It is reasonably well known that many species, such as bees and some types of birds as examples can see into the ultraviolet (UV). How is the structure of their eye different to humans to allow this?
Also, how are they shielded from some of the harmful effects of ocular UV exposure?
Firstly most UV perceiving organisms only perceive far UV (~300-400nm) which is less damaging. There are different opsins, which get activated by different wavelengths. Like other light UV is perceived by opsins sensitive to UV wavelength [ref].
I am not sure about this but opsin sensitivity to UV must be high so that low irradiation is sufficient for perception and excess is filtered off. UV filter mechanism in reported for human and squirrel eye; mostly uses kynurenine derivaties. May be similar mechanism is present in other organisms too.
The relevant structure of the eye only differs in one way, the types of cones present, different types of cones contain different opsins (light sensitive pigments). Reptiles and birds have 4 cone types, Mammals have 2 (primates have 3). Early mammals lost two of these as modern mammals are descended from early mammals who were nocturnal, one was the one that responds to UV light. Modern mammals as their descendants are stuck with this reduction since the genes to make those other cones is no longer present. Primates evolved a third cone, (a mutant variant of one of the two they had before) Primates did this because many a frugivores and color is excellent for determining when fruit is ripe.
Vision is the most important sense for birds, since good eyesight is essential for safe flight. Birds have a number of adaptations which give visual acuity superior to that of other vertebrate groups a pigeon has been described as "two eyes with wings".  Birds likely being descendents of theropod dinosaurs,   the avian eye resembles that of other reptiles, with ciliary muscles that can change the shape of the lens rapidly and to a greater extent than in the mammals. [ citation needed ] Birds have the largest eyes relative to their size in the animal kingdom, and movement is consequently limited within the eye's bony socket.  In addition to the two eyelids usually found in vertebrates, it is protected by a third transparent movable membrane. The eye's internal anatomy is similar to that of other vertebrates, but has a structure, the pecten oculi, unique to birds.
Some bird groups have specific modifications to their visual system linked to their way of life. Birds of prey have a very high density of receptors and other adaptations that maximise visual acuity. The placement of their eyes gives them good binocular vision enabling accurate judgement of distances. Nocturnal species have tubular eyes, low numbers of colour detectors, but a high density of rod cells which function well in poor light. Terns, gulls, and albatrosses are among the seabirds that have red or yellow oil droplets in the colour receptors to improve distance vision especially in hazy conditions.
Animal UV vision - Biology
Thanks to UV vision, birds see the world very differently than we do
IN THE EARLY 1970s, A RESEARCHER testing the ability of pigeons to discriminate colors discovered by accident that the birds can see ultraviolet (UV) light. The finding was deemed curious but not too important. &ldquoIt was natural for scientists to assume that bird vision is like human vision,&rdquo says Geoffrey Hill, an Auburn University ornithologist and the author of Bird Coloration. &ldquoAfter all, birds and humans are both active by day, we use bright colors as cues. . No one really imagined birds might see the world differently.&rdquo
But during the following decades, systematic testing of bird vision revealed something unexpected: Many bird species&mdashnot just pigeons&mdashcan see UV light. Indeed, with the exception of night-flying birds such as owls, the eyes of most birds probably are even more sensitive to ultraviolet light than they are to what we call visible light. Scientists also have learned that many birds have plumage that reflects UV light. Together, these discoveries &ldquomade us realize there could be new answers to old questions,&rdquo says Drake University biologist Muir Eaton. Birds rely on vision to choose mates, find food and scan for predators, for example. &ldquoIf you assume birds see exactly what we see, you could have the wrong framework for understanding bird behavior,&rdquo Eaton says.
Consider how birds choose mates. &ldquoAfter the first studies on birds and UV came out, people started saying, &lsquoMaybe your study of mate choice isn&rsquot valid because you scored the feather colors with the naked eye,&rsquo&rdquo says Peter Dunn, a University of Wisconsin&ndashMilwaukee biologist who studies active little warblers called common yellowthroats (below). Adds Hill, who has researched mate choice in house finches, bluebirds and indigo buntings: &ldquoWhen I started working, back in the 1980s, we used to hold up color charts against the birds&rsquo feathers&rdquo&mdashthe same square paint chips that are an industry standard for graphic designers and interior decorators.
During the past three decades, a flurry of studies has tested the intriguing notion that mate choice and other bird behaviors may be shaped by secret visual signals humans cannot see. Though the premise was exotic, what facilitated this explosion of research was prosaic: Technology got better and cheaper. In particular, the increased availability and decreased cost of a lab device called the spectrophotometer&mdashwhich precisely measures light reflected or absorbed by a surface&mdashlet scientists, if not see like a bird, at least quantify what birds are seeing.
Initially, many researchers turned their spectrophotometers on birds that do not use flashy feathers to attract mates. A team of Swedish scientists, for example, looked at the blue tit, a European relative of the chickadee. As with many bird species, male and female blue tits look alike to humans. &ldquoStandard literature describes the plumage as closely similar between the sexes,&rdquo says Staffan Andersson, a professor of animal ecology at the University of Gothenburg. &ldquoThe main problem with this conclusion is that it is based on the UV-blind and yellow-biased human eye.&rdquo Using a spectrophotometry probe to scan the feathers of wild-caught birds, Andersson and his colleagues discovered that blue tits themselves have no problem telling males from females: Males have a patch of feathers on the crown of the head that strongly reflects UV light females do not.
Blue tits are not alone. In 2005, Eaton used a spectrophotometer to scan the plumage of museum study skins of 139 songbird species in which males and females appear alike, from cedar waxwings to barn swallows to mockingbirds to western meadowlarks. Though scientists previously had classified these birds, along with 70 percent of all songbird species, as sexually monochromatic (males and females looking identical), a full 90 percent of the species Eaton scanned actually were sexually dichromatic: different once you took into account the better discrimination of colors (including ultraviolet) by birds and the amount of UV light feathers reflect. &ldquoTo the birds themselves, males and females look quite different from one another,&rdquo Eaton says.
Such findings led some researchers to speculate that the primary role of avian UV vision is to select mates. Indeed, in laboratory tests, Andersson and his colleagues found that female blue tits strongly preferred males with the brightest &ldquoinvisible&rdquo crowns&mdashevidence that the UV-reflecting feathers humans cannot see were serving their function.
