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Why is mammal coloration so dull?

Why is mammal coloration so dull?


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I have seen species of birds, insects and fishes with splendid colours. But when it comes to mammals (including us humans), they almost always appear in shades of brown, grey, orange, or in black and white. I don't recall seeing any pink, green and purple. Why is that so?


I don't think, there is a precise answer about the evolutionary mechanisms, but "mechanically":

  • mammals have principally just two types of pigments: eumelanin and pheomelanin, both of which have their color variants, but within a known range. Bird pigments, besides melanins, include carotenoids and porphyrins. Arthropods generally have carotenoids, melanins and ommochromes [and other pigments?]. E.g. carotenoids and ommochromes alone can create rather "exotic" coloration from a mammal point of view (green, pink, violet).
  • both birds and insects actively utilize iridescence. With fur it seems to be technically much more difficult to achieve than with feather or scales.
  • many (most?) mammals do not differentiate colors. Birds have much better vision abilities in this respect. From a selectionist point of view this cuts out a considerable part of selection acting upon coloration, which could otherwise produce broader spectrum of phenotypes.

In addition to the arguments presented in the accepted answer, I would like to add that most species of mammals are red/green colour-blind. This has to do with the fact that for much of their evolutionary history, mammals were nocturnal creatures and so the ability to see different colours wasn't necessary.

This dates back to the mesozoic period when mammals had nocturnal lifestyles in order to avoid predation from dinosaurs.

Here's an article that summarizes recent research on this topic: http://www.utexas.edu/news/2012/10/29/effects-prehistoric-nocturnal-life-mammalian-vision/


The Answer is simple… EVOLUTION… The colour of an living organisms depends on 2 things 1) attracting other sex. 2) escape from the predators.

In birds and insects: They doesn't much depend on other sex for living(except for the process of reproduction)and there is no need t exhibit physical work for attracting other sex… For example if you take a peacock the male needs to be colourful to attract female. but there is no physical work… Let The Other example be some Insect,then if it is colourful… a predator may not eat for its bright colour… this made insects and birds evolute into colourful…

But in mammals. the attracting and escape both depend on physical work… a lion being handsome cannot attract lioness… :P It need to be physically healthy to escape from predators and give an enough competition to other lions in his territory… this made evolution for mammals much depend on their capability of body… rather than physical attractiveness… for example in a forest a tigress will obviously pair with a Tiger which has more territory than a more attractive handsome tiger without territory(this is not a case with insects)…


Why is mammal coloration so dull? - Biology

A tiger’s stripes allow him to disappear into the jungle shadows.

The plant and animal kingdoms abound with bright colors, from the lush green of photosynthesizing plants to the bold black and orange stripes of tigers. Color plays a multitude of roles in the natural world, used to entice, to camouflage, or to warn other creatures. Colors signal harvest time, breeding conditions, and the change of seasons, from the first greens of spring to the brilliant reds and browns of the fall.


Why is mammal coloration so dull? - Biology

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    Female Birds Aren’t So Dull After All

    In the bird world, flashy colors and boisterous songs are generally reserved for males. Females tend to be comparatively dull-colored and often stay quietly hidden in the background. Charles Darwin himself noted this difference between the sexes and proposed that it was mainly caused by past evolutionary changes in males, in a process he called sexual selection—a theory still widely upheld today.

    Yet, these understated female birds have a hidden past that overturns something researchers have assumed for over 150 years.

    In a recent study, biology researchers J. Jordan Price of St. Mary’s College of Maryland and Muir Eaton of Drake University provide evidence that, despite current appearances, most past evolutionary changes in color have occurred in females, not males. Many of today’s drab females had brightly colored ancestors, and current differences between the sexes are largely due to females losing bright colors rather than males gaining them.

    “I think Darwin would have been very surprised by this idea,” said Price. “Darwin was a strong advocate for the idea that sexual dimorphism is due to sexual selection. Many male traits undoubtedly are the products of sexual selection, as he suggested, but our evidence suggests that differences between the sexes are largely due to selection on females.”

    Price and Eaton focused on a group known as the New World blackbirds, which includes a variety of songbird species ranging from Canada to Argentina. In New World blackbirds some females are dull in color compared to their male counterparts and some are just as brightly colored as males. Using relationships based on DNA sequence data, they were able to reconstruct the likely evolutionary histories of each sex, which revealed the surprising differences in the speed with which males and females have evolved.

