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In most biomes on earth, and certainly in the most densely-populated, the best camouflage color or pattern for an animal would be a variant of green, so as to blend in with the leaves and grasses of the surrounding environment. Whether predator or prey, the art of not being seen is hugely important in the natural world, and yet to the best of my knowledge there isn't a single mammal on the planet with fur that is even partially green.

Is there a reason for that? It seems like animals have evolved some pretty crazy color patterns to break up their silhouette or blend into the background, but why haven't any mammals taken the obvious route of "look like leaves"?

Is it really THAT not-easy being green?

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    $\begingroup$ I don't know if this is related, but I've done enough computer graphics programming to know that brown is hard color to reproduce on a computer. It's a mixture of red and green. If animals don't have good color vision, they might not be able to tell the difference between red and green, and it all looks brown anyway, kind of like colorblind men. However, I don't know how many animals have this issue, I think lack of blue vision is more common than lack or red or green. $\endgroup$ – user137 Sep 22 '14 at 19:02
  • $\begingroup$ i wonder if its because small mammals started out as terrestrial - bound to the ground where green is not as much a color. certainly birds can be green when they need to be. $\endgroup$ – shigeta Sep 22 '14 at 22:23
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    $\begingroup$ But we clearly need it for the Green-beard effect $\endgroup$ – fileunderwater Dec 9 '14 at 23:24
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The technical answer is: Because the coloration of skin and hair is done by the two forms of melanin: Eumelanin, which is dark brown to black and Pheomelanin which is yellow to red. This enables colors from white (not pigmentation) to black (dense eumelanin pigmentation) and also colors in between by different ratios of the two pigments.

The evolutionary answer is that it obviously made no evolutionary advantage to have other colors. If you look closer at this, you will see that most habitats are not green or only for a part of the year. If you think about african plains, these are mostly brown, something like artic landscapes are only green for a very short timeframe in the summer, for the rest of the year these are white to brown. And even tropical rainforests are mostly dark to brown on the ground. They would clearly stick out of the environment with green colored hair.

For all these different habitats the animals living there have adapted their hair color. It is not uncommon for artic animals to change their hair color with the course of the year, dark to brown in the summer and white in the winter (artic foxes and snow hare) for example. Animals which live in deserts will have a paler color than those which live in dark colored mountain areas.

See this article for more information: "The Adaptive Significance of Coloration in Mammals"

Also interesting in this context is this article which looks at the reason for zebra stripes: "The function of zebra stripes"

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  • $\begingroup$ Whoever downvoted me, can you please explain why? If there is criticism on my answer, it will be helpful to know about it, so I can improve the answer. $\endgroup$ – Chris Sep 22 '14 at 9:42
  • $\begingroup$ +1, I am not a biologist, but I would also expect a kind of an evolutionary baggage argument. Reptiles tend to be in similar habitats and grow to the same sizes in comparison to plants as many mammals, yet they developed chromatophores and mammals just melanocytes. It would also be interesting to understand bio/chemically why melanogenesis produces such a limited variety of colors. $\endgroup$ – Void Sep 22 '14 at 20:59
  • $\begingroup$ I am not much into chromatophores, I know this only for zebrafish. There three different pigments are available, dark melanophores, blue xantophores and silvery iridophores. It doesn't happen there, but it is imaginable to combine blue and yellow (pheomelanin) to get green. Still it didn't happen evolutionary. $\endgroup$ – Chris Sep 22 '14 at 21:34
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Just to add a different dimension to the answer from @Chris. Not all animal colouration is produced by melanin. A whole range of bright colours in insects, birds and reptiles comes under the heading of structural colouration, which basically involves having a repeating structure at the microscopic level to interact with light. This is the basis for macaw colours, peacock feathers, and some butterfly wing colours.

It also occurs in mammals - see for example the mandrill. In this case the coloured patches are bare skin containing ordered collagen fibres to create the effect, (science here). I have no idea if collagen could be structured to produce a green colour.

Finally, an anecdote: a number of years ago a polar bear in the San Diego zoo was seen to develop green patches. Upon investigation this was found to be due to damaged hair shafts in areas where the animal rubbed against surfaces in its enclosure. In the polar bear the hair shafts are hollow, for insulation, and the damaged hairs were being colonised by algae. So you could imagine a green mammal could evolve by using this effect.

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    $\begingroup$ That algae colonize the sloth happens quite regulary. But I would see this under the pigmentory aspect. See here for details. $\endgroup$ – Chris Sep 22 '14 at 16:04
  • $\begingroup$ I would be surprised if collagen structures could make red and blue but not green, as green is between the two in wavelength. I had no idea mammals did structural colouration. $\endgroup$ – Resonating Sep 23 '14 at 0:11
  • $\begingroup$ Polar bears have hollow hair shafts? Forget alpaca and mink, I want to knit polar bear yarn! $\endgroup$ – rumtscho Sep 23 '14 at 10:17
  • $\begingroup$ Tropical tree-dwelling sloths often have green fur, the pigments due to possibly symbiotic algae; see this. $\endgroup$ – mgkrebbs Oct 18 '16 at 20:15
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A dimension not explored by the other (excellent) answers has to do with color perception under trees. Leaves are green while on the tree, which tends to make mostly green light available to the understory.

Viewed under green light, a green-furred animal would appear bright green, roughly the same as a white creature viewed under green light. A red or brown creature would appear grey-brown or black, and reflect much less light.

Orangutans apparently do this for camouflage, according to this book I haven't read:

Russon, Anne E. 2000. Orangutans: wizards of the rain forest. Firefly Books, Buffalo, New York, USA.

which I was told about by these guys. It's not the first place I've heard this factoid, but it does make first-pass sense.

There is a difference between looking the same as a background of green leaves under sunlight and looking the same as a background of tree trunks or leaf litter under a greenish light.

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  • $\begingroup$ Very good point. $\endgroup$ – Chris Sep 23 '14 at 6:19

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