Over time, however, scientists have concluded that blue tits are the exception to the rule. Very few bird species use UV light only&mdashwith no other visual cues&mdashto attract and choose mates. &ldquoIn general, ultraviolet reflectance simply reinforces the plumage color patterns we humans already can see,&rdquo says Dunn. Among his study subjects, &ldquoyellowthroat females do prefer males that are brighter, but not because of the UV reflectance alone. It&rsquos more the brightness of the feathers overall.&rdquo
Foiling Nest Parasites
So, how do birds use their power of UV vision? In a surprising number of ways, scientists propose. Many songbirds, for example, are pestered by nest parasites: birds such as cuckoos and brown-headed cowbirds that dump their eggs in a host nest and leave the hard work of childcare to the unwilling adoptive parents. It turns out that some potential hosts are able to recognize and reject eggs that, to human eyes, look like their own. Might birds be responding to UV signals rather than to colors visible to people?
The evidence so far is suggestive but inconclusive. In one 2007 study in the Czech Republic, song thrushes rejected experimental eggs researchers had designed as perfect mimics. It turned out the scientists&rsquo eggs had a UV reflectance different from the thrush eggs. But a Canadian study of 11 species parasitized by cowbirds found no correlation: Some species accepted eggs that were a UV match others rejected them.
Signals From Hungry Chicks
Scientists also are investigating whether UV signals play a role after eggs hatch. Think of hardworking parent birds, ferrying caterpillars to a nestful of hungry chicks. Which chick gets fed first? In some species, parents cue in on a hatchling&rsquos size or how loudly and energetically it begs. But color also is a factor&mdashthe brightness of the gape (edge of the mouth) or the head seems to stimulate a parent to proffer food. Some researchers suggest UV color may enhance this effect.
Newly hatched European rollers, for instance, have a patch of bare skin on the foreheads that reflects UV light. Their parents face a particular challenge as they dole out centipedes and other treats: Because roller clutches hatch over a period of days, first-hatched chicks are larger and need more food than chicks that hatch later. In a 2011 study, Spanish researchers noted that heavier chicks tend to have the least UV-reflective forehead patches lighter chicks had more reflective foreheads. To test whether this difference helps parents decide who to feed the most, the scientists smeared a sunblocklike lotion on the foreheads of some chicks, using a control lotion on others. The chicks with the blocker gained less weight than their unblocked nestmates&mdashclearly showing they got less food when they could not advertise their nutritional status with UV signals.
Parent birds may rely on UV signals when they&rsquore off finding food as well. Many insects, including moths and butterflies, have body coatings that strongly reflect UV light. Many seeds also are reflective, and berries and fruits develop a highly reflective waxy coating as they ripen. On the other hand, most green leaves do not reflect UV light. So even if a red berry seems quite visible against a green leaf to human eyes, for birds this contrast is enhanced.
&ldquoI think the biggest thing to come from the discovery that birds see in the ultraviolet is our understanding of how some predatory birds find their prey,&rdquo says Hill. Picture, for example, a kestrel (American kestrel, right) perched high on a telephone wire, surveying a field far below. &ldquoI always wondered how a bird of prey gets enough to eat,&rdquo he says. &ldquoAfter all, you can walk through a grassy field 20 times and never see a mouse.&rdquo
But that&rsquos because we do not see what the birds see. It turns out that one key prey for common kestrels, the meadow vole, behaves like a tiny dog, using squirts of urine to mark its trails through tall grass. About 15 years ago, Finnish researchers from the University of Turku discovered that vole urine reflects UV light&mdashwhich kestrels soaring over open fields can plainly see. &ldquoOnce you realize raptors can follow the trail right to the animal, it makes a lot more sense,&rdquo Hill says.
Indeed it does. While people long have wondered what it would be like to soar like a bird, the more interesting question&mdashparticularly for biologists&mdashmay be: What would it be like to see like a bird?
Cynthia Berger is a Pennsylvania-based writer and the former managing editor of Living Bird magazine.
Birds and UV Light: The Eyes Have It
How do birds detect ultraviolet (UV) light? To answer this question you must understand avian eye structure. The human retina has three kinds of cone cells (receptors used for color vision): red, green and blue. By contrast, birds active during the day have four kinds, including one that&rsquos specifically sensitive to UV wavelengths. There&rsquos another difference: In birds, each cone cell contains a tiny drop of colored oil that human cells lack. The oil drop functions much like a filter on a camera lens. The result is that birds not only see UV light, they are much better than humans at detecting differences between two similar colors.
What does the world look like to a bird with UV vision? &ldquoWe can&rsquot imagine,&rdquo says Auburn University ornithologist Geoffrey Hill. Since birds can detect more colors than humans can, scenes may appear more varied. And colors that already are bright to human eyes are&mdashif amplified by UV reflectance&mdashprobably even brighter to birds.
Bird Research Yields Consumer Products
In the grand U.S tradition, entrepreneurs are beginning to capitalize on new knowledge about bird vision to invent clever consumer products. Here are a few examples:
A Better Duck Decoy: Waterfowl hunters know that the more realistic a duck decoy is, the better it works. A life-long duck hunter, ornithologist Muir Eaton notes, &ldquoWhen I got into this UV research, I said, &lsquoHoly moly, I should invent UV-reflecting paint for my decoys!&rsquo&rdquo Someone beat him to it. Most major manufacturers of mass-produced decoys now offer UV-reflecting paint as an option on their products.
Avoiding Collisions . . . and Cats: Each year up to 1 billion North American birds die after colliding with windows, says Muhlenberg College researcher Daniel Klem. One way to warn birds that an invisible but solid barrier blocks their flight path is to decorate windows with decals. &ldquoBut that&rsquos hardly visually satisfying,&rdquo Klem notes. A more pleasing option for consumers would be windows that reflect UV light&mdashvisible to birds but not people&mdasha project Klem is working on and hopes to convince manufacturers to produce commercially. Hundreds of million of birds also fall prey each year to outdoor cats. One entrepreneur is capitalizing on birds&rsquo ability to see UV to combat the problem by marketing a collar that claims to make feline predators more visible to birds.
Camouflage Clothing for Birders: Some avid bird-watchers are reconsidering their fashion choices now that they know birds see in the UV. Many modern clothing dyes reflect UV, as do the &ldquobrightening&rdquo agents in some detergents. Today birders can choose from a variety of sprayable fabric treatments that will make their favorite jackets less showy as the clothing absorbs (rather than reflects) UV wavelengths.
Goose Be Gone: A flock of Canada geese winging overhead can be a prelude to a mess. One way to repel so-called &ldquonuisance&rdquo geese is by spraying the grass with a bad-tasting but harmless chemical derived from grapes. Research shows this treatment is even more effective when coupled with a second spray: a compound that reflects UV light. Invisible to human eyes, the spray makes a swath of treated grass quite obvious to geese, a visual cue that reinforces the lesson, &ldquoThis food tastes bad&mdashstay away.&rdquo
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How Do You Know If an Animal Can See Color?