    “We mapped out differences using a phylogenetic tree to figure out what happened in the past and to look at color differences over time, and we found that although evolution occurs along the same period of time for both sexes, females have covered a lot more evolutionary ground,” said Price.

    To paint the picture of the female birds’ rapid, teeter-totter evolution—away from and toward male colors—Price said to think of something many do every day. “It’s like when you are out walking your dog and you cover a distance and the dog covers three times the distance by running back and forth,” he explained.

    “What’s often assumed is that sexual selection operates mainly on male appearance, and the result is that males then look different from females,” said Eaton. “Our results strongly suggest the opposite. Females, with their dull colors, are under strong natural selection to not stand out, thus they look very different from males. Perhaps this is because they spend more time on or near the nest and must be inconspicuous.”

    Eaton also noted that the color differences in male and female birds go beyond what the human eye can see. “Our use of objective measurements of feather coloration, and quantification of color differences from the perspective of how birds see color differences, allowed us to uncover these complex evolutionary patterns that might otherwise go unnoticed,” he said.

    The study, entitled “Reconstructing the Evolution of Sexual Dichromatism: Current Color Diversity Does Not Reflect Past Rates of Male and Female Change,” is available online today in the journal Evolution.


    Warming seas might also look less colorful to some fish: Here's why that matters

    When marine biologist Eleanor Caves of the University of Exeter thinks back to her first scuba dives, one of the first things she recalls noticing is that colors seem off underwater. The vivid reds, oranges, purples and yellows she was used to seeing in the sunlit waters near the surface look increasingly dim and drab with depth, and before long the whole ocean loses most of its rainbow leaving nothing but shades of blue.

    "The thing that always got me about diving was what happens to people's faces and lips," said her former Ph.D. adviser Sönke Johnsen, a biology professor at Duke University. "Everybody has a ghastly sallow complexion."

    Which got the researchers to thinking: In the last half-century, some fish have been shifting into deeper waters, and climate change is likely to blame. One study found that fish species off the northeastern coast of the United States descended more than one meter per year between 1968 and 2007, in response to a warming of only about one degree Celsius.

    Could such shifts make the color cues fish rely on for survival harder to see?

    Previous research suggests it might. Scientists already have evidence that fish have a harder time discerning differences in each other's hues and brightness in waters made murkier by other causes, such as erosion or nutrient runoff.

    As an example, the authors cite studies of three-spined sticklebacks that breed in the shallow coastal waters of the Baltic Sea, where females choose among males -- who care for the eggs -- based on the redness of their throats and bellies. But algal blooms can create cloudy conditions that make it harder to see, which tricks females into mating with less fit males whose hatchlings don't make it.

    The turbidity makes it harder for a male to prove he's a worthy mate by interfering with females' ability to distinguish subtle gradations of red or orange, Johnsen said. "For any poor fish that has beautiful red coloration on his body, now it's like, 'well, you're just going to have to take my word for it.'"

    Other studies have shown that, for cichlid fish in Africa's Lake Victoria, where species rely on their distinctive colors to recognize their own kind, pollution can reduce water clarity to a point where they lose the ability to tell each other apart and start mating every which way.

    The researchers say the same communication breakdown plaguing fish in turbid waters is likely happening to species that are being pushed to greater depths. And interactions with would-be mates aren't the only situations that could be prone to confusion. Difficulty distinguishing colors could also make it harder for fish to locate prey, recognize rivals, or warn potential predators that they are dangerous to eat.

    In a study published April 21 in the journal Proceedings of the Royal Society B, Caves and Johnsen used mathematical models to determine what the colors of the underwater world might look like as fish in the uppermost layer of the ocean shift to new depths.

    They were able to show that, while the surface waters may be bursting with color, descending by just 30 meters shrinks the palette considerably.

    "It's like going back to the days of black and white TV," Johnsen said.

    When sunlight hits an object, some wavelengths are absorbed and others bounce off. It's the wavelengths that are reflected back that make a red fish look red, or a blue fish blue. But a fish sporting certain colors at the surface will start to look different as it swims deeper because the water filters out or absorbs some wavelengths sooner than others.

    The researchers were surprised to find that, especially for shallow-water species such as those that live in and around coral reefs, it doesn't take much of a downward shift to have a dramatic effect on how colors appear.

