Alexey Veraksa, Ph.D.
assistant professor
University of Massachusetts

Adam

Why is there no mammal with green fur? Green is a good camouflage color, and nonmammals (for example, frogs, snakes, and birds) do come in green. Why not mammals?

Alexey Veraksa
assistant professor,
Biology Department,
University of Massachusetts,
former HHMI predoctoral fellow

I know from my general zoology classes that mammals can make only two kinds of pigment: melanin (black or brown pigment) and the reddish-yellow pigment that red-haired people have. So my first idea for a simple explanation of the nonexistence of green mammals was to say that mammals just can't make green pigment for some reason. This is of course not a good evolutionary explanation, but at least it superficially answers the question.

As I started researching this a bit more, I was astounded to discover that frogs, birds, and others in the tetrapod, or four-legged, world can't make green pigment, either! Or blue, for that matter. It turns out that all color variation that we see in tetrapod animals is the result of different combinations of patterns of deposition and refraction of the same two types of pigments: black and yellow-red. A chameleon's color changes because of rapid shape changes in refractory cells in its skin, not by rapid production or release of an actual pigment. Frogs are green because of the pattern of refraction of blue light by special cells in their skin, which blends with their yellow pigment.

The colors of bird feathers are also generated by light refraction but by a different mechanism. The surface of feathers has microscopic ridges that form ordered tracks, much like the surface of a CD. The spacing of the ridges and the size and orientation of the pigment granules they contain determine the feathers' brilliant greens and blues (see links in reference 2). Tiny air pockets in feathers can add to color variation in birds. Light refraction is also responsible for the color of the human iris, which can range into deep blue or green hues. Another example is the sky-blue nose of the mandrill. For an exploration of the ways animals make different colors, see reference 1.

As you can see, the question is really why mammals don't have the ability to generate a variety of colors in their fur. This is a difficult question, and until we master time travel—to be able to trace back the evolution of mammals—we won't know the answer with certainty. But it's interesting to speculate, so here are some possible explanations.

1. I think that the most important difference is in the lifestyle of mammals. Being warm-blooded animals, they are constantly in search of food and are moving a lot. Even if an animal has the best camouflage, as soon as it starts moving, it is more easily detected. In contrast, many amphibians and reptiles can spend a long time being motionless, and here looking the same as a leaf nearby helps more. I believe that during evolution, mammals have "invested" more into working out active escape mechanisms (for example, running away and hiding in burrows) rather than into developing passive camouflage techniques.

2. Scientists believe that the first mammals were relatively small, ratlike creatures that ate insects. Their habitats were probably different in terms of background color patterns compared with the habitats of amphibians and reptiles. Mammals, of course, originated on land, while amphibians never leave the source of water. More green is around amphibians, and therefore we find more green amphibians.

In contrast to their early ancestors, many mammals are now large, and for them, a factor of size comes into play in determining coloration. As we know from classical statues, it's difficult to cover a whole mammal with a fig leaf (but it is quite possible for a frog). A solid-green mammal would in fact stand out in real habitats rather than be hidden. For a large animal, being dappled helps more than being green.

3. Most mammals, except whales and dolphins (which in fact can develop bluish green coloration), are covered by some form of fur. Like bird feathers, mammalian hairs are keratinized structures that extend far out from the surface of the skin and can be of different colors because of varying patterns of pigment deposition during hair development. But in contrast with bird feathers, mammals have not developed an ability to produce intricate microstructures in their growing hairs that would reflect light in such a way as to create green (perhaps with the exception of a few species such as the African green monkey, whose coat can approach various shades of yellow to olive green on the back). And forget about changing skin color at will like a chameleon—nobody will notice that under your fur, anyway.

4. Now think about what mammals need to camouflage themselves against (predators) or for (to hunt prey). Most predators are in fact other mammals, and most mammals do not have good color vision, so they don't really care if their prey is green or not. They are much better at distinguishing patterns and differences in light intensity than in colors. That's probably why most mammals have patterns such as spots, stripes, or blotches on their fur, which help them blend in.

5. Why are there green birds? Here, it seems, a different evolutionary scenario was at work. By being able to fly, birds can escape most predators, and it is thought that striking colors in birds have developed for display and mating purposes rather than for camouflage. Accordingly, birds have exceptional color vision, possibly superior to that of humans. If you think about it, though, birds that do live close to the ground and rarely fly (and are therefore within reach of mammalian predators) are frequently earth colored and dappled, striped, or spotted, just like mammals.

6. Curiously, there is in fact a group of mammals that is green—three-toed sloths. This appears to be a secondary evolutionary acquisition and results from the growth of green algae in their fur (they rarely move and apparently never wash themselves). Maybe this also helps them blend into the leaves.

So it seems that a combination of evolutionary forces (and constraints imposed by such mammalian features as high metabolism and fur coat) can explain the lack—or rather, extreme scarcity—of green mammals.

Finally, some scientists were so disappointed by the absence of green mammals that they decided to create their own! A few years ago, a group of researchers in Japan inserted a gene encoding green fluorescent protein (GFP) from jellyfish into the mouse genome to make mice that fluoresce bright green when illuminated with a blue light. The long evolutionary wait for green mammals is now over. For a stunning picture of "three green mice," see reference 3.

References

1. The nature of color in different animal groups, including frogs.

Wallin, M. 2002. Nature's palette: how animals, including humans, produce colours. Bioscience Explained 1 (2),

http://www.bioscience-explained.org/ENvol1_2/index.html

2. Bird coloration.

For an essay on the coloration of birds, go to:

http://www.stanford.edu/group/stanfordbirds/text/essays/Color_of_Birds.html

For information on feathers, including colors, go to:

http://www.whozoo.org/ZooPax/ZPFeather.htm

For an interesting paper describing the discovery of the generation of iridescent color patterns in peacock feathers, see:

Zi, J., et al. 2003. Coloration strategies in peacock feathers. Proceedings of the National Academy of Sciences USA 100 (22): 12576-78. http://www.pubmedcentral.nih.gov/articlerender.fcgi?
tool=pubmed&pubmedid=14557541 .

The findings about peacock feathers are also discussed here, with some amazing microscopic pictures:

http://hyperphysics.phy-astr.gsu.edu/hbase/vision/peacock.html

3. Green mice.

For the paper describing the creation of green mice, see:

Okabe, M., et al. 1997. "Green mice" as a source of ubiquitous green cells. FEBS Letters 407:313-19.

Images and even a movie showing green mice can be found here:

http://kumikae01.gen-info.osaka-u.ac.jp/tg/tg-ad.cfm

07/21/11 22:00