Eyes are used to capture light and the optic nerves then send signals to the brain where the information is processed into an image. Click to enlarge and to read additional details.
This question can be answered pretty easily. If an animal eye has cones they will be able to see some color. What is difficult to know is which colors an animal can see and how strong or weak the color will appear to the animal.
Scientists can study an animal eye and find out if it contains cones and what colors of light the cones can detect. It is also possible to count the number of cones and their location in the retina to understand how strong or weak a color might appear to an animal.
But, what color does the animal see? Vision, like all of our senses, is processed in the brain. Without being able to get into the head of an animal, it is only possible to know what colors can be detected and not how they "look" to the animal.
This is also true for a more familiar animal: the human. Two people may say they see a painted wall as a particular color, but do they see it the same way? The answer is not known at this point.
How do animals see the world? We can study animal eyes but we may never be able to know exactly what different animals see. Images left to right: Jumping spider by Opoterser. Rattlesnake by Karla Moeller. Owl by Woodwalker and Poxnar. Cat by Guylaine Brunet.
Nature’s Most Amazing Eyes Just Got A Bit Weirder
Eyes are testaments to evolution’s creativity. They all do the same basic things—detect light, and convert it into electrical signals—but in such a wondrous variety of ways. There are single and compound eyes, bifocal lenses and rocky ones, mirrors and optic fibres. And there are eyes that are so alien, so constantly surprising, that after decades of research, scientists have only just about figured out how they work, let alone why they evolved that way. To find them, you need to go for a swim.
This is the eye of a mantis shrimp—an marine animal that’s neither a mantis nor a shrimp, but a close relative of crabs and lobsters. It’s a compound eye, made of thousands of small units that each detects light independently. Those in the midband—the central stripe you can see in the photo—are special. They’re the ones that let the animal see colour.
Most people have three types of light-detecting cells, or photoreceptors, which are sensitive to red, green and blue light. But the mantis shrimp has anywhere from 12 to 16 different photoreceptors in its midband. Most people assume that they must therefore be really good at seeing a wide range of colours—a “thermonuclear bomb of light and beauty”, as the Oatmeal put it. But last year, Hanna Thoen from the University of Queensland found that they’re much worse at discriminating between coloursthey’re much worse at discriminating between coloursthey’re much worse at discriminating between coloursthey’re much worse at discriminating between coloursthey’re much worse at discriminating between colours than most other animals! They seem to use their dozen-plus receptors to recognise colours in a unique way that’s very different to other animals but oddly similar to some satellites.
Thoen focused on the receptors that detect colours from red to violet—the same rainbow we can see. But these ultra-violent animals can also see ultraviolet (UV). The rock mantis shrimp, for example, has six photoreceptors dedicated to this part of the spectrum, each one tuned to a different wavelength. That’s the most complex UV-detecting system found in nature. Michael Bok from the University of Maryland wanted to know how it works.
Like us, mantis shrimps see colour with the help of light-sensitive proteins called opsins. These form the basis of visual pigments that react to different wavelengths of light, allowing us to see different colours. If a mantis shrimp has six UV receptors, it should have at least six opsins that are sensitive to different flavours of UV.
Except it doesn’t. Bok could only find two.
How could there possibly be six types of photoreceptors with only two opsins? There was one possibility. Something could be filtering the light hitting the different receptors.
Here’s an analogy: say you’ve got a big crowd lining up in front of six security guards, each of whom must shout out when they spot someone with a specific name. One recognises Adams, another targets Bobs, and so on. But the guards aren’t too bright they wouldn’t know Adam if he introduced himself. So you make their job easier. You rig the queuing system so that only Adams line up in front of Adam-blocking guard, only Bobs reaching the Bob-blocker, and so on. The guards shout pretty much indiscriminately, but they still do their jobs correctly. They’re not specific you impose specificity onto them.
That’s exactly what happens in the mantis shrimp’s eye. When light enters the units in its eye, it must first pass through a crystalline cone, which lies over the receptors. Bok found that these cones contain UV-blocking substances called MAAs (or mycosporine-like amino acids, in full). There are four, possibly five, of these, which block slightly different wavelengths of UV. Combine these filters with the two underlying opsins, and you get six different classes of UV receptor.
Many marine animals have one or two MAAs. They use these as sunscreens to block UV from reaching their skin and eyes, and causing damage that could eventually lead to cancer. The mantis shrimps also use MAAs to block UV but for a unique purpose: to turn their eyes into incredibly sophisticated UV detectors.
Where do the MAAs come from? It’s not clear. No animal can make these chemicals themselves, so they must get them from their environment, possibly from their diet or from microbes. But two of the MAAs that Bok discovered have never been seen before, so it’s possible that the mantis shrimps can somehow change any incoming MAAs into five different types.
“We presented these results last summer at a big vision conference and one of my colleagues said: Now, you’ve solved all the problems. What are you going to do next?” says Tom Cronin, who led the study. “We sort of feel that way. The big problem now is: What does this all have to do with vision?” Why do mantis shrimps have such ridiculously complicated eyes? That’s the big question, and no one really knows.
The team are now trying to study how mantis shrimps react to different UV signals. For example, they find some short wavelengths of UV so repulsive that they’ll avoid food that’s paired with those wavelengths. Maybe this has something to do with aggressive signals? Mantis shrimps have rich social lives and they might communicate with ultraviolet patterns reflecting off their bodies.
“That’s the leading hypothesis but it has its own problems,” says Cronin. “Signals don’t evolve unless you have the visual system to see them. So you generally don’t have a system in place to see signals unless it’s there to see something else.” So the team are also looking at the patterns of UV light in the places where mantis shrimps live. But even if that line of research pans out, many animals share the same waters, and none of them have such a complex eye. So why does the mantis shrimp?
When I spoke to Marshall last year, he said that the mantis shrimp’s style of vision might help it to process images very quickly without much contribution from its brain. That might be useful to a predator that uses some of the fastest strikes in the animal kingdom. But of course, that’s still a hypothesis.
And there’s another baffling layer of complexity: the receptors that detect red to violet colours are connected to different nerves than the ones that detect UV, and both streams lead to different parts of the brain. The mantis shrimp didn’t just evolve an absurdly over-engineered way of seeing, it did it twice.
Ultraviolet Vision and Avoidance of Power Lines in Birds and Mammals
The avoidance by mammals and ground-nesting birds of habitat up to several kilometers from high-voltage power lines is a major consequence of infrastructure development in remote areas, but the behavior is perplexing because suspended cables are neither an impenetrable physical barrier nor associated with human traffic (e.g., Vistnes & Nellemann 2008 Pruett et al. 2009 Degteva & Nellemann 2013 ). Moreover, avoidance may persist >3 decades after construction (Nellemann et al. 2003 Vistnes et al. 2004 ), suggesting behavioral reinforcement. Integration of new information on visual function with the characteristics of power line function provides compelling evidence that avoidance may be linked with the ability of animals to detect ultraviolet light (UV).
