Plumage. An incredible world, for an incredible phenomenon. Say it with me now: plumage. Picture the colours, their variety and richness. Picture, while you’re at it, some other stuff relevant to this week’s Giz Asks, such as bugs that look shaped from stained glass and sea creatures that look like they’ve been doused in neon paint.
Now picture a person. Any person will do. Pathetic, right? We — and, really, mammals generally — just do not stack up. Below, read along as a panel of experts explain why.
Assistant Professor, Biological Science, University of Tulsa
To understand why brilliantly coloured mammals are uncommon, we must consider the world in which mammals first evolved. This was a world dominated by dinosaurs that were most likely active during the day and had excellent colour vision, much like modern birds. Many of these dinosaurs were predators and a major threat to the relatively small and defenseless early mammals. In response, it appears that early mammals evolved inconspicuous coloration, and nocturnal and underground ways of life. In these dark environments, their senses of smell and hearing took priority. In the dark, colour vision is of little use and early mammals lost much of their colour vision.
This “nocturnal bottleneck” is apparent in the visual capabilities of mammals today. Most living mammal species perceive colour with just two types of photoreceptors tuned to sense either the blue or the red portion of the light spectrum. Therefore, most mammals have limited ability to discriminate colour, and many hyper-vivid hues would not be discerned by them. In contrast, bird colour vision involves four different photoreceptor types sensitive to ultraviolet, blue, green, or red portions of the spectrum allowing them much finer discrimination of the colours they use to attract and intimidate one another. Interestingly, some primates, like humans, have regained more complex colour vision capabilities and among these species vividly coloured skin patches have evolved, such as the colourful faces of mandrills.
The “nocturnal bottleneck” may have also limited the palette of pigments available to mammals. Whereas the bodies of birds, insects, and reptiles are coloured with a range of pigments, mammal hair and fur is coloured with just one class of pigment molecules, melanins, which produce a limited variety of colours ranging from black to brown and auburn. While other animals employ melanin in complex optical nanostructures to create iridescence in their feathers and scales, it is exceedingly rare in mammals. Like complex colour vision, it is likely that these mechanisms of vivid-colour production were lost as our distant ancestors lived their secretive nocturnal lives in the shadow of the dinosaurs.
Professor, Ornithology, Ecology, and Evolutionary Biology, Yale University
This relates to what we call sensory ecology: how the sensory system is used in the lives of different organisms.
Colour is really about communication — all these beautiful, brilliant colours are really communication devices. It’s about what animals prefer, what they like, what’s memorable. A prerequisite for this sense, which I’d argue is aesthetic, is cognitive ability, and the ability to choose — to make social choices, or sexual choices. Who you mate with, who you hang out with, etc. All these things are in play when you’re talking about why a certain group has the colours it does.
These groups will use what they have available to make colour, which means pigments, i.e. molecules that differentially absorb visible light, like paint or dye; or structural colours, which are made by optical interactions of light with the material of an object (so: blue skies, rainbows, etc.). So you’ve got, on the one side, the biology of cognition and social behaviour, and on the other you’ve got what a given group has to work with materially to make colour.
Birds are tetrachromatic — they can just blow us away in terms of colour vision. They can see colours we can’t even imagine, like ultraviolet yellow or ultraviolet green. Plus, they’ve got rich social lives — they hang out, they fly around, and most of them are engaged in mate choice. They’ve also got a certain freedom of choice. So what do birds do with that freedom? They choose beautiful things — things that are subjectively desirable. And that evolves, and leads to what I call aesthetic radiation (as distinct from adaptive radiation, which is just variations in phenotype that function in a material way to do something new).
So that’s half the question: it explains why birds are so colourful to begin with. But what’s going on in mammals?
Well, they’ve got the cognitive capacity, but what they don’t have are the other things — colour-vision and, frankly, opportunity for choice. The ancestor of placental mammals was walking around in the dark for like 100 million years trying to keep from being eaten by dinosaurs. During that time, they lost the capacity for complex colour-vision. And if you can’t see a colour, you’re not going to evolve social signals that are colourful, period. Also, most mammals are still going around at night: bats and rodents comprise the majority of mammals, and they’re both nocturnal.
That said, one lineage of placental mammals did evolve colour-vision: old-world primates. They don’t match up to your average lizard or bird or even goldfish, but they do have a lot of colour: the face of a mandrill, for instance, is blue and African macaques have vividly blue scrotums.
Professor and Curator of Birds at Auburn University
Humans are atypical mammals. Most mammals are nocturnal — most mammals are rodents, who live in burrows. When you deal with your dog, you kind of get the impression that they perceive the world differently than we do — they want to smell you first, which is typical of most mammals, whose world is less vision-based and more smell-based. Most mammals don’t even have full colour vision like humans do. We have three-dimensional vision (which is why you put three colours in a printer). Dogs, cows, and most other mammals only have two-dimensional vision — they don’t see colours like we do. What all of this means is that they don’t display very often with colour. Where we do see some colours are in primates; apes like mandrills are among the most colourful animals. So that’s basically it: mammals are more prone to smell, and humans and birds are more prone to look.
