Why are mammals limited in color compared to other animals?

[grisly]

How come mammals are so "drab", compared to what is typically seen in the other major animal taxa?

In a bit more detail, what I mean is this: The feathers of birds, the skins of amphibians, the scales of reptiles and fish, the carapaces of insects and spiders, all of those seem to fundamentally have access to more or less the whole colour gamut, both in hue and in saturation. The fur of mammals, by contrast, seems to be confined to a relatively small region of the spectrum ("redwards" of yellow?) and to saturations from moderate to low. To put it another way, if you give a painter a palette with nothing more than black and white and a medium red and a medium yellow, they'd still be able to mix all the tints, tones, and shades they need to paint any mammal they can think of.

Discussions of both the physiological and the evolutionary circumstances (if any) explaining this would be very much appreciated! :)

 

[256bits]

For birds,
the feather color is due to several factors, including pigmentation and the behavior of light upon a surface.
In many cases, males usually display more prominent colors than the females.
Females may need to be less noticeable if she is the only one nesting the eggs.
See Coloration under the wiki
https://en.wikipedia.org/wiki/Feather

Skin color for other animals can also be attributed to blending into the background.

 

[grisly]

In many cases, males usually display more prominent colors than the females.

That did occur to me, but why would mammals be any different?

As far as I know, mating display is the main intraspecific evolutionary reason for extravagant colouration. The main interspecific evolutionary reason is as a warning sign, usually in the context of poison. And there are no poisonous mammals, so that one does signify, where the difference in question is concerned.

By itself, it seems insufficient somehow, though.

 

[Ygggdrasil]

Part of the answer may have to do with the fact that most mammals tend to have only dichromatic vision (red-green colorblindness). Primates are the exception in having trichromatic vision. Of course, this doesn't explain the evolutionary reasons mammals have not evolved better color vision or more colorful pigmentation.

 

[grisly]

Oh, good point. I did not make that connection.

IIRC; the sub-par colour vision is believed to be the byproduct of improved low-light vision, which was of particular import for the earliest (nocturnal insectivore) mammals. Is that about right?

If so, it actually may go quite some way to answering the question. The reason they were nocturnal would presumably have been that they had to conceal themselves from predators. For that, muted colours are generally best (with the exception of particular species which spend much of their lives in a setting that is itself brightly coloured, such as leaf-green insects or birds with sky-blue bellies). With both of those adaptations in place, the "return" to a state of both a more colourful appearance and the ability to appreciate it becomes a less trivial proposition.

 

[BillTre]

there are no poisonous mammals

The platypus, although drab, is a poisonous mammal (fang-like spur on its back leg).

Compared with birds, many mammals are way more scent based (chemical sensing) in the information they get about the world. This could downgrade the relative importance of vision for many mammals. That combined with an evolutionary history of nocturnality and the advantages of camouflage could probably explain a lot.

Birds (dinosaur subgroup) and primates (mammal subgroup) both have color vision. They both have a long history in an extended 3D environment where accurate and informative vision would be highly adaptive, since failures could lead to death.

Birds have feathers which can produce iridescence based colors from interference patterns based on the structure of their feathers (alluded to be @256bits). I don't think mammals can do that (maybe hollow polar bear hairs?). But there are artificial enhancements (which seem to be either mating or maybe status signals).

Another thing birds can do is turn their displays on and off by doing things like spreading their wings or fanning out their feathers or bunching them together on top of each other. This can make flashy displays less of an adaptive downer when it comes to survival.

Baboons come to mind as being rather colorful at times (but only at the ends (head and butt)), probably for mating purposes or maybe as a group status display.

 

[grisly]

The platypus, although drab, is a poisonous mammal (fang-like spur on its back leg).

Thanks for pointing that out! I think I may have heard about this previously, but I'd completely forgotten. Even better, the wikipedia article has a link to a more general one on venomous mammals, which further include the vampire bat (including anticoagulants under the umbrella term "venom"), the common mole (thus able to incapacitate worms without killing them, for the purposes of building up a food stash - another bit of information I think I may already have come across at some point but didn't retain), and some shrews and similar small insectivores.

More startlingly, there's this: "[Some species of loris] are considered indirectly or secondarily venomous, because their venom is produced in the brachial gland on the inside of the elbows, licked, and injected into the victim by a specialized tooth comb. A protein in the secretion, which is similar to the allergen protein isolated from the domestic cat, may be introduced by the bites of these lorises, resulting in anaphylaxis."

All that being said, I think it doesn't substantially counter my earlier argument, because the prominent colouration seen as a warning sign in non-mammals usually means "don't bite me, I'm poisonous" rather than "don't make me bite you, I'm venomous".

Another thing birds can do is turn their displays on and off by doing things like spreading their wings or fanning out their feathers or bunching them together on top of each other. This can make flashy displays less of an adaptive downer when it comes to survival.

True. But, again, no reason this technique couldn't by utilized by hypothetical brightly-coloured mammals just the same.

Baboons come to mind as being rather colorful at times (but only at the ends (head and butt)), probably for mating purposes or maybe as a group status display.

Ohhhh, yes, another exception I'd not considered. And aren't there other monkeys which employ even more lurid colours? ... Yes, there are indeed, a wikipedia search for "monkeys with colourful faces" pointed me straight to the article on mandrills, which describes them as having "an olive green or dark grey pelage with yellow and black bands and a white belly", "[a] hairless face [...] with distinctive characteristics, such as a red stripe down the middle and protruding blue ridges on the sides[,] red nostrils and lips, a yellow beard and white tufts", "areas around the genitals and the anus [which] are multi-colored, being red, pink, blue, scarlet, and purple", and "pale pink ischial callosities". "Drab" indeed, heh.

That find prompted me to run a second search for simply "colorful mammal", but that one failed to produce any more such outliers.

So, it looks like we keep coming back to primates, which, I'd say, strongly supports the notion that the prevailing drabness outside of that taxon may indeed be closely linked to the lack of full colour vision.

That answers my question more fully than I'd really hoped, on that front - but don't let that stop you from contributing additional information, needless to say. :)

---

Now, as for the physiological angle, anyone know anything about how Mandrills are able to put on such a spectacular display?

It sounds to me as though the really unusual colours are those of areas of naked skin, though the "olive-green pelage" by itself invalidates (or at least somewhat extends) my original "redwards of yellow" characterization.

If I ever thought about it at all, I suppose I took the red buttocks of baboons to be simply the result of a more pronounced form of the sort of reddening of skin that occurs in humans as well, as the result of blood(flow) closely beneath the skin, or irritation of that skin, or some such. But it seems quite clear to me that that's insufficient in this case. I suppose blood by itself can produce quite a range of colours, ultimately (think aging bruises), but many of the tones seen in the photos in the article seem to me to be far too saturated to be accounted for that way. So it has to be "genuine" skin pigmentation, I suppose? And if so, are those pigments something that other mammals could quite easily produce as well, had they a reason to, or is that ability something mandrills had to "re-invent" from scratch?

 

[grisly]

Birds (dinosaur subgroup) and primates (mammal subgroup) both have color vision. They both have a long history in an extended 3D environment where accurate and informative vision would be highly adaptive, since failures could lead to death.

Just to clarify: By "extended 3D environment", do you mean that they're fliers and climbers rather than full-time ground-dwellers?

 

[BillTre]

Just to clarify: By "extended 3D environment", do you mean that they're fliers and climbers rather than full-time ground-dwellers?

Yes. Extended, involving depth perception, which is probably benefited by having color vision for greater contrast.