Ultraviolet discharges on power lines occur both as standing corona along cables and irregular flashes on insulators. The discharge spectrum (200–400 nm Maruvada 2000 ) is below the normal lower limit of human vision, UV being attenuated by the human cornea and lens, but in birds, rodents, and reindeer/caribou (Rangifer tarandus) (hereafter reindeer) the cornea and lens are UV permissive. The former have specific UV sensitive opsins (Bowmaker 2008 ) and, hence, power line corona may be assumed visually salient in these. Reindeer have no specific UV opsin, but we obtained robust retinal responses to 330 nm mediated by other opsins (Hogg et al. 2011 and unpublished) and propose that corona flashes are both visually salient and a cause of this species avoiding power lines.
Recent demonstration of UV responses in reindeer retinae was based on electrophysiological corneal recordings (Hogg et al. 2011 ). These, however, are approximately 3 log units less sensitive than psychophysical measurements of visual perception (Ruseckaite et al. 2011 ). They demonstrate an ability to see UV discharge but are poor indicators of visual threshold and underestimate visual sensitivity. Furthermore, reindeer and some birds have a reflective surface directly behind the retinal photoreceptors (the tapetum lucidum) which ensures that light not captured as it passes through them is reflected back for a second pass, consequently, increasing retinal sensitivity in dark (i.e., very low light) environments (Johnson 1968 ). In reindeer, the winter adapted tapetum scatters light among photoreceptors rather than reflecting it which enhances photon capture and increases retinal sensitivity by approximately 3 log units at winter threshold (Stokkan et al. 2013 ).
Other factors increase the likelihood that reindeer see coronal discharges in the dark. First, retinal sensitivity is maximized in reindeer because their retinae are almost permanently dark adapted during the extended dusk of Arctic winters, and, given that the mammalian visual range is approximately 9 log units, fully dark adapted eyes are capable of responding to the stimulus of a single photon. Second, the reindeer eye is larger than the human eye and thus provides greater image magnification, and the pupil, which dilates to 21 mm compared with approximately 10 mm in humans, is likely to be permanently dilated in winter consequently increasing retinal sensitivity approximately 4-fold. Third, dilation exposes more of the peripheral retina that is sensitive to sudden changes in the visual environment.
The stimulus is also important. Ultraviolet discharge is both strongly (approximately 90%) reflected and scattered by snow. Hence, in a snowy landscape the corona is likely to appear brighter to animals responsive to UV than in conventional imaging which focuses on source discharge. Second, and crucially, the pattern of occurrence of corona flashes is temporally random, which is likely to impede habituation.
These observations constitute a strong argument that reindeer, like birds and rodents, may see corona UV. By extension, we suggest that in darkness these animals see power lines not as dim, passive structures but, rather, as lines of flickering light stretching across the terrain. This does not explain avoidance by daylight or when lines are not transmitting electricity—although, interestingly, electrically earthed cables are more hazardous to galliformes (which detect UV to 355 nm Lind et al. 2014 ), perhaps precisely because without corona definition is lost (Bevanger & Brøseth 2001 )—but it may be an example of classical conditioning in which the configuration of power lines is associated with events regarded as threatening.
Humans don't see colours very well, or even at all, in low light. This is because our cone cells function best in relatively bright light.
Other cells in our eyes, called rod cells, help us see in dim light. But because rod cells only have a single light-sensitive pigment, at night we see in shades of grey.
Geckos, on the other hand, have excellent colour vision at night - a useful advantage for a nocturnal hunter. Their eyes have evolved to be up to 350 times more sensitive to colour at night than ours.
What If Humans Had Eagle Vision?
If you swapped your eyes for an eagle's, you could see an ant crawling on the ground from the roof of a 10-story building. You could make out the expressions on basketball players' faces from the worst seats in the arena. Objects directly in your line of sight would appear magnified, and everything would be brilliantly colored, rendered in an inconceivable array of shades.
The more scientists learn about eagle vision, the more awesome it sounds. Thanks to developing technologies, some aspects of their eyesight may eventually be achievable for humans. Others, we can only imagine.
Eagles and other birds of prey can see four to five times farther than the average human can, meaning they have 20/5 or 20/4 vision under ideal viewing conditions. Scientists have to cook up special experiments to judge eagles' eyesight &mdash your optometrist's alphabet eye charts are of no use, after all &mdash and one common setup involves training the birds to fly down a long tunnel toward two TV screens. One screen displays a striped pattern, and the birds get a treat when they land on it. Scientists test their acuity by varying the width of the stripes and determining from what distance the eagles begin to veer in the correct direction.
According to William Hodos, a distinguished professor emeritus at the University of Maryland who has studied the visual acuity of birds since the 1970s, two eyeball features confer eagles' sharper vision. First, their retinas are more densely coated with light-detecting cells called cones than human retinas, enhancing their power to resolve fine details just as higher pixel density increases the resolving power of cameras.
Second, they have a much deeper fovea, a cone-rich structure in the backs of the eyes of both humans and eagles that detects light from the center of our visual field. "Our fovea is a little shell or bowl, while in hawk or eagle it's a convex pit. Some investigators think this deep fovea allows their eyes to act like a telephoto lens, giving them extra magnification in the center of their field of view," Hodos told Life's Little Mysteries.
On top of sharp focus and a central magnifier, eagles, like all birds, also have superior color vision. They see colors as more vivid than we do, can discriminate between more shades, and can also see ultraviolet light &mdash an ability that evolved to help them detect the UV-reflecting urine trails of small prey. But there's no way to know what these extra colors, including ultraviolet, look like. "Suppose you wanted to describe the color of a tomato to someone who was born blind. You couldn't do it. We can't even guess what they're subjective sensation of ultraviolet light is," Hodos said. [Red-Green & Blue-Yellow: The Stunning Colors You Can't See]
Life with 20/5 vision
Eagle vision wouldn't change how we perform most daily activities &mdash such as reading computer screens or the newspaper, or finding milk in a crowded refrigerator &mdash but how we perceive the world and use our eyes would certainly be different. It's perhaps easiest to consider our new powers in the context of how eagles use them: for hunting.
On top of the ability to see farther and perceive more colors, we would also have nearly double the field of view. With our eyes angled 30 degrees away from the midline of our faces like an eagle's, we would see almost all the way behind our heads with a 340-degree visual field (compared to normal humans' 180 degree field) this would confer a clear advantage in hunting and self-defense.
