinking. In 1991 Roland Frey of Germany’s Freiburg University reported a number of features of blood vessels of scrotal testes that ensure more constant pressure, possibly to avoid impaired blood drainage during galloping. The specific adaptations are different between marsupials and the rest of us but seem aimed at the same goal.
The galloping hypothesis would be a case of evolutionary compromise—the dangers of scrotality being a necessary price for the greater advantages of a new and valuable type of movement.
There are many theories in evolutionary biology. Often there’s great pleasure in the detectivelike process of piecing together the available, incomplete evidence into a coherent story, but the big challenge for this science is actually testing these ideas. One exciting recent development that might provide relevant evolutionary data has been the identification of the signal that controls the testicles’ initial descent from the kidney region to the undercarriage.
When the testes and ovaries are young, they are held in place by the so-called cranial suspensory ligament, while holding on loosely is a second, measly ligament termed the gubernaculum. To begin their roller-coaster ride, testicles secrete a signal that causes the suspensory ligament to degenerate and the gubernaculum to grow capable of dragging them to the base of the abdomen.
To study the evolution of this signal, a molecule related to insulin, Teddy Hsu and colleagues at Stanford University turned to the duck-billed platypus. They found that the platypus has a single gene for the prototype version of the signal, and that it was this gene’s duplication in subsequent mammals that allowed one version to evolve a function in testicular descent and the other in nipple development.
It’s a beautiful example of a genetic event in biological history that produced mammalian specialization. However, elephants and their nondescended cousins all have the duplicated genes, so the story’s not complete. A crucial next step will be determining the genes required for forming the inguinal canal and making the scrotum. Probably the best place to look will be in those mammals that have backtracked on externalization, where these genes have likely changed.
It’s rather humbling to realize that this basic aspect of our bodies remains a mystery. The fact that such a ridiculous appendage evolved twice surely means we should be able to get a handle on it. A successful theory will have to explain the full diversity of mammalian testicle positions, not just the scrotum’s existence. I like Chance and Frey’s galloping hypothesis, but could a scrotum really be the only way to deal with undulating abdominal pressure? In addition, do scrotal sperm really differ fundamentally from internally generated tiddlers? Can we definitively prove temperature sensitivity evolved after the expulsion of the scrotum? And signaling is still an outside bet, but if scrotums were really sexually selected, where’s the mammalian peacock, some species toting a pair of soccer balls?
Talking of which, while we wait for a final answer, the scrotality totality, us soccer goalkeepers should probably look to our baseball-playing friends who use evolution’s gift of a large brain and opposable thumbs to don a protective cup.
Correction, July 9, 2013: This article originally misspelled the name of the vervet monkey.
Hippo Testicles Are So Mobile They Make Castration Tricky
The hippopotamus is among the world’s largest creatures on land. Only elephants and some species of rhino are larger. Hippos also are very aggressive. Legendarily ill-tempered and entirely unafraid of humans, they are responsible for the majority of wildlife deaths in Africa.
These are some of the facts no doubt borne in mind during the work performed by a team of veterinary researchers who have developed a method for the delicate operation of castrating a hippo. The problem is that, unlike a quick visit to the vet to have a pet spayed, castrating a one-and-a-half tonne animal with powerful jaws and thick, rubbery hide is not easy. Their recent paper has revealed how this tricky task is complicated by the fact that hippos testicles are not only hard to find, but actually move around. Hippos are blessed with, as lead author Chris Walzer of the University of Veterinary Medicine in Vienna put it, “highly mobile testicles.”
Unlike humans, hippos' testicles are not external, nor are they tucked inside the abdomen. Instead they are located inside the inguinal canal, a space in the lower front part of the body. But their exact location in the canal varies widely, sometimes minute by minute. “Hippo testicles are retractable, and can vary in depth by around 40cm, which makes them quite hard to find,” Walzer said, adding that there had been in the past several documented efforts that tried and failed to locate them, and at least one paper that declared that it was “not known” where they are.
Without the right leg position, the testicles perform a disappearing act.
The team led by Walzer, veterinary professor of wildlife population health at the university’s Research Institute for Wildlife Ecology, developed a technique for applying the initial anaesthetic, and for the castration itself. Ultrasound scanning is used to locate the testicles, and if they are retracted the scanner is wrapped in a sterile bag and inserted into the incision in order to get a better reading from the inside on where they’ve gone.
The anaesthetic procedure was also difficult. “The thing about hippos is that no one wants to work with them as they’re so dangerous, which is why we developed the anaesthesia protocol. The problem was delivering the right amount through their skin to keep them down,” Walzer said.