    "You really don't have to go very far from the surface to notice a big impact," said Caves, who will be starting as an assistant professor at the University of California, Santa Barbara, this fall.

    Precisely which colors lose their luster first, and how quickly that happens as you go down, depends on what depths a species typically inhabits and how much deeper they are forced to go, as well as the type of environment they live in -- whether it's, say, the shallow bays or rocky shores of the Atlantic, or a tropical coral reef.

    In clear ocean water, red is the first color to dull and disappear. "That's important because so many species use red signals to attract mates or deter enemies," Johnsen said.

    The team predicts that some species will be more vulnerable than others. Take, for instance, fish that can't take the edge off the heat by relocating toward the poles of the planet. Particularly in semi-enclosed waters such as the Mediterranean and Black seas or the Gulf of Mexico, or in coral reefs, which are stuck to the sea bed -- these species will have no option but to dive deeper to keep their cool, Caves said.

    As a next step, they hope to test their ideas in the coral reefs around the island of Guam, where butterflyfishes and fire gobies use their vivid color patterns to recognize members of their own species and woo mates.

    "The problem is only accelerating," Caves said. By the end of this century, it's possible that sea surface temperatures will have heated up another 4.8 degrees Celsius, or an increase of 8.6 degrees Fahrenheit, compared to the 1896-2005 average.

    And while warming is happening faster at the poles, "tropical waters are feeling the effects too," Caves said.

    This research was supported by the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement (No 793454).


    Innes Cuthill

    Professor, Behavioural Ecology, University of Bristol

    One factor — in reference to things like iridescence (peacocks, morpho butterflies, jewel beetles) — is the nature of their pelage. Hair is pretty unstructured (tubes of keratin with melanin pigment), whereas bird feathers, butterfly wings and insect cuticles in general, are complex multi-layer structures — which is just what you need to generate structural colours created by interference of light rays.


    Why are there so few blue, green, or purple mammals?

    Why do we see only really see white/black/red/orange/yellow/grey/brown/pink but little to no blue/purple/green based mammals? With Green and Blue being such prominent colors in nature, I would think they would be go to colors for camouflage. I honestly couldn't think of any mammals except the Blue Whale, but they are more grey-ish than blue.

    Any insight would be welcomed.

    -Blue- Whales, Dolphins, Mandrills

    This column has an in-depth answer, focusing specifically on the absence of the color green. But as it turns out, mammals are only capable of generating brown-black pigment (the melanin that gives skin its tan) and reddish-yellow. These are the dyes which, in various combinations or absences, are responsible for the limited mammalian color palette.

    It turns out, amazingly enough, that other terrestrial vertebrates can't create blue or green pigments either! The array of cool colors we see in birds, amphibians, and the like (as well as many brilliantly-colored insects like tropical butterflies) are actually caused by microscopic structures in their feathers or skin. These structures refract the light that bounces off of them, like a prism or a CD, causing us to perceive them as a particular color.

    This Mammal seems to be capable of creating blue pigment, no?

    Edit: image url changed. Points to all who knew what it was anyway!

    The array of cool colors we see in birds, amphibians, and the like (as well as many brilliantly-colored insects like tropical butterflies) are actually caused by microscopic structures in their feathers or skin.

    The bit about birds you spoke of isn't completely true. It is a combination of pigments and the refractory processes of which you speak. And some of the pigments are quite colorful arising from porphyrins and ingested pigments found in plants.

    A lot of birds coloration has to do with their diets also. The most famous example is the Pink Flamingo. They are only pink because of their diets. But there are other examples. The house finch is usually a dull brown bird with very little red plumage on it, but during mating season they eat berries that are full of carotenoids (http://www.birds.cornell.edu/pfw/AboutBirdsandFeeding/HoufinColorVariants.htm) which is what accounts for most of the red coloration in birds today. Most do not naturally produce red pigments in the depth that you see in nature it comes from the berries they eat.

    Not all colors in bird feathers are caused by micro structures. Some are plain old pigments.

    This is a great answer! But I have a follow-up question regarding color blindness vs sightedness. I think its important to distinguish between Predatory and Prey mammals.

    Do Prey mammalia have enough color distinction to benefit from vibrant colors to sufficiently impact Sexual Selection?