With eagle eyes, we would swivel our heads constantly. To locate prey or any other object of interest in the distance, you'd periodically turn your head to the side to sweep your fovea (telephoto lens) across your field of view. After spotting what you're looking for in this manner, you'd redirect your head toward it and use stereoscopic vision &mdash combining the viewpoints of both eyes to gauge distance &mdash to calibrate the speed of your approach.
Enhanced perception and hunting prowess would likely come with a few drawbacks. "I would say that birds probably have a greater proportion of their brain volume devoted to visual processing than other groups of animals. Now the question of what it comes at the expense of: most birds appear not to have a well-developed sense of smell or taste," Hodos said.
It's more difficult to say how your more sophisticated cognitive processes would fare. "Birds have areas that seem to function like the cortex [the part of our brains responsible for memory, language and complex thought], but it's arguable. But in terms of their ability to solve problems and so on, they match what many mammals can do. Many birds have superb memory," he said. [The 5 Smartest Non-Primates on the Planet]
Maximizing our potential
Eagles' high-flying lifestyle requires better vision than humans need, and the physical properties of our eyeballs limit us to 20/10 or 20/8 vision at best. Natural vision that good is extremely rare, but research by David Williams, director of the Center for Visual Science at the University of Rochester, and his colleagues may soon enable laser eye surgeons to achieve 20/10-or-better vision for a large percentage of patients, placing their visual acuity halfway between that of humans and eagles.
Williams and his colleagues use an instrument called a wavefront sensor to detect distortions in human vision. They shoot light into the eye and observe how it bounces back through hundreds of tiny lenses in the sensor. The aberrations in patterns created by those lenses serve as a map of the eye's mistakes. Customized surgical techniques are being developed to implement the results of patients' wavefront measurements, in order to correct their vision beyond 20/20.
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Animal UV vision - Biology
By studying rats' eyes and behavior, scientists have a pretty good idea of how a rat sees the world. In a nutshell, rats are dichromats: they perceive colors rather like a human with red-green colorblindness, but their color saturation may be quite faint, and color appears to be far less important to them than brightness. Rat vision is quite blurry, around 20/600 for normally pigmented rats. Albino rats, however, are probably blind or severely visually impaired, with about 20/1200 vision.
- What do normally pigmented rats see?
- Color vision
- Color vision in the retina
- Color perception
- What is the function of UV vision?
- Depth of focus
- Would tiny eyeglasses help a rat see better?
- Field of vision vs. binocular depth perception
- Depth perception using motion parallax
- Visual orientation
- Vision and aging
- Low acuity
- Impaired vision in bright light: dazzling
- Impaired vision in low light
- Few rods, few photoreceptors
- Delayed dark adaptation
- Night blindness
WHAT DO NORMALLY PIGMENTED RATS SEE?
Human and rat retinas have two types of light receptors: cones are sensitive to bright light and color, and rods are sensitive to dim light and cannot see color. Human and rat retinas differ, however, in the types and density of cones in the retina, which has implications for color vision.
Color vision in the retina: Humans have three types of color cones in our retinas. We have "trichromatic" vision, consisting of short-wavelength "blue" cones, middle-wavelength "green" cones and long-wavelength "red" cones.
Rats have just two types of cones (called "dichromatic" vision): a short "blue-UV" and the middle "green" cones (Szel 1992). The "green" cones' peak sensitivity is around 510 nm (Radlwimmer 1998), but the "blue" cones are shifted toward even shorter wavelengths than human blue cones, peaking at 359 nm. This means rats can see into the ultraviolet, they can see colors we can't see (Jacobs et al. 1991 2001).
About 88% of a rat's cones are the middle "green" type, and 12% are the long blue-UV cones (Jacobs et al. 2001), the blue-UV cones are located in a zone at the bottom of the retina (Szel et al. 1996). For more on how ultraviolet and red-green color vision evolved, see Shi et al. 2001 Yokoyama and Radlwimmer 1999, 2001 Shi and Yokoyama 2003.
Color perception: So, the rat's retina is sensitive to greens and to blue-ultraviolet. Can the rat actually perceive different colors, and distinguish between them? For a long time, rats were throught to be completely colorblind (e.g. Crawford et al. 1990). Recent behavioral experiments, however have shown that rats can indeed perceive ultraviolet light, and with training can distinguish between ultraviolet and visible light, and between different colors in the blue-green range (Jacobs et al . 2001).
What would such vision look like? Animals with red-green colorblindness would be able to distinguish blues from greens, but reds would appear dark to them. They would also have a "neutral" point in the blue-green area of the spectrum: they cannot distinguish these blue-green hues from certain shades of gray. The rat's color vision merges into the ultraviolet, however, so they can see ultraviolet shades that we cannot (see flowers under ultraviolet light to get an idea of what UV looks like).
Rats don't have many cones, though -- 99% of the rat retina consists of rods, which sense only light and dark, and only 1% consists of cones (LaVail 1976), compared to a human's 5% (Hecht 1987). So the rat's perception of color may be fainter than ours, and color cues may not be very important to rats. In fact, brightness appears to be far more important to rats than color. It is easy to train rats to behaviorally differentiate brightnesses, but difficult to train them to behaviorally differentiate colors (Jacobs et al. 2001).
So, while rats are physically capable of distinguishing between ultraviolets, blues, and greens, such differences may not be very meaningful to them. This gets into the whole "just noticeable difference" vs. "just meaningful difference" concept, first introduced by Nelson and Marler (1990).
What is the function of ultraviolet vision? The function of ultraviolet vision in rodents is not well understood yet and is currently an active area of research. Here are some possibilities:
Urine-mark visibility: Urine is visible under ultraviolet light (humans can see rat urine using a black light). Therefore, when rodents leave urine marks in their environment, these marks may visible as well as smellable (e.g. degus, Chavez et al. 2003 voles, Koivula et al. 1999 mice, Desjardins et al. 1973). Unfortunately, these urine marks may also be visible to predators, such as dirunal raptors. Using ultraviolet cues from the urine marks, kestrels can discriminate between active and abandoned vole trails, thus increasing their hunting success (Viitala et al. 1995).
The body under UV : different parts of an animal's body may reflect different amounts of ultraviolet light. In degus, for example, the belly reflects more UV light than the back. Therefore, when a degu stands up on its hind legs it exposes its belly to other degus, and ultraviolet vision may come into play. When it stands on all fours its low-reflectance back could help make the degu less visible to predators (Chavez et al. 2003).
