Here’s a green polar bear for ya. (All right, so like the sloths, he’s cheating with algae. Still. Green bear.)
That doesn’t sound plausible to me. If you look at a general camouflage uniform, it’s small patches, not a uniform shade. Broad shapes of one colour get you noticed sooner, not small irregular patches. So that would work in the animals favour.
Except for those mammals where sexual preference for a colour rates higher than hiding.
I still would like to know how it would be biochemically possibly - that even birds cheat with light refraction, and that monkeys (if I understand the earlier posts correctly) aren’t really blue, but naked means that blue and green are almost impossible to produce with proteins. But why? Leaving aside the question of why it hasn’t happened yet, if some mad scientist wants to switch genes, or splice something into the human genome (like that terrible new Hulk movie, where at the beginning Banner puts in biolumincence as marker to “explain” why the Hulk is green), is there any way he could get it?
How about incorporating a certain mineral or other element by food that in conjunction with an enzyme lets hair or skin look green or blue? We already have slightly blue blood in human veins, copper-based green blood in vulcans, and real blue blood in snails.
I wonder something similar to that, why couldn’t chlorophyll be absorbed from plants and distributed to the skin or hair. IIRC don’t flamingos do something similar with the red pigments from the creatures they eat so unless it has to do with feather structure…
How about thiscritter?
Flamingos do get their pink coloration from the carotenoids in their diets (from algae and brine shrimp). If they don’t get them, they are white. Zoo flamingos are sometimes fed supplements to keep them brilliant pink for the visitors. The same supplement is sometimes fed to farmed salmon so that they are a nice pink color when they become filets.
I don’t know how cholorophyll is absorbed or broken down once ingested.
Yes, which is why I said it would be less important for NON-camouflage purposes such as sexual signaling. Green isn’t necessary if reddish brown will do the job.
A parasite which causes 100% fatalities goes extinct along with its host. It’s beneficial to the parasite to evolve a modus vivendi where it can live off its host while letting the host survive in a competitive state. Taken to an extreme, it would not be totally incorrect to say that parasites evolve towards being symbiotes, where having the ‘parasite’ is actually a pro-survival trait because the ‘parasite’ is actually a symbiote that positively contributes to the host’s well-being.
Honestly, the value of green for camouflage is highly overestimated. Just look at snakes, which clearly are capable of green colors. And yet, most snakes still tend toward black, brown and gray. This makes a lot of sense for snakes as well as mammals.
First, you have the problem of seasons. There are some places with year-round green leaves, but many forests and every grassland are green only part of the year.
Even in evergreen forests or rainforests, you have to think about what the animals are in contact with. Those that live in the trees are too heavy to hold onto the green leaves; they get around by holding onto brown trunks and branches. Those that live in the ground also do not spend much time on green plants. Instead, they move around between the plants on ground that is mostly brown leaves and dirt.
So Kermit was correct all along.
A monkey infected with Sakutia may be green, but such cases are very rare. 
That is no answer at all. It amounts to saying that mammals have not evolved green hair because they have not evolved green hair.
Green monkey disease, by contrast, has spread beyond the DC Comicverse.
there were the Blue People of Kentucky (Appalachia) that were affected with Methemoglobinemia.
I used to work with a guy named Fugate before I learned about the disease. To the best of my knowledge, his complexion was like a pair of well-bleached blue-jeans.
Lots of interesting posts here I’m enjoying reading. I should have clarified in my original post I was really thinking about green or blue fur/hair, not skin. Not sure why, it just seems different somehow.
I was kind of unconvinced by Cecil’s column that a couple of people linked to, but a lot of the other posts are really helpful – thanks!.
I don’t entirely see why we can’t answer “why not” about evolution when we are looking at something that happened to all other kinds of animals. But perhaps it would be more useful to break each of those groups down into smaller groups (orders, families etc) as it’s not as though all birds are blue or all fish are green.
I think you need to go back and rad my post again.
Because evolution isn’t directed. It’s ultimately based on a random process. The only real answer to “why didn’t this obviously better strategy evolve?” is just that it never arose through the process of random mutation. You can dissect why it might be difficult for it to arise, but ultimately, that’s what it reduces to.
If you believe in natural selection as an essentially random process of reproduction, mutation and survival, then asking “Why not?” is about as useful as a craps player asking “Why haven’t I rolled a 7 yet?” Investigating the situation might show that the dice are loaded, that the player is cheating or that some other factor does explain why not. But most of the time, you can’t do anything better than “Because it just hasn’t happened yet.”
I did, and it is still tautologous. I think you need to think about what you wrote a bit harder. To say that mammals cannot produce a pigment that will color their hair green is just to say they have not evolved green hair. It is no explanation of why they have not evolved the ability to produce such a pigment (and, consequently, have such hair). If they had evolved green hair, they would have evolved such a pigment. All the stuff about skin and diffraction patterns is irrelevant to this purely logical point.
Evolution works by re-adapting existing organs, tissues, biochemicals, etc. A forelimb can adapt as a leg for quadrupedal walking and running, an arm and hand for arboreal locomotion or manipulation in bipeds, a wing for flight, a flipper for aquatic propulson, etc. The same muscles a child uses to wrinkle up its nose are greatly expanded to produce prehensileness in an elephant’s or tapir’s trunk. The air-filled swim bladder used for buoyancy and balance by primitive fish turns, on the one hand, into the oil-reservoir swim bladder of advanced fish, and into the lungs of lungfish and tetrapods. Carotene has multiple functions in various creatures, depending on their needs (pigmentation, nutrient storage, building block for vitamin synthesis, etc.)
What is being said, I believe, is that there is nothing that is or can be adapted to a green or blue pigment in either hair follicles or skin. It’s not just the tautological “there’s no pigment because there’s no pigment” that you are reading it as – no matter how useful grass- or leaf-green coloration might be to a small plains- or tree-living herbivore, there is nothing that can be easily adapted to provide the proper coloration. Most pigments have other purposes – oxygen transport for hemoglobin, the uses noted above for carotene, photosynthesis for chlorophyll, etc. – and their use for coloration becomes important only as an advantage when they are present for their other physiological purpose.
So, yeah, it’s true that “there’s no green or blue pigment present because there’s no green or blue pigment present” – but the point is that there’s no other physiological reason for them to be present, so that they can be adapted for protecvtive coloration. Carotene is useful for food storage – so orange/red/brown coloration can be easily done by concentrating the carotene present. There’s no substance which could function/be adapted as a green or blue pigment similarly present.