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.
Director for Undergraduate Research in Life Sciences and Co-Director of Animal Behaviour PhD Program at Arizona State University
There are in fact some very colourful mammals! Golden lion tamarins and red pandas have bold red-orange pelage, Honduran white bats have vivid yellow facial skin, and several primates, such as mandrills, have quite ornate red and blue skin colours on their face. Moreover, a handful of mammals like skunks and zebras have bold coat patterns (i.e. contrasting black and white). That said, the vast majority of mammals do in fact have a drab appearance, consisting mostly of shades and earth tones. This is largely thought to be a product of their use in camouflage and disruptive coloration, and also linked to the history of mammals having poor colour vision (i.e. constraining their use of diverse colour signals).
Professor, Evolution, Ecology, and Behaviour, University of Illinois
There ARE at least a few brightly coloured and distinctly patterned mammals. These vary from the blue-red-yellow facial and anal regions of male mandrills to the best known black-and-white stripes of zebras, but also the deep orange pelage of the golden lion tamarins and the algal-green fur of long-lived sloths.
Body parts of mammals can also be bright focally, especially testicles (blue in vervet monkeys, white in howler monkeys, red in Japanese macaques), and, of course, the eyes, varying from light blue to deep brown in humans and huskies, and orange-reds in slender loris.
Finally, we shouldn’t forget the aquatic mammals: the conspicuously white-and-black patterned orcas and the bright pink river dolphins of the Amazon.
Professor, Anthropology, Penn State
and George Chaplin
Senior Research Associate, Anthropology, Penn State
First of all, there are some mammals with vivid coat colours, specifically vivid yellow and orange coats. Think of the golden lion tamarins of Brazil and the golden langurs of Bangladesh, for instance.
Second, there are mammals who have vivid-coloured skin. Male mandrills and grivets (both African monkeys) and golden monkeys from China have bright blue skin on their noses and backsides. These blue colours are optical colours, not produced by pigment, but by highly oriented bundles of collagen in the skin which scatter visible light. This process, called Rayleigh scattering, is the same process which makes the sky appear blue. The same animals also have bright red skin on their backsides (and mandrills on their noses, too) produced by hemoglobin in the superficial blood vessels. Hemoglobin also makes the estrous swellings of many female primates appear bright red.
Back to coat colours: Basically, mammalian hair gets its colour from two melanin pigments, the yellow-red pheomelanin and the intensely dark brown eumelanin. The melanin producing cells in mammalian hair follicles produce different mixtures of these pigments at different times to produce coats of many different colours. This makes it possible for mammals to produce anything from bright yellow to near black, as well as the absence of pigment — white, and almost every conceivable shade of grey, brown, reddish-brown, and greeny brown, in between. In contrast to birds, mammals lack the ability to produce coloured hairs containing pigments called carotenoids. This is why you don’t see mammals coloured like cardinals or scarlet tanagers. But, interestingly, the blue colours of bird feathers are also produced by optical effects, produced by light impinging on oriented collagen bundles.
Professor, Ecology and Evolutionary Biology, UCLA
I think that can largely be explained in terms of the opportunity that animals have to use colour signals in communication. Most colourful vertebrates are diurnal (active during the day) and have excellent colour vision, while most mammals are nocturnal or crepuscular, lack colour vision, and rely more on scent and sound for communication. Primates are an exception that supports the hypothesis — most primates are diurnal and have colour vision, and some are quite colourful. Male mandrills are probably the best example.
Not all colourful invertebrates have colour vision, but the ones that do not are probably signalling to potential predators that do have colour vision, rather than to each other (e.g., warning coloration of bees and wasps). However, the conventional wisdom that insects do not see in colour is wrong. Many do, including butterflies and dragonflies.
Professor Emeritus, Wildlife, Fish and Conservation Biology, UC Davis, whose research focuses on animal coloration, among other things
Compared to birds or insects, mammals show a paucity of sexually selected coloration. In most species of mammal, female and male coloration does not differ (called sexual dichromatism); neither sex changes colour at time of breeding; and neither sex is particularly flamboyant or gaudy in appearance.
Sexual selection consists of two processes: intersexual selection, most commonly between males, over access to females or resources on which females depend, and intrasexual selection, usually by females choosing males with whom to mate. Both processes can drive flamboyant coloration in (usually) males.
There are factors relating to sexual selection predisposing mammals to be drab. In contrast to birds, most species of mammals are polygynous, with males employing coercion to corral females, which limits female choice of gaudy males. Most female mammals are philopatric (they don’t disperse before breeding), limiting females’ ability to choose mates. Mammals are limited in their movement compared to birds, restricting females’ ability to sample males for extra-pair copulations. In addition, male mammals rarely care for offspring, excluding one of the factors on which female birds base their mate choice. So, a priori, we might expect rather little flamboyant ornamentation in male mammals.
