Eagles. They get freaking huge in South America.
Not a mammal, or even a vertebrate, but cool as all heck…
For no particular reason. It was random.
At the base of the evolutionary process are genetic variations (mutations and recombinations of DNA), unrelated in any causal way to the expression of the gene(s), i.e. characteristics of the individual carrying it/them. In this sense, the emergence of the genes is random.
Only after the genes are expressed (the blue-hair mammal exists), evolutionary mechanisms act on it in terms of selection / propagation. So it only could have been beneficial for some mammals somewhere, if those mammals had already had the genes required for producing bright blue colouration of their hair. They didn’t. Yet. Randomly. They might in future, they might not.
That’s probability for you. It’s non-deterministic. There is no particular reason.
This is the short answer. In the long answer the actual probabilities play a role - an already existing gene, which can be changed by fewer mutation to produce the effect will bump up the likelyhood of the change happening. It helps if the die produced is not toxic, even better if it has additional benefits. And so on. It all goes into the specification of some mammal somewhere, making it wider or narrower, adjusting the likelihood of the blue-hair mammals presence in the jungle near you. But the gene must be there first.
The largest mammals in the world are blue. Gray/blue, but blue nonetheless. Blue whale photographs, Balaenoptera musculus photos
Except that mammals do have blue pigments, at least to some degree, as evidenced by the facial and genital displays of some primates. What’s to stop some critter from having the same pigment all over that a mandrill has in its butt?
A lot of people have blue eyes too.
Those aren’t blue pigments. As mentioned several times already in this thread, they are produced by refraction and scattering.
- It’s not a pigment.
- As I noted in my first post, the fact that the animal would have to be bald. For the vast majority of mammals, the loss of hair needed to be able to produce a visible blue colouration would be fatal.
And once again, this is the result of refraction/scattering, not a pigment.
OK, I knew that that was the explanation for blue feathers, but I didn’t see that that was the explanation for mandrill butts, too.
Well, that is your explanation, and it may have been what Blake was thinking, but it was not what he said. You have given an explanation of why they do not have green pigment that he did not give.
But anyway, I do not buy it. Organisms produce huge numbers of chemical substances with all sorts of chemical properties. The properties of proteins alone run an enormous gamut. Furthermore, pigment is not a chemical type. It just means a substance that is colored, and all sorts of chemically unrelated substances can be colored in all sorts of different ways. It may be the case that, given current mammalian biochemistry, there is no way that a single mutation (or a small number of mutations that would not otherwise individually be selected against) could give rise to the production of a suitable green pigment, but I do not believe that either you, or Blake, or, indeed, anyone, knows that for a fact. Nobody knows mammalian biochemistry in anything like enough detail to know such a thing. That being so, your explanation, though not a tautology like Blake’s, is still nothing more than speculation, and speculation that is probably, in practice, quite unverifiable.
Anyway, arguments to the effect that something cannot evolve because you can’t get there from here have a pretty bad record in evolutionary theory. That is the sort of argument that creationists use when they say that eyes (or flagella, or whatever) could not have arisen by natural selection because it would have taken too many steps, and the intermediate steps would not be adaptive. The argument, of course, is fallacious, and we now have a fairly good idea about the multiple adaptive stages through which eyes actually did evolve. I am confident that it would take a lot fewer steps to get from organisms with no green pigment to organisms with green pigment. If there was a selective advantage to it, it would almost certainly have happened somewhere. The fact that it has not happened in mammals strongly suggests that it would not confer any selective advantage, and a proper explanation would be one that explained why being green would confer no selective advantage, or even a selective disadvantage, to mammals, given the other facts about their ecology. Several people in this thread have offered speculations along these lines. If anyone cared enough, they could probably be tested by the normal methods of evolutionary biology.
Refraction on the skin? How does that work. It can’t be Rayleigh scattering then, I would think.
It’s due to phase change interference between light bouncing off different parts of a non-random structure of skin. (I was googling yesterday because I was curious about this, I found an article in Yale science? or scientist? or something).
I am human. I ask the OP: How would being green help me?
The point: There are traits that are much more key to my survival than skin/hair/fur color.
The same applies to other mammals.
Nonsense. Novel traits do not originate solely on the basis of potential usefullness; there has to be an actual pathway for them to evolve. There are some traits that simply cannot evolve because of developmental constraints (e.g., we will never see a 6-limbed vertebrate here on Earth, no matter how advantageous it might be). Blue and green pigments in mammals seem to fit into this category. Sure, they exist in other groups, such as marine invertebrates, but that doesn’t mean the pathway is available in tetrapods. Indeed, even those terrestrial animals which do possess green and/or blue colors do so via refraction or reflection rather than pigmentation, which would seem to indicate that the developmental - and thus evolutionary - pathway to evolve such pigments may not even be present in tetrapods.
I don’t know! (Camouflage? I guess it wouldn’t work for mate attraction because most mammals don’t see green.) If it’s useful for snakes and birds and frogs then couldn’t it be useful for mammals too? 
I realize that there has to be an available evolutionary pathway and I gave reasons why I think it probable that there is one (or even several). To put it briefly: there is lots of lots of biochemistry that could be tweaked a little to make substances we don’t make now, and lots and lots of ways to make colored substances. It may, conceivably be the case that there is no way to evolve a biochemical pathway that would produce any green pigment without going through some seriously maladaptive stages, but it is effectively impossible to prove that that actually is the case.
What we are missing is any suggestion of a reason why there should not be an evolutionary pathway to the making of green pigment in mammals (and I am not holding my breath waiting for one). In the absence of any hint of such a reason it seems more reasonable that best explanation of their actual lack of such pigment will be in terms of its lack of adaptiveness.
This is quite unlike the case of 6 limbed vertebrates, where we do have a plausible account of why you can’t get there form here.
Did you read the rest of the thread, where your question was answered fairly thoroughly?
It’s not a matter of “should” or “should not” - it’s a matter of what is or is not. Very few animals (if any) have evolved actual green or blue pigments; the vast majority of animals possessing those colors produce them via structural means. This would imply that evolving those pigments is a difficult, if not impossible, process, even when the coloration may be adaptive. One would think, for example, that it would have been “easier” for at least certain frogs to evolve green pigments, rather than the complex system of tri-layered xanthophores, iridophores and melanin to achieve the same effect.
Since we lack many good examples of green or blue pigments for animals, it’s difficult to say how easy or difficult, in theory, it would be to evolve them from existing pigments, or their precursors. Therefore, it’s not really possible to answer why a viable evolutionary pathway to them may not exist.