Twilight ultraviolet vision: Ultraviolet light is not available at night, but is abundantly available during the day. Interestingly, there is a significant increase in the ratio of ultraviolet to visible light in the morning and evening twilight hours (Hut et al. 2000). Rats are nocturnal, but they are also active during the twilight hours, starting just before sunset and ending just before sunrise (Robitaille and Bovet 1976). Ultraviolet vision would be advantageous at these twilight times of day. It is therefore possible that ultraviolet sensitivity is retained in rats because it is useful during the twilight hours.
The rat's world is very blurry. Visual acuity is measured in cycles per degree (cpd), a measurement of the number of lines that can be seen as distinct within a degree of the visual field. Acuity of humans is about 30 cpd, normally pigmented rats is 1 cpd, and 0.5 for albino rats (Prusky et al. 2002, 2000also see Birch and Jacobs 1979 who found 1.2 cpd for pigmented rats and 0.34-0.43 cpd for albino rats). If we translate Prusky's cpd measurements into vision chart measurements, a normally pigmented rat has about 20/600 vision, and an albino rat has about 20/1200 vision.
Rat acuity can also be measured by examining the density of ganglion cells in the retina. The denser the ganglion cells, the higher the acuity at that point of the retina. In the rat, the densest area of ganglion cells (defined as the region encompassing 75% of maximum ganglion cell density) is 52.8º wide and is located slightly above and temporal to the optic disk. The maximum density of this area is 6,774 cells/mm 2 . This isn't very dense -- the densest area of the human retina, the fovea, has up to 38,000 cells/mm 2 (Curcio and Allen 1990). The low density of ganglion cells of the rat's retina suggests a maximum visual acuity of 1.5 cpd, which is consistent with the measures found in behavioral acuity experiments (Heffner and Heffner 1992).
Depth of focus: Combined with poor visual acuity, rats have an enormous depth of focus. Depth of focus is the range of distances at which an object is in equivalent focus for an unaccomodated eye. In humans, the depth of focus is from 2.3 meters to infinity (Campbell 1957). In rats, the depth of focus is from 7 centimeters to infinity (Powers and Green 1978), which may be due to the small size of the rat's eye and its poor acuity (Green et al. 1980).
One consequence of this difference in depth of focus is that humans perceive blurriness after a change of about 1/3 diopter, but rats require a 14 diopter change to perceive any blurriness (Powers and Green 1978).
Would tiny eyeglasses help a rat see better?
To understand why, imagine that you put lenses of different strengths in front of a human's eyes and a rat's eyes, like the optometrist does when fitting you for glasses. A human can perceive slight differences in the strength of these lenses: a difference as low as 0.3 diopter. A 0.3 diopter lens is a weak lens -- it's weaker than a 0.5 diopter lens, which would correct the vision of a slightly nearsighted person with 20/25 to 20/30 vision.
Therefore, a person with perfect 20/20 vision who puts on 0.3 diopter glasses will detect just a little blurriness. That's the lowest change in lens strength that humans can perceive.
A rat, however, due to its enormous depth of focus, couldn't perceive such a small 0.3 diopter difference in lens strength. So, if you put tiny 0.3 diopter glasses on a rat it would perceive no change in blurriness. It would be the same with 2 diopter glasses, or 6 diopter glasses, or even 10 diopter glasses. In fact, for the rat to perceive any change in blurriness at all, you would need to put thick 14 diopter glasses on it. With such strong lenses the rat would probably perceive a slight increase in blurriness.
The upshot of this is that rat vision is naturally very poor and cannot be corrected with glasses even if you could make glasses that small. Rat eyes are not capable of 20/20 vision. Their eyes' enormous depth of focus, combined with tiny optics, the coarse grain of the retina, their inability to change the shape of their lens to adjust focus, all add up to poor vision. The rat's 20/600 vision is probably as good as it gets. Give a rat immensely strong lenses and its vision would not improve its vision would become, if anything, slightly blurrier.
Field of vision vs. binocular depth perception: Rats have eyes on either side of their heads. This position allows for a large field of vision but less binocular vision (Block 1969).
Specifically, the position of the eyes on the head represents a tradeoff between field of vision and binocular vision, which is used in binocular depth perception. Laterally-placed eyes (on either side of the head) scan separate portions of the world, and the field of vision at any one point of time is enormous. Because this position allows the animal to detect threats from many directions at once, it is most common in prey animals (think horses, pigeons, gophers etc.).
Eyes placed on the front of the head have much more overlap. This means a smaller visual field, but more binocular vision. Depth perception is useful for predators who need to coordinate their movements to capture prey, so predators often have forward-facing eyes (think raptors, cats, dogs etc.).
Here's how binocular vision provides depth perception. Each eye sees a slightly different image. The closer an object is, the more different it will look to either eye. The brain processes the disparity between the two images and this provides a sense of depth. Also, focusing on a nearby object makes the eyes converge slightly, and the brain uses information from the eye muscles to calculate how far away the object must be. So, the move overlap between the visual fields of the two eyes, the more binocular depth perception.
Rats don't have as much overlap as we do: their field of binocular vision is only about 76º, while ours is about 105º (Heffner and Heffner 1992). Therefore, rats have a larger field of vision than we do but poorer binocular depth perception.
Depth perception using motion parallax: However, there are many other ways to perceive depth besides binocular vision (e.g. image displacement, motion parallax, and loom). Motion parallax is one of these non-binocular methods of perceiving depth. When one moves one's head from side to side, objects seem to change position relative to each other. Close objects seem to move more than objects that are far away. This is called relative motion parallax , and the brain uses it to calculate relative distance between objects.
An image of the object also moves across the retina. The object's apparent movement across the retina, combined with the amplitude of the head movement are used to calculate absolute distance between the obeserver and the object. This is called retinal or absolute motion parallax (Kral 2003).
Rats use motion parallax to estimate depth. Legg and Lambert (1990) counted the number of vertical head bobs before rats jumped between two platforms. As the gap between the platforms increased, trained rats performed more, and larger head bobs before jumping. They may be adjusting the size of their head movement until they produce a detectable amount of motion parallax (Ellis et al. 1984). As the gap widens, larger head movements are needed. Legg and Lambert (1990) found that that rats could jump accurately even if only the leading edge of the platform was visible, indicating that rats use absolute motion parallax to estimate depth. However, the fact that the rats were reluctant to jump to just a leading edge indicates that they may prefer to use relative motion parallax.
Visual orientation: Rats orient themselves using distant visual cues (Hebb 1938, Lashley 1938). For example, Carr (1913) found that rats trained in a sideless maze lost their way if the maze was rotated, but if the the visual environment was rotated along with the maze their performance didn't suffer (Higginson 1930). The presence of visual cues in mazes speeds up the maze-learning process, while the removal of visual cues causes even a trained rat to make mistakes (Honzik 1936). Rats can discriminate between visual stimuli 51 cm away (Mostafa et al. 2002) and can tell the difference between a blocked and an open maze pathway up to 75 centimeters away (Robinson and Weever 1930).