The team has performed the surgery on 16 hippos across Europe, and with the publication of the procedure now encourages zoo staff to tackle the job themselves.
Why the hippo has evolved a set of retractable testicles is not exactly known, but it is possibly a defense mechanism. “One of the theories is that when male hippos really fight – not just the display of bravado when they yawn and stretch their mouths open – they will go for the testicles and try and crush them with their teeth,” said Walzer. “If you can destroy your rival’s testicles, then that’s a evolutionary, reproductive advantage.” The ability to yank them more than a foot further into the body is certainly one defence against the probing of hippos' extremely long, self-sharpening teeth.
While listed as vulnerable in the wild by the IUCN, hippos breed well in captivity. Rather too well for many zoos, as they are large, expensive animals to keep an adult female hippo may have perhaps 25 offspring over a 40 year lifespan.
Hippos in the wild rarely fight to the death, with the weaker animal backing off once the stronger hippo has demonstrated his dominance. But in captivity, the restricted space changes their behavior. Walzer said: “It’s important that young bulls are castrated before they become breeding adults, because otherwise two adult males will kill each other.” So in addition to cutting down unwanted hippo calves, castration leaves males much more placid, meaning they can share space with others.
This article was originally published at The Conversation. Read the original article.
Some mammals have internal testes (Elephants, Rhinoceroses, Cetaceans), how do they get around the difficulties that body heat imposes on sperm production?
Additionally, with the exception of cetaceans, which obviously evolved a streamlined body shape, why do these few mammals have internal testes, when most other mammals get along fine with external testes?
This has more to do with the sperm than the testes.
Temperature tolerances of proteins can vary quite a bit with very small changes in the amino acid sequence used to make them. The bonds between amino acids not linked together by peptide bonds are typically pretty weak so the extra movement with temperature (the average kinetic energy of a substance) can rattle this long twisted string apart.
Animals with external testes have sperm that have proteins with lower heat tolerances and require a lower temp to be viable.
If the proteins of the sperm have a high enough tolerance, the testes can remain inside, which reproductively is advantageous (since they're less likely to be accidentally lost).
7 Answers 7
Humans already have the ability (though unconscious mostly) to extend and retract their testicles. So just increase this ability so you can retract all the way inside yourself, and they only descend when aroused, therefore they start producing sperm at that time. You could add some sort of sphincter muscle that closes behind them and protects them when stressed so it's still an automated biological response. Technically you would not be damaging the sperm because they simply aren't being created while the testicles are being stored.
Mating rituals would be modified to last an hour or so to allow a large enough number of sperm to be created before they are needed.
However, there is a problem. Too much heat is deadly to sperm. The body's temperature would kill the millions of sperm being produced, effectively making males sterile.
How can I get past this conundrum?
By doing what nature does with species that have internal testes but need to keep the sperm from dying of excess heat.
The basal condition for mammals is to have internal testes. The testes of the non-boreotherian mammals, such as the monotremes, armadillos, sloths, and elephants, remain within the abdomen.[not in citation given] There are also some marsupials with external testes and Boreoeutherian mammals with internal testes, such as the rhinoceros. Cetaceans such as whales and dolphins also have internal testes. As external testes would increase drag in the water they have internal testes which are kept cool by special circulatory systems that cool the arterial blood going to the testes by placing the arteries near veins bringing cooled venous blood from the skin.
Just do what marine mammals do: keep them well inside the body!
This article describes some ancient research that revealed that cetaceans keep their testes cool by a curious arrangement of blood vessels in that region. Basically, your humans could do the same: develop a network of blood vessels designed to locally regulate the temperature of the tissues surrounding the testes.
Dolphins possess a countercurrent heat exchanger that functions to cool their intra-abdominal testes. Spermatic arteries in the posterior abdomen are juxtaposed to veins returning cooled blood from the surfaces of the dorsal fin and flukes
Your post is based on the assumption that testes evolved to keep sperm cool. There is actually no evidence for this. It could well be the opposite, that sperm evolved to like it cooler because it's in the testes.
Given how much of a vulnerability testes (not just the final form, also how they develop) are, it seems much more economical to evolve sperm with higher heat resistance. Elephants and birds, for instance, have a high body temperature and no respective cooling mechanism.
Testes evolved independently in marsupials and placentals, so they seem to be solving an important problem. But if that problem was temperature, then birds and elephants should have them, too.
TLDR: The cooling problem is easily solved, but we don't really know why humanoids (in particular) need testes, so your species might suffer from other problems.