    Do Predatory mammalia have enough color distinction to be attracted to any mammals that develop these colorations?

    But this explanation raises the question why mammals never developed that ability or lost it. One explanation is that at some point in the past mammalian ancestors developed diminished capacity to perceive color.

    But there is one major exception: primates. Most other mammals have only two types of color-perceiving cones in their eyes primates, including humans, have three.

    Not surprisingly then, certain monkeys developed quite brilliant coloration, though usually confined to small areas of their bodies. Examples: Mandrills, Tamarins, Lesulas.

    These structures refract the light that bounces off of them, like a prism or a CD, causing us to perceive them as a particular color.

    I think you meant diffract instead of refract?

    Incidentally, I once encountered a pseudo-green monkey while working at a veterinary clinic. It had bluish-black hairs and yellow hairs mixed, which at a distance created an impression of greenness. I don't know if it was the same species mentioned in the article you quoted. Sadly, it had to be put down, as it had become quite unmanageable, putting its claws through its owner's lower lip.

    The Moray eel also appears green, but as I recall that's because it secretes a blue slime, and has yellow skin.

    This may be a stupid question, but would it not be possible for there to be a mutation in a mammal that would allow them to be green or blue? I mean, couldn't certain mammals evolve to be green if it was beneficial to their species?

    That's definitely the most direct, technical answer. But (probably because I've been reading about a lot recently) I think it's also worth noting vibrant colors are generally used for two things. Mate attraction and aposematism. Birds definitely come to the forefront as examples of coloration in mate selection. But from what I can think off the top of my head, no mammal is actually toxic if ingested, so I don't thinking aposematism is represented in mammals at all.

    That page is no longer available. Here's what it said:

    Alexey Veraksa
    assistant professor,
    Biology Department,
    University of Massachusetts,
    former HHMI predoctoral fellow
    I know from my general zoology classes that mammals can make only two kinds of pigment: melanin (black or brown pigment) and the reddish-yellow pigment that red-haired people have. So my first idea for a simple explanation of the nonexistence of green mammals was to say that mammals just can't make green pigment for some reason. This is of course not a good evolutionary explanation, but at least it superficially answers the question.

    As I started researching this a bit more, I was astounded to discover that frogs, birds, and others in the tetrapod, or four-legged, world can't make green pigment, either! Or blue, for that matter. It turns out that all color variation that we see in tetrapod animals is the result of different combinations of patterns of deposition and refraction of the same two types of pigments: black and yellow-red. A chameleon's color changes because of rapid shape changes in refractory cells in its skin, not by rapid production or release of an actual pigment. Frogs are green because of the pattern of refraction of blue light by special cells in their skin, which blends with their yellow pigment.

    The colors of bird feathers are also generated by light refraction but by a different mechanism. The surface of feathers has microscopic ridges that form ordered tracks, much like the surface of a CD. The spacing of the ridges and the size and orientation of the pigment granules they contain determine the feathers' brilliant greens and blues (see links in reference 2). Tiny air pockets in feathers can add to color variation in birds. Light refraction is also responsible for the color of the human iris, which can range into deep blue or green hues. Another example is the sky-blue nose of the mandrill. For an exploration of the ways animals make different colors, see reference 1.

    As you can see, the question is really why mammals don't have the ability to generate a variety of colors in their fur. This is a difficult question, and until we master time travel—to be able to trace back the evolution of mammals—we won't know the answer with certainty. But it's interesting to speculate, so here are some possible explanations.

    I think that the most important difference is in the lifestyle of mammals. Being warm-blooded animals, they are constantly in search of food and are moving a lot. Even if an animal has the best camouflage, as soon as it starts moving, it is more easily detected. In contrast, many amphibians and reptiles can spend a long time being motionless, and here looking the same as a leaf nearby helps more. I believe that during evolution, mammals have "invested" more into working out active escape mechanisms (for example, running away and hiding in burrows) rather than into developing passive camouflage techniques.

    Scientists believe that the first mammals were relatively small, ratlike creatures that ate insects. Their habitats were probably different in terms of background color patterns compared with the habitats of amphibians and reptiles. Mammals, of course, originated on land, while amphibians never leave the source of water. More green is around amphibians, and therefore we find more green amphibians.