Some solutions: One possibility is little opportunity for sexual selection as presented above. An alternative argument is that the visual system of most mammals is dichromatic, whereas birds are tri- or tetrachromatic, so there is less necessity to rely on visual as opposed to olfactory or auditory sex differences. Another related explanation, again untested, is that most mammals are nocturnal but most birds are diurnal. At present these possibilities are difficult to tease apart; moreover more than one may be relevant.
EPSRC Innovation Fellow & Proleptic Lecturer in Animal Sensing and Biometrics at the University of Bristol
The colours of mammalian hair are primarily determined by the presence of two natural pigments: eumelanin and pheomelanin. Eumelanin is responsible for black and brown colours, while pheomelanin can create yellow and reddish hair. This means that, for mammals, it is biochemically impossible to manufacture vivid colours like bright green or deep blue using hair. The orange coat of a tiger or an infant langur is as far as it goes.
Vivid colours often evolved through sexual signalling in order to impress the opposite sex; the bright feathers of a peacock would be one of the most famous examples. However, most mammals are actually red-green colour blind, meaning they cannot tell apart red and green. This is true to almost all mammals, except us, Old World monkeys and some New World monkeys. Therefore bright vivid colours would not be much use to find mates.
Another strong selective force is camouflage, however brown and grey coats seem to work at most places. Many mammals can turn white during winter and remain hidden in the snow. Often prey is colour blind, too: While an orange-brown tiger might be easy to spot against green vegetation by humans, a deer (the animal a tiger is really trying to hide from, not us) will struggle to tell apart the orange coat from green leaves for the reasons outlined above.
Professor, Functional Zoology, Lund University
There are a few exceptions, like an African monkey, of which Darwin already stated, “no other member in the whole class of mammals is coloured in so extraordinary a manner as the adult male mandrill’s”. With their red and blue faces, this exception brings me to the first two answers to the question.
Brilliant colours are often used for communication with conspecifics, and most mammals use chemical and acoustic signals for purposes such as finding the best mate or marking a territory rather than visual signals. So there is less need of bright colours as signals among mammals. So why is the mandrill different?
A main reason is that old-world monkeys (and howler monkeys) differ in their visual system from other mammals in a way that makes them more similar to birds. Birds (as lizards, crocodiles and many fish) have highly sophisticated colour vision based on four visual pigments allowing them to see more shades of colour than us humans. Most mammals only have two pigments, and that reduces their colour world dramatically, so even if they were coloured, they would not appreciate it. The old world monkeys (the branch including us humans) had a duplication of one of the pigment genes and mutations, which together gave us three pigments, and a much more colourful world — even though not as colourful as that of birds, as we are missing all shades of ultraviolet which many birds can see.
So why do mammals only have two visual pigments? This gets us to the third answer: Unlike birds, early mammals survived the age of dinosaurs as mostly nocturnal and often subterranean animals. Their eyes evolved to see well at night, on the cost of seeing the brilliant colours in bright light, which they rarely saw. If these early mammals ever had brilliant colours, they must have lost them back then, as they had no need for them if they would not be not seen. And most often, production of these colours is costly, so they are easily lost.
Many bugs don’t have brilliant colours to attract mates, but to warn predators who want to eat them. Many bugs are eaten by birds so they are for birds to see and avoid them, either because they taste badly, are poisonous, or mimic other bugs that are.
Back to mammals and why they have their “dull” colours. The most mechanistic answer then is that we mammals mostly use brown, black and red pigments (melanins) and lack both colourful pigments and sophisticated mechanisms to produce the structural colours that underlie many green, blue and violet shades in butterflies, beetles and birds. However, the mandrill shows that this can evolve again when an animal evolved a better sense of colour vision, within less than 30 million years.
Associate Professor, Biology, Ghent University
A few mammals have bright colours, for example the bright red and blue faces and rumps of mandrills. However, such bright skin colour is certainly rare, and I only know of a few examples of somewhat brightly coloured hair, for example the iridescent colours of some golden moles. So why indeed are bright colours so rare in mammals? From a proximate (mechanistic) perspective, they may have fewer ways to produce colour. The only pigment that mammals use is melanin, which can produce a variety of blacks, browns and greys. Thus, they lack the pigments such as carotenoids needed to produce bright reds and yellows. Outside of pure pigmentary colours, birds and insects can produce so-called structural colours, produced by the nanoscale arrangement of materials within their feathers or cuticle. These make some of the brightest colours in nature, such as the flashing feathers of hummingbirds, but are virtually non-existent in mammals. Perhaps the chemistry, morphology, or developmental patterns of hairs preclude them from making these nanostructure. From an evolutionary perspective, mammals evolved as nocturnal animals, and in the shadows of dinosaurs, so perhaps camouflage has been more significant than mate choice in the evolution of their colours.
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