For a humanoid species (that is, bipedal but not related to homo sapiens) you really don't have to worry. You can just make testicles internal. We have no good idea why Earth mammals commonly, but not always, have external testes. Therefore we have to reason to generalise and expect external testicles in all bipedal body plans.
The heat hypothesis is popular in pop-sci articles but has no useful evidence nor mechanisms to explain its evolution. For starters: mammals begin 220 million years ago and the scrotum evolves twice in lineages about 70 and 100 million years later, so we have many millions of years of hot, internal testicles to explain.
For my money, the better bet is that testicles are damaged by the fluctuating abdominal pressures of running and galloping mammals (usefully aligns with current populations of scrotum/not-scrotum mammals and the rough evolution of these gaits).
Therefore you can explain that this species didn't evolve such fragile reproductive organs, or that they are located in a protective casing of some sort.
Why Chimpanzees Have Big Testes, and Mandrills Have Small Ones
Apr 16, 2019
ABOVE: Chimpanzees lack fancy ornamentation, but have large testes in relation to their body size.
© ISTOCK.COM, IMPALASTOCK
B irds are well known for extravagant, sexually selected features such as peacocks’ tails, but primates too go out of their way to show off their good looks to potential mates. Some species display flashy ornaments designed to signal their dominance or attract females. Take the large cheek flanges of orangutans or the oversized noses of proboscis monkeys. But the ornaments don’t always match what’s under the hood, so to speak.
A comparative study of more than 100 primate species finds that males tend to have either large testes or flashy ornaments, but not both—suggesting an evolutionary trade-off between the two. The situation is different for canine teeth, which some species use as weapons: in fact, the larger the testes, the longer the canines tend to be, scientists reported April 10 in Proceedings of the Royal Society B.
“It’s great that this study has been done,” says Antje Engelhardt, a behavioral ecologist who studies primates at Liverpool John Moores University and wasn’t involved in the study. “We still know far too little about sexual ornaments, weapons, and also testes and sperm competition in primates.”
The authors—a group of Australian and Swiss researchers—turned to previous studies to gather data on sexually selected traits across a range of primate species, from bonobos to lemurs. These included figures on average testes size relative to body mass, as well as canine length and the expression of sexual ornaments in males relative to females. They also noted a number of social traits, such as the degree to which males monopolize access to females in a social group.
Using a statistical phylogenetic model, they uncovered several striking correlations in the data. Importantly, they found that larger testes tended to correlate with less pronounced sexual ornaments. “Bonobos or chimpanzees have huge testes relative to their body size, but they are otherwise quite modest-looking,” having no noticeable ornamentation at all, explains coauthor Stefan Lüpold, an evolutionary biologist at the University of Zürich.
Although some have taken this finding to mean that men with beards may have smaller testes, and the analysis did include humans, Lüpold stresses that this isn’t the case: the study compares variation across species and doesn’t draw any conclusions about variation within individual species, he warns. Besides, “you can shave the beard, and your testes don’t change.”
The researchers suppose the correlation has to do with the animals’ social mating systems: Chimpanzees and bonobos, which for the most part lack ornamentation, are by nature promiscuous species, females typically mating with multiple males. This means that within the female reproductive system, there’s competition among sperm to fertilize the egg. To secure better chances of outcompeting the gametes of rival males, it pays off to produce more sperm, and growing larger testes is one way to do that, Lüpold explains.
Paradoxically, the team found the opposite pattern when they examined the relationship between gonad size and weapons: Testes tended to be larger in species where males have longer canines relative to females, such as in long-tailed macaques. Both sexes tend to mate with multiple partners, and while the alpha male has the greatest access to females, he doesn’t entirely monopolize access to all of them. “Consequently, they have enlarged weapons—through priority access to females—but also relatively large testes because of some level of sperm competition,” Lüpold says.
Why ornaments trade off against testes size is unclear. The team suggests that canines and testes may not compete for the same developmental resources, whereas ornaments and testes would. In addition, fleshy ornaments or colorful skin could be more costly to grow and maintain than teeth, which once established don’t require much additional resources.
Engelhardt expressed surprise about the positive relationship between canines and testes size. Having both long canines and large testes might make sense for some primates, such as long-tailed macaque males, “but I would not expect that this is then something that comes out as a general trend,” she remarks.