    In contrast to their early ancestors, many mammals are now large, and for them, a factor of size comes into play in determining coloration. As we know from classical statues, it's difficult to cover a whole mammal with a fig leaf (but it is quite possible for a frog). A solid-green mammal would in fact stand out in real habitats rather than be hidden. For a large animal, being dappled helps more than being green.

    3. Most mammals, except whales and dolphins (which in fact can develop bluish green coloration), are covered by some form of fur. Like bird feathers, mammalian hairs are keratinized structures that extend far out from the surface of the skin and can be of different colors because of varying patterns of pigment deposition during hair development. But in contrast with bird feathers, mammals have not developed an ability to produce intricate microstructures in their growing hairs that would reflect light in such a way as to create green (perhaps with the exception of a few species such as the African green monkey, whose coat can approach various shades of yellow to olive green on the back). And forget about changing skin color at will like a chameleon—nobody will notice that under your fur, anyway.

    4. Now think about what mammals need to camouflage themselves against (predators) or for (to hunt prey). Most predators are in fact other mammals, and most mammals do not have good color vision, so they don't really care if their prey is green or not. They are much better at distinguishing patterns and differences in light intensity than in colors. That's probably why most mammals have patterns such as spots, stripes, or blotches on their fur, which help them blend in.

    5. Why are there green birds? Here, it seems, a different evolutionary scenario was at work. By being able to fly, birds can escape most predators, and it is thought that striking colors in birds have developed for display and mating purposes rather than for camouflage. Accordingly, birds have exceptional color vision, possibly superior to that of humans. If you think about it, though, birds that do live close to the ground and rarely fly (and are therefore within reach of mammalian predators) are frequently earth colored and dappled, striped, or spotted, just like mammals.

    6. Curiously, there is in fact a group of mammals that is green—three-toed sloths. This appears to be a secondary evolutionary acquisition and results from the growth of green algae in their fur (they rarely move and apparently never wash themselves). Maybe this also helps them blend into the leaves.

    So it seems that a combination of evolutionary forces (and constraints imposed by such mammalian features as high metabolism and fur coat) can explain the lack—or rather, extreme scarcity—of green mammals.

    Finally, some scientists were so disappointed by the absence of green mammals that they decided to create their own! A few years ago, a group of researchers in Japan inserted a gene encoding green fluorescent protein (GFP) from jellyfish into the mouse genome to make mice that fluoresce bright green when illuminated with a blue light. The long evolutionary wait for green mammals is now over. For a stunning picture of "three green mice," see reference 3.

    The nature of color in different animal groups, including frogs.

    Wallin, M. 2002. Nature's palette: how animals, including humans, produce colours. Bioscience Explained 1 (2),

    For an essay on the coloration of birds, go to:

    For information on feathers, including colors, go to:

    For an interesting paper describing the discovery of the generation of iridescent color patterns in peacock feathers, see:

    Zi, J., et al. 2003. Coloration strategies in peacock feathers. Proceedings of the National Academy of Sciences USA 100 (22): 12576-78. http://www.pubmedcentral.nih.gov/articlerender.fcgi? tool=pubmed&pubmedid=14557541 .

    The findings about peacock feathers are also discussed here, with some amazing microscopic pictures:

    For the paper describing the creation of green mice, see:

    Okabe, M., et al. 1997. "Green mice" as a source of ubiquitous green cells. FEBS Letters 407:313-19.


    Variability of eye coloration in humans and animals

    Eye colour in wild species tends to be fixed, whereas humans and domesticated animals show multiple eye colours. Dr Juan J. Negro, lead author of a recent hypothesis published in Frontiers in Zoology, takes us through why this might be.

    Did you know that eye coloration only varies in human populations and their domestic animals? Wild animal species, with few exceptions, have just one type of eye coloration, be it light or dark. By the way, when we say eye color, we refer to the iris encircling the pupil, which is always dark and expands and contracts rapidly depending on light conditions.

    In the case of humans, it is common knowledge that the eye color palette is remarkably large. What is not known, however, is how and when this variation emerged in the evolutionary history of Homo sapiens. And the same can be said for domestic animals, both of feather and fur. There are blue-eyed dogs, cats, horses, goats, camels and llamas. And some of these species also have yellow-eyed variants.