Rats use their vision for shorter distances, too. For example, rats can jump from a raised platform to another surface over a distance of 30 cm (Lashley 1930).
At very short distances, however, rats may trust their whiskers more than their eyes. In one experiment, rats were placed on a sheet of glass. Half the glass was over a platform, and the other half was over a void. This is called a visual cliff experiment. Animals that rely on visual information to perceive depth, like human children, choose to step onto the glass above the platform instead of the glass over the dropoff. Rats, however, chose the deep and shallow side of the glass in equal amounts: they walked fearlessly on the glass suspended over a dropoff. Their whiskers told them there was a solid surface to walk on. In contrast, rats with clipped whiskers stayed away from the dropoff and chose the shallow side, indicating that without their whiskers they were forced to rely on vision to perceive depth (Schiffman et al. 1970).
Vision and aging: As rats age beyond age two, the rat's retina loses many of its cells, and parts of the retina become enlarged and thickened. The capillaries that feed the retina also become greatly thickened (Weisse 1995). Therefore, old rats probably do not see as well as young rats.
Normally pigmented rats have panoramic, blurry vision with faint greens, blues and ultraviolets. These colors may or may not be meaningful to the rats. This kind of vision probably works just fine for rats: rats can see food and other rats a short distance away, but are still sensitive to incoming hawks or dogs and distant orientation cues.
Albinos have a number of differences in their visual systems compared to normally pigmented animals. In a nutshell, albinos have no pigment -- no melanin -- in their eyes. This means no pigment in the iris. That's why the iris looks red -- the only color left comes from blood in the capillaries. Albinos also lack pigment deeper in the eye, pigment which normally absorbs light. Without it, light inside the eye scatters. The consequence is that the albino's eye is flooded with light. Over time, the light causes retinal degeneration. In addition to all this, albinos have abnormal neural connections between the eyes and the brain. The end result of all this is that albino rats have very poor vision. Here are the specifics:
Low visual acuity : The albino rat's inability to control levels of incoming light, the scattering of light inside the eye, and gradual retinal degeneration lead to very poor visual acuity. Albino rats have poorer visual acuity than pigmented rats (Prusky et al. 2002), estimated at about 20/1200 vision.
Impaired vision in bright light: dazzling: Albino rats can't control levels incoming light. Normally-pigmented animals have a pigmented iris that surrounds the pupil and controls how much light shines onto the retina. Albinos lack pigment in their irises, so light passes through the iris, dazzling the retina. In bright light, albinos may not see anything at all because their retinas are overwhelmed by the incoming light.
Impaired vision in low light:
Few rods, few photoreceptors: Rods require a melanin precursor to develop ( dopa ). Albinos can't make this. Without it, about 30% of the rat's rods fail to develop (Ilia et al . 2000). Not only does the albino rat have fewer rods, but even early in life the rods it does posess have fewer rod photoreceptors (called rhodopsin ) than the rods of pigmented rats (Grant et al. 2001). Rods and their photoreceptors are useful for detecting low levels of light, so albino rats may have trouble seeing in low light conditions.
Delayed dark adaptation: Albino rats take longer to adapt to the dark than pigmented rats. Specifically, pigmented rats adapt to the dark within 30 minutes, but albino rats take about three hours (Behn et al . 2003). This delay comes from the albino rats' lack of melanin in their eyes. Eyes that lack melanin have reduced bio-availability of calcium (Drager 1985). Calcium plays a key role in a retina's ability to adapt to the low light conditions (called dark adaptation ) (Fain et al. 2001).
Night-blindness in albino rats: conflicting reports : Once dark adaptation is achieved, Balkema (1988) reports that pigmented rats have a much lower dark-adaptation threshold than albinos. In other words, Balkema reports that pigmented rats can see under conditions of much lower light than albino rats. However, Green et al. (1991), Herreros et al. (1992), and Munoz et al. (1994) found no such differences in dark-adaptation thresholds between albino and pigmented rats, indicating no albino night blindness.
Problems coordinating what the two eyes see: There are even further visual differences between albinos and normally pigmented animals, involving the eye-to-brain connection. In normal mammals, the left side of each eye is connected to the right hemisphere of the brain, and the right side of each eye is connected to the left hemisphere. Albinos have a much simpler connection: most of the left eye is connected to the right hemisphere, while most of the right eye is connected to the left hemisphere (Silver and Sapiro 1981). In addition, the deeper neural projections involved in vision are disorganized (Creel et al. 1990). The consequence is that albinos may have trouble coordinating and processing what their two eyes see.
Poor depth perception : The albino rat's poor visual acuity leads to poor visual depth perception. In the visual cliff experiment by Schiffmann et al. (1970) described in the previous section, rats were placed above a sheet of glass over a ledge and dropoff. Pigmented and albino rats with intact whiskers relied on their whiskers instead of their eyes and chose to walk on the glass over the dropoff as often as the glass over the ledge. When the whiskers were clipped, however, the rats were forced to rely on visual cues. Pigmented rats with clipped whiskers chose the glass over the ledge. Most whiskerless albino rats also chose the glass over the ledge, but a large percentage of them (20%-33%) made no choice at all but stood stock still. This failure to choose indicates that albino rats do not use visual information to perceive depth as readily as pigmented rats do. Albinos appear to be more impaired by whisker removal than pigmented rats, probably because their fallback sensory system -- vision -- is so poor.
Pet owners often note that albino rats bob their heads and sway frequently. This bobbing and swaying may be the albino's attempt to increase its perception of depth using its greatly impaired vision.
Poor motion perception: Albino rats have greatly impaired motion perception. They are not motion blind, but they have poor motion perception when compared to pigmented rats. Albino rats require about twice to three times the coherence level to distinguish coherent motion patterns from dynamic noise.
Specifically, Hupfeld and Hoffman (2006) presented rats with moving dot patterns in which dots moved randomly on a screen. A coherent moving pattern was created by a proportion of the dots moving to the right. The percentage of dots moving to the right was called the "percentage of coherence." So, a 100% coherence meant all dots moved to the right 70% coherence meant 70% the dots moved to the right while 30% moved randomly, and so forth. Both pigmented and albino rats could distinguish between a random pattern and a 100% coherence pattern. When the coherence was reduced, discrimination performance declined in both pigmented and albino rats. Pigmented rats tended to do better, but not significanly so, down to 30% coherence. Below that coherence level pigmented rats did significantly better than albinos. In sum, pigmented rats could discriminate a pattern of 12% coherence from dynamic noise, while albino rats needed about 30% coherence to make the distinction.
