|MadSci Network: General Biology|
I am so glad you asked this question because it had never even occurred to me that green mammals don't exist. Mammals produce two types of pigment for colouration: eumelanins that produce a brown-black colouration; and phaeomelanins that produce yellow-red colouration. I do not think it is possible to produce green with a combination of these two pigments. Incidentally, most birds do not produce a green pigment either. They produce green feathers by combining yellow pigments with refracted light. It is through the same method of light refraction that most birds create blue feathers without actually producing blue pigment.
You may then well ask why mammals over time would not develop a green pigment to help them camouflage themselves (In nearly all diurnal animals camouflage is an important issue). The answer is because they do not need to. Most mammals, especially predators, have limited colour vision. Mammals are generally large and are better camouflaged by taking advantage of the differences in light intensity and the varied background colours that exist in our natural world. A zebra is a good example of this. If you were able to look at zebras in the same way that lions see them, then you would soon appreciate that, as far as cats are concerned, zebras are not particularly easy to see. Many mammals have lighter undersides and blotches or patches of colour that help to break up their form when viewed against the background of vegetation.
In other animals, green may be used for camouflage such as in some lizards and frogs. It is their lifestyle and their size which allows them to do this. These animals are small enough and remain still for long enough to blend in.
In birds, green may be used for identification and showing off to the opposite sex. They can do this because birds have great colour vision which in some species is superior to human colour perception. Camouflage in these animals may not be an issue. Examples that come to mind include the parrots and the Resplendent Quetzal (claimed by some to be the most beautiful bird in the world).
Thank you for posting this interesting question on the madsci network.
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