    Cat breeds in particular show a remarkable variation in eye coloration. In a majority of domestic breeds, however, brown is the default color. As it should be expected for domestic animals under selective breeding, the emergence and fixation of variants in both coat or plumage, as well as eye coloration, started at the early stages of domestication in the Neolithic due to the cherry-picking of rare color mutants.

    Curiously enough, eye color variants for humans may have also started very recently (about 8,000 years ago), concurrent with sedentarism and domestication of plants and animals, and only (or mainly) in Europe. As of today, eye color variation in humans may be described as continuous, with numerous shades from very light blues to very dark browns.

    In wild animals, and also in the ancestors of domestic animals, eye coloration does not tend to vary. The few reported cases of eye color variation in wild species, mostly in birds, correspond to changes associated to maturation with age and some rare instances of sexual dimorphism (as with certain duck species such as the common pochard Aythia ferina).

    Perhaps blue- and green-eyed individuals were preferred as mates and left more descendants spreading their eye color in the populations.

    Bird species in which the adults have bright yellow or red eyes may have a darker, brownish color, in the juvenile phase. This seems to imply that certain color types require some time for the individual to accumulate the necessary pigments that provide the definitive coloration of the adult eye. Melanins, by the way, are responsible for the color differences in the eye color of humans: dark eye colors contain eumelanin and pheomelanin, green eyes contain mainly pheomelanin, and blue eyes contain practically no melanin. Today it is possible to get blue eyes with a surgical procedure that removes melanins from the iris. And the acquired color is permanent because the melanin is never replaced.

    Sexual selection can be discarded as a driving force for eye color variation in domesticated species, and natural selection does not act on them as heavily as in the case of their wild ancestors. But, what is the case for humans? Maybe it is a case of sexual selection, after all. Perhaps blue- and green-eyed individuals were preferred as mates and left more descendants spreading their eye color in the populations.

    In wild animals with no variation in eye color, it seems that this trait is adaptive and fixed by natural selection. It may well be that all Darwinian processes would be at work regarding eye colors: artificial selection in domestic animals, natural selection in wild animal species, and perhaps sexual selection when humans became farmers.


    Warming seas might also look less colorful to some fish. Here's why that matters.

    As warming oceans drive fish into cooler, deeper waters, the colors they rely on for survival could become harder to see. A mere 20-meter drop in the water column turns this fire goby from magnificent to muted. Credit: Nazir Amin.

    When marine biologist Eleanor Caves of the University of Exeter thinks back to her first scuba dives, one of the first things she recalls noticing is that colors seem off underwater. The vivid reds, oranges, purples and yellows she was used to seeing in the sunlit waters near the surface look increasingly dim and drab with depth, and before long the whole ocean loses most of its rainbow leaving nothing but shades of blue.

    "The thing that always got me about diving was what happens to people's faces and lips," said her former Ph.D. adviser Sönke Johnsen, a biology professor at Duke University. "Everybody has a ghastly sallow complexion."

    Which got the researchers to thinking: In the last half-century, some fish have been shifting into deeper waters, and climate change is likely to blame. One study found that fish species off the northeastern coast of the United States descended more than one meter per year between 1968 and 2007, in response to a warming of only about one degree Celsius.

    Could such shifts make the color cues fish rely on for survival harder to see?

    Previous research suggests it might. Scientists already have evidence that fish have a harder time discerning differences in each other's hues and brightness in waters made murkier by other causes, such as erosion or nutrient runoff.

    As an example, the authors cite studies of three-spined sticklebacks that breed in the shallow coastal waters of the Baltic Sea, where females choose among males—who care for the eggs—based on the redness of their throats and bellies. But algal blooms can create cloudy conditions that make it harder to see, which tricks females into mating with less fit males whose hatchlings don't make it.

    The turbidity makes it harder for a male to prove he's a worthy mate by interfering with females' ability to distinguish subtle gradations of red or orange, Johnsen said. "For any poor fish that has beautiful red coloration on his body, now it's like, "Well, you're just going to have to take my word for it."

    From dazzling to drab: A reef trigger fish in the shallow coastal waters of the Indo-Pacific, and how it might look if it had to shift just 10 meters down in the water column to escape warming surface waters. Credit: Bernard Spragg.

    Other studies have shown that, for cichlid fish in Africa's Lake Victoria, where species rely on their distinctive colors to recognize their own kind, pollution can reduce water clarity to a point where they lose the ability to tell each other apart and start mating every which way.