Retinal degeneration: In addition to dazzling them, ambient light (even at low intensities) can cause irreversible retinal degeneration (green light Noell1966). Rods, because they are so sensitive to light, degenerate more easily than cones (Cicerone 1976, Lanum 1978), which reduces the rat's ability to see in low light. Twenty four hours of ambient light is enough to cause some degeneration, and a few weeks is enough to completely degenerate the outer retina (Lanum, 1978), by causing loss of photoreceptors and cell bodies (Wasowicz et al. 2002).
Retinas more easily damaged: When the rat's eyes are subjected to pressure, the albino rat's retina sustains more damage than the pigmented rat's retina (Safa and Osborne 2000).
Lens fiber anomalies: Albino rats also have abnormal lens fibers compared to pigmented rats. When the lens fibers of pigmented and albino rats are observed under the electron microscope, pigmented rat lens fibers have many "ball and socket" joints between them, but albino rats have few of these joints. The membranes of albino rat lens fibres are often ruptured (Yamada et al. 2002).
A note about foveas: Many normally pigmented mammals like humans and other primates have an area of the retina that is packed with cones, called the fovea . The fovea is surrounded by an area with many more rods than cones. The fovea therefore has very sharp, color-sensitive vision that is useful in bright light, while the area surrounding the fovea has blurry, mostly monochromatic vision that is useful in low light conditions. Albinos of these species don't develop a fovea, so they are missing this patch of sharper color vision. However, in rats, even normally-pigmented rats don't develop a fovea (Reese 2002), so fovea hypoplasia in albinos isn't an issue in rat vision.
What if albinos are provided with a missing enzyme during developement? Albinos have a mutated tyrosinase enzyme. This enzyme is essential for the complex chemical reaction leading to melanin. This chemical reaction passes through I-tyrosine, L-DOPA, and I-dopaquinone on its way to melanin. L-DOPA itself plays an important role in retinal development. Lavado et al. (2006) examined transgenic albino mice that had the gene for tyrosine hydroxylase spliced in to their genome. Tyrosine hydroxylase is an enzyme that will do part of what the missing tyrosinase does: it will oxidize I-tyrosine to L-DOPA, but it won't go any further toward melanin. The resulting animals were phenotypically albino, but they did not have many of the visual problems of albinos: they had normal photoreceptors, normal neural pathways in the brain, and improved visual function.
Pigmented rats see a blurry world with only faint color, ranging from green to ultraviolet. The function of such color vision, and whether it is meaningful to rats, is currently unknown.
Pigmented rats rely less on vision and more on smell and hearing than we do. They navigate extensively by whisker touch. Having poor vision is not as great a handicap for the rat as it is for a human. Rats live in a rich world of sound and smell and touch that enables then to navigate effectively in their world.
Albino rats are probably severely visually impaired or blind within a few weeks of opening their eyes. Their retinas degenerate, their brains have trouble coordinating images from their two eyes, and they see poorly in both bright and dim light.
The albino rat will therefore need to compensate for its near-blindnes by using its other senses. Unfortunately, albino rats also appear to have an impaired sense of smell when compared to pigmented rats. However, albinos do appear to have normal hearing.
We extracted genomic DNA from quill bases of feathers, blood, muscle and other tissue material either with a GeneMole® automated nucleic acid extraction instrument (Mole Genetics), the DNeasy Blood and Tissue Kit (QIAGEN) or with Chelex. Standard procedures were applied except for the quill bases, which were lysated with 1% DTT. Feather material was sampled from a European green woodpecker Picus viridis killed by traffic. Live animals were not sampled for this study. Other tissue material was borrowed from museum collections and from the collections of colleagues, the National Veterinary Institute SVA in Uppsala and Uppsala City Council. We performed mtDNA barcoding with COI, following the Stockholm protocol outlined in , to confirm labelling of selected tissue samples and to identify species Ramphastos tucanus from an unspecified toucan tissue sample.
Using the degenerate primers SU80a , SU149a, SU161a, SU193a  or SU200Ca, combined with SU304b  or SU306b  we amplified a gene fragment coding for residues from aa sites 81–94, located in the 2nd α-helical transmembrane region of the SWS1 opsin. We conducted PCR on an Eppendorf MasterCycler Gradient or a PE Applied Biosystems Geneamp® PCR System 9700 with reactions containing 0.5-2.5 ng/μl DNA extracts, 1 unit Taq-polymerase (Applied Biosystems) plus reaction buffer, 0.4 pmol of forward and reverse primers, 0.2 mM of each dNTP, and 2 mM MgCl2. Each PCR reaction contained 0.5–2.5 ng/μl total DNA extracts, 1 unit Taq-polymerase (Applied Biosystems) with reaction buffer, 0.4 pmol of forward and reverse primers, 0.2 mM of each dNTP, and 2 mM MgCl2. For some reactions, PuReTaq™ Ready-To-Go™ PCR beads (GE Healthcare) replaced separate volumes of Taq-polymerase, dNTP’s and MgCl2. Initially, the reaction conditions followed  (i.e. 90 s at 94°C, 5 × (30 s at 94°C, 30 s at 54°C and 1 s at 72°C), 38 × (15 s at 94°C, 30 s at 54°C and 5 s at 72°C) and 10 min at 72°C) but were later optimized to exclude the extension phase in order to minimize nonspecific amplification of longer fragments. The final version of thermocycling started with 90 s at 94°C, was followed by 48 × (5 s at 94°C, 15 s at 54°C) and ended with 1 s at 72°C. We used a different protocol for the primer pair SU80a/SU306b, namely 2 min 30 s at 95°C, 40 × (30 s at 95°C, 30 s at 54°C and 10 s at 72°C) and 1 min at 72°C. Two percent agarose gel electrophoresis for 90 min at 80 V confirmed amplification and expected fragment length. When there were extra fragments present we sometimes performed a second PCR on the products using internal primers.
The PCR products were purified with EXOsap-IT (USB). Macrogen Inc. (South Korea) then performed double-stranded sequencing using the same primers as above plus SU200a , SU200Ga , and SU296b 5 ′ -AAG AYR AAG TAD CCS YGS G-3 ′ , which we designed for this study with the help of Primer3 online software (http://frodo.wi.mit.edu/) .
We translated our DNA sequences into aa’s to identify the spectral tuning sites 86, 90, and 93 [5, 10]. From the aa residues presents at these sites we estimated λmax values following Wilkie et al. , Yokoyama et al.  and Carvalho et al.  as outlined in .
- Color vision