The Green Dog
Why aren't mammals green?
Many years ago, we had a dog, a white puff ball with scattered black markings. We called him Domino. I took him out for a walk one day and he got away from me. Ran straight to a nearby lazy river and jumped in. He thought better of it and jumped right back out. What emerged was the creature from the black lagoon, fur from head to tail saturated with bright green algae. One of the strangest sights I've seen.
But that raises a question: why is a green dog so strange? Why is green so rare for animals, especially mammals? It would seem to be an ideal color for camouflage. And we know green is possible - like frogs, katydids and parrots, not to mention plants.
There are a few reasons, both physical and functional.
It is not easy being green
Green is not the "easiest" color to make. In plants, the green color comes from chlorophyll, a key chemical involved in photosynthesis, the process that converts sunlight into chemical energy. But most animals do not use photosynthesis and do not have chlorophyll. They achieve green in other ways.
Feathers and lizard skin can look green, but often not because they contain a green pigment. Instead, the color can come partly from microscopic structures that scatter light. This is similar to how we see greens and other colors in an oil slick or soap bubble. The oil or soap is not green itself; the color comes from how light interacts with the physical structure of the material.
Mammal coloration is based mainly on melanin, which gives a range of browns, oranges, and reds (and white, lack of pigment). There is no easy way to get green fur from that color system.
Color is not always seen the same
Not all animals see color the same way we do. What might not look like the best camouflage to us might work very well against another animal. Human vision uses three kinds of color-sensitive cones, roughly red, green, and blue. But most mammals, including many ungulates (hoofed mammals like deer), have only two, roughly green and blue. Essentially they are red-green colorblind. That means to them, a tiger's orange coat, which looks almost ridiculously obvious to us, blends in much better with green vegetation.
Gray and brown - good enough?
Mammals do a pretty good job with their existing color palette. They have great camouflage against tree trunks or branches, dried grass, dirt, rocks, snow, and so on. There is a lot of green in nature, but when you look closer browns and grays are common too, especially as vegetation changes over the year.
Evolution works with what exists
We should also think about how evolution works. It doesn't just say, "make green." It starts with what exists, then those features can change through random mutations. If a mutation increases the likelihood that the organism reproduces, then it can spread through the population. So a green fur mutation would have to happen in the first place, and it would have to be beneficial overall (not a given since there could be negative effects from the same mutation). And there may just be no simple mutation that results in green fur.
Man-made camouflage
Consider the situation without the constraints requiring mutations or natural selection: man-made camouflage. The U.S. Army has spent considerable effort designing and testing camouflage patterns. Many of them include muted green along with grays and browns. So perhaps in an ideal world there would be more green fur. But even then it would depend on the immediate visual environment the animal inhabits.
But one time I did have a green dog.
Related ideas:
The World's Largest R&D Lab
Local Optima: Survival of the Imperfect