    The researchers say the same communication breakdown plaguing fish in turbid waters is likely happening to species that are being pushed to greater depths. And interactions with would-be mates aren't the only situations that could be prone to confusion. Difficulty distinguishing colors could also make it harder for fish to locate prey, recognize rivals, or warn potential predators that they are dangerous to eat.

    In a study published April 21 in the journal Proceedings of the Royal Society B, Caves and Johnsen used mathematical models to determine what the colors of the underwater world might look like as fish in the uppermost layer of the ocean shift to new depths.

    They were able to show that while the surface waters may be bursting with color, descending by just 30 meters shrinks the palette considerably.

    "It's like going back to the days of black and white TV," Johnsen said.

    When sunlight hits an object, some wavelengths are absorbed and others bounce off. It's the wavelengths that are reflected back that make a red fish look red, or a blue fish blue. But a fish sporting certain colors at the surface will start to look different as it swims deeper because the water filters out or absorbs some wavelengths sooner than others.

    The researchers were surprised to find that especially for shallow-water species such as those that live in and around coral reefs, it doesn't take much of a downward shift to have a dramatic effect on how colors appear.

    From dazzling to drab: A reef trigger fish in the shallow coastal waters of the Indo-Pacific, and how it might look if it had to shift just 10 meters down in the water column to escape warming surface waters. Credit: Bernard Spragg.

    "You really don't have to go very far from the surface to notice a big impact," said Caves, who will be starting as an assistant professor at the University of California, Santa Barbara, this fall.

    Precisely which colors lose their luster first, and how quickly that happens as you go down, depends on what depths a species typically inhabits and how much deeper they are forced to go, as well as the type of environment they live in—whether it's, say, the shallow bays or rocky shores of the Atlantic, or a tropical coral reef.

    In clear ocean water, red is the first color to dull and disappear. "That's important because so many species use red signals to attract mates or deter enemies," Johnsen said.

    The team predicts that some species will be more vulnerable than others. Take, for instance, fish that can't take the edge off the heat by relocating toward the poles of the planet. Particularly in semi-enclosed waters such as the Mediterranean and Black seas or the Gulf of Mexico, or in coral reefs, which are stuck to the sea bed—these species will have no option but to dive deeper to keep their cool, Caves said.

    As a next step, they hope to test their ideas in the coral reefs around the island of Guam, where butterflyfishes and fire gobies use their vivid color patterns to recognize members of their own species and woo mates.

    "The problem is only accelerating," Caves said. By the end of this century, it's possible that sea surface temperatures will have heated up another 4.8 degrees Celsius, or an increase of 8.6 degrees Fahrenheit, compared to the 1896-2005 average.

    And while warming is happening faster at the poles, "tropical waters are feeling the effects too," Caves said.


    Why are male birds more colorful than female birds?

    Males are more colorful or ornamented than females in most, but not all, bird species. Understanding this phenomenon requires a basic grasp of the evolutionary forces that shape the behavior and morphology of individuals and species. Charles Darwin developed much of the theory that helps explain this. He proposed that traits promoting survival in individuals are favored by the process of natural selection, whereas traits that help the individuals of just one sex (usually the males) compete for mates are favored by sexual selection. Sexual selection is responsible for many of the features unique to one sex in a given species. These features can be divided into two general categories: those acting as weapons that allow males to fight for access to females (antlers on deer, for example) and those acting as ornaments that attract the attention of females, such as long tails on birds.

    Darwin concluded that color differences between sexes in birds (also known as sexual dichromatism) result largely from female preference for bright colors in males. This general rule has received much support since Darwin's time, but other influences have also been noted. For example, females of species that are exposed to predators while incubating tend to have dull colors, although both sexes may be brightly colored in species that nest in tree hollows because the females are less visible to predators. Color can also aid individuals in recognizing members of their own species. And in species that are not good to eat, colors can provide a warning to potential predators.

    Color is also used in contests between males over mates or resources such as territory. Conspicuous colors can help show that an area is already occupied and that the occupant is in good condition and prepared to fight. The red shoulder patch on red-winged blackbirds provides an excellent example. The patch is coverable and is shown to males and females of the same species but never to predators. Males who had their patch experimentally covered tended to lose their territories more often than did uncovered birds. Similar results have been shown in other species such as scarlet-tufted malachite sunbirds, confirming that the brilliant badges function primarily in male-male competition over territories.