Less surprising to Engelhardt is another correlation that Lüpold and his colleagues uncovered: that canine length and ornamentation go hand in hand. When they examined the relationship between canine length and the expression of ornaments alone, they found them to be positively correlated. Male mandrills and gelada baboons, for instance, both have relatively long canines as well as striking ornamentation compared to females, but also have smaller-than-average testes compared to other primates. This makes sense, says Engelhardt, because by using ornamentation to signal one’s strength, “the higher-ranking males would signal to the lower ranking ones that it wouldn’t make sense to challenge them, which would save both energy and potential injuries,” she says.
However, whether this interpretation holds water depends on the function of male ornaments, which isn’t well understood in primates. In some cases, ornaments could serve as signals of fighting ability or social status—so-called “badges of status”—but they could also signal attractiveness and viability to females, or both. The authors didn’t distinguish between these possibilities, Engelhardt cautions. “I would not necessarily mix those in one analysis, because the function is completely different,” she says.
Marion Petrie, an evolutionary biologist at Newcastle University, would like to see more within-species work exploring what information such ornaments convey to females. “Most of the literature on mating systems talks about the ability of males to monopolize females and the environmental factors that might lead to this. However, I am also interested in the female perspective,” she writes in an email to The Scientist. “Of course, one really needs to consider the costs and benefits to males and to females under both scenarios, but we seem to be a long way from understanding the drivers that result in one system predominating over the other.”
For her, the study illustrates the diversity of ways in which sexual selection operates across the animal kingdom. “We are only beginning to touch the surface of understanding it. There is so much we still don’t know about the evolution of the natural world and the sad thing is, we may never know as we are in real danger of losing it before we will ever find out.”
S. Lüpold et al., “Sexual ornaments but not weapons trade off against testes size in primates,” Proc R Soc B, doi:10.1098/rspb.2018.2542, 2019.
Descended testicles: DNA study drops new hints on secrets of low hanging glands
The scrotum is a mystery. Why do most male mammals have their reproductive glands so vulnerably located in a sack of skin and muscle outside the body? According to new research, the answer might be found in those unusual mammals that have testicles located inside the abdomen. These includes elephants, aardvarks and others from a group that originated in Africa, known as the Afrotheria.
Testicles function best when slightly below body temperature. But we can't say for sure this is why mammals evolved descended testicles, not least because males without scrota can still successfully reproduce. External testes may also be a way of showing off to potential mates, or to protect them from pressure inside the body created by movement.
The new study, published in PLOS Biology, examined 71 placental mammals for two key genes – RXFP2 and INSL3 – that are needed for the development of ligaments involved in testicular descent. They found that in many afrotherian mammals without external testes these genes had mutated to the point where they would no longer function.
This data showed that the common ancestor of all afrotherian mammals, which may have lived 70m to 90m years ago, as well as the much older ancestor of all placental mammals, did have descended testicles. The research also showed this trait was reversed at least four separate times throughout the evolution of certain afrotherian animals. This suggests that studying Afrotheria and their ancestors in more detail might reveal exactly what the evolutionary advantages and disadvantages of having testes outside the body really are.
Scientists often try to work out what extinct animals were like and how they evolved, especially when there are no available fossils, by looking at their living descendants. If all the modern descendants of a particular species have a certain physical or behavioural trait, then it's likely the ancestral species did too.
Reconstructing details of species like this is known as extant phylogenetic bracketing. But this approach relies on solid evidence for how groups of living species are related and, in the case of placental mammal groups, attempts to draw an accurate family tree are still controversial.Balls in. Credit: Shutterstock
Studying the DNA of the Afrotheria gets around this problem because we don't have to know how they are related more broadly to other placental mammal groups. Instead, the researchers looked for evidence of how the animals' genes had changed as they evolved. They found that only modern afrotherian mammals had mutated versions of genes involved with testicular descent. All the other groups of mammals had these genes intact.
The fact that dysfunctional remnants of the RXFP2 and INSL3 genes are found in mammals without external testicles strongly suggests their ancestors had functioning copies of these genes. Over the course of evolution, when it was no longer an advantage to have external testes for whatever reason, mutations in these genes occurred without reducing the animal's chances of reproduction. These mutated genes were then passed on to the next generation.
The researchers were also able to date roughly when these gene mutations occurred using what's known as the molecular clock, the idea that gene mutations build up at specific rates. If you know what an original functioning gene looks like, you can work out how many mutations have occurred in mutated genes present in other animals. Using the molecular clock, you can then work out how long it would have taken for these mutations to build up.
What we still don't know is why these mutations were successfully passed on and why the Afrotheria evolved so their testicles were no longer descended (testicondy). The researchers suggest that either testicondy is beneficial in some unknown way, that it results from another beneficial trait, or that it results from the constraints of some other bodily development.