    Some studies have shown that females use the brightness of a male's color as an important indicator of his health and vitality. House finches provide one of the best examples of this tactic. This species is monogamous and males exhibit orange or red in their crowns and elsewhere in their plumage. The extent and brightness of the color in individuals is directly related to carotenoid pigments that are picked up from high quality seed. Extensive field studies have shown that artificially brightened males were much preferred by females and that naturally brighter males were better at providing food to the female and her chicks. Not all plumage colors derive from diet, however. Blues and greens consist of structural pigments that are manufactured by the birds themselves. They, too, may provide good indicators of a bird's health and abilities, but this has not yet been clearly demonstrated.

    Researchers realized only quite recently that birds see a much wider range of color than people do. They even have colors in their plumage that are invisible to the human eye. Birds have four color cones in their eyes (compared to three in humans), which allow them to see the ultraviolet part of the color spectrum. Scientists using spectroradiometers to measure the extent of ultraviolet coloration have found that males in many apparently monochromatic species (those with similarly colored sexes, such as European starlings) in fact sport bright ultraviolet colors that females use extensively in their choice of mate.

    Males are usually the most colorful sex because females are more likely to be in short supply due to the extra work involved in incubation and chick rearing. Males must thus compete for the chance to mate with them. In an interesting twist, a handful of species are known to have reversed sex roles in which males incubate the eggs and females defend territories and fight amongst themselves for access to the males. These species provide the exceptions that prove the rule, because they demonstrate that the competitive sex is the one most likely to have bright colors. Phalaropes, sandpipers and button quail are good examples of species in which the females are more colorful.

    My recent study of eclectus parrots showed for the first time that bright colors can evolve in both sexes simultaneously. In this species the bright green males and red-and-blue females look so different that they were originally thought to be separate species. Our eight-year study in northern Australia, published in the July 22, 2005, issue of Science, demonstrated that the sex roles are not reversed--females incubate eggs and protect the young. The sexes differ where the females do not join the males in foraging for food and instead defend the nest hollow for up to 11 months each year. Each female relies on up to five males to supply all the food required by her and the chicks. Males face a higher predation risk from hawks while they are foraging, and their colors have evolved to blend in with the leafy foliage. Meanwhile, their shiny green stands out and appears very bright to other parrots against the wood at the nest hollow. In addition, the green is laced with ultraviolet pigments, which the parrots can see much better than predatory hawks can. Their colors are therefore a clever compromise between camouflage and showiness. The females, however, are under less predation pressure, and their red and blue appears as a long range signal to other females of their presence at the hollow.


    Why are birds, animals, and fish so much more colorful near the equator than farther away from it?

    The first thing to notice is that feathers and scales can be a wider variety of vibrant colors than hair can. So mammals may have exciting stripes and spots, but the bright colors are going to be on reptiles, birds, and fish.

    The truth is, as far as birds are concerned, we don't have reason to believe that there is a larger percentage of colorful birds in the tropics: https://www.asianscientist.com/2016/11/in-the-lab/bird-plumage-evolution-australia/

    There are just more species in total in the tropics, and so the total number of colorful bird species is greater.

    There are definitely more reptile species in the tropics and subtropics than in more poleward regions. I've seen no study comparing the percentage of them that are colorful, but the same general notion would apply either way---there are more species in total to be colorful. Though there certainly are some colorful species in temperate regions---like the Lacertids in Europe, which can be vibrant greens and blues, the Garter snakes and smooth green snakes of the USA, etc.

    I think you're primarily thinking of near-surface coral reefs.

    It's really just piggybacking the coral reefs. There happen to be these organisms which thrive in very low-nutrient, clear, shallow, warm water----creating diverse ecosystems. And within those diverse ecosystems, there will be a diversity of species all evolving bright colors for signaling things, because the bright colors can be seen in the clear, shallow, brightly lit water.

    But there are other aquatic environments in the tropics----more sediment-laden waters, murky rivers, mangroves, etc. and there are also highly oligotrophic (clear, low-nutrient) waters in temperate regions where you may find colorful fish like the tangerine darter.

    So is life in the tropics actually more colorful? I don't see that as an obvious conclusion. But the sheer total diversity of species in the tropics certainly allows for more possible expression in general.


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