Clearly much remains unknown and these ideas need testing. But what the latest study also shows is how scientists can use this form of molecular palaeobiology to work out what extinct organisms were like, even when we don't have any hope of finding their complete physical remains as fossils.
This article was originally published on The Conversation. Read the original article.
Why Do the Testes of Most Mammals Hang Outside Their Bodies?
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Answer by Adriana Heguy, professor of pathology:
The reason why many mammals evolved to require a cooler temperature to produce viable sperm is not precisely known, and it has been a subject to debate for decades, mainly because having the testes exposed creates a vulnerability that seems to be the opposite of an adaptation. And interestingly, not all mammals have dangling testes and scrota, exceptions being not just pinnipeds (seals and sea lions) and cetaceans (whales and dolphins) but also elephants, which keep their testes inside the body.
There are many theories out there as to why sperm needs to be kept cooler and why the testes of most mammals hang outside their bodies, inside the scrotal sac. In my opinion, the least likely is the “handicap principle.” The easiest way to illustrate this principle is the peacock’s flashy feathers: The animal displays his genetic fitness by being able to survive in spite of those cumbersome feathers that attract the predator’s attention. Male mammals with dangly testes signal to the female mammals that they have good genes if they could keep their hanging, exposed, sensitive gonads safe. The reason why this hypothesis is unlikely is because it has not led to increasingly ornate, highly visible scrota, with a few exceptions of mammals with brightly colored scrota (for example, the vervet monkey).
Evolutionary psychologist G.G. Gallup wrote a very detailed paper a few years back, reviewing the current hypotheses and proposing his own, known as “activation hypothesis.” According to his hypothesis, when the sperm cells are ejaculated into the vagina, they experience a sudden rise in temperature, as the female’s reproductive tract is at body temperature (37 degrees Celsius). This elevation in temperature activates sperm, temporarily enabling them to increase their motility to go through the cervix and reach the fallopian tubes. The lower temperature in the descended testicles serve to prevent the premature activation of sperm by keeping testicular temperatures below body temperature. Indeed, human sperm that are placed at body temperature become more motile for about one hour, which is the time it would take them to reach the oocyte, and then they slow down. Gallup also speculates that the reason for the extreme sensitivity of the testes and the extreme pain that ensues if they are hit are a result of the activation adaptation. Pain compels male mammals to protect their testes from possible damage. The cremaster muscle, which pulls the testes closer to the body, contracts not only when the outside temperature is too cold, when having an erection (possibly warming the sperm cells a bit, in preparation for the big swim of their lives), and also when there is a hint of possible damage to the testes. All these adaptations make sense in light of the activation hypothesis. This could explain why marine mammals do not need to keep the testes in the scrotum, because they are inside the body but close to the skin and the water temperature may keep them cold enough. But it does not explain the elephant case.
I also came across a recent article that proposes another hypothesis that I like the most, perhaps because it highlights one aspect of evolution: that something that evolved a long time ago, as an adaptation to an environment or physiological conditions that no longer exist, may have become “fixed,” and that’s why they are still around. It’s called “endothermic pulses hypothesis,” and it’s described in a heavy-duty evolutionary biology paper, authored by the aptly named South African evolutionary biologist B.G. Lovegrove.
This hypothesis argues that the evolution of the scrotum was driven by increases in physiological body temperature (endothermic pulses) that occurred in Boreoeutheria (a clade of mammals, supported by genomic data, that comprises the rodents, primates, lagomorphs such as hares and rabbits, carnivores, bats, and ungulates) during the Cenozoic. These pulses occurred as an adaptation to climate changes, and as an adaptation to cursoriality (the ability to run, which increases body temperature). The model proposes that selection maintained an optimum temperature for spermatogenesis and sperm storage throughout the Cenozoic, at the lower levels of body temperature that prevailed in ancestral mammals for at least 163 million years. The lower temperatures also favor a reduced rate of mutation during spermatogenesis, and evolutionary processes ended up stabilizing these lower temperatures for sperm production and storage. Evolutionary stasis may have been driven by reduced rates of germ-cell mutations at lower body temperatures. The fitness advantages of an optimum temperature of spermatogenesis ultimately outweighed the costs of testes externalization and resulted in the evolution of the scrotum. This hypothesis would explain why elephants do not have external testes (they are not Boreoeutheria, but belong to another clade, Afrotheria, comprising tenrecs, hyraxes, and elephants.
We may never know why exactly evolution favored cooler sperm and dangling testes in a scrotum, but it is fun and useful to hypothesize about the reasons for this adaptation.