Life's Great Mysteries - Why don't we ever see any green mammals?

Someone once asked why I go to so much trouble to answer such weird questions. To paraphrase George Carlin… The reason I answer these questions myself is because I’m the only one whose answers I accept. :)

Okay, think about this. There are plenty of green birds, green reptiles, green amphibians, green fish, green insects… why aren’t there any green mammals? Wait… the Grinch is green. Is the Grinch a mammal? Honestly, I don’t know what he (or she?) is supposed to be. Anyway, the Grinch is fictional, so we still have no green mammals. There may be some mammal species humans haven’t discovered yet, but it’s fair to assume none of them will be green, based on the approximately 6,640 mammal species known so far that are not green.

The closest we have to a green mammal is the two-toed and the three-toed sloths. Their fur sometimes appears to be green. But this isn’t part of the sloth’s natural pigmentation. Instead, sloths have a symbiotic relationship with green algae. The sloths provide the algae with a nice place to live, in the special grooves of their hairs where there is moisture. In return, the algae makes the sloth’s fur appear green, which serves as camouflage, protecting the sloth from predators (such as the massive harpy eagle, which loves to eat sloths).

Okay, so besides the fictional grinch, and besides sloths that look green but are not really green, why are there no green mammals?

The color of skin and hair in mammals is determined by two forms of melanin (pigment). Eumelanin varies from dark brown to black. Pheomelanin varies from yellow to red. Various combinations of these allow mammals to have colors ranging from white (no pigmentation at all) to black (dense quantities of eumelanin), with all ranges of brown and blond in between.

However, it’s also important to understand that not all animal coloration is produced by melanin. Many animals have what is called structural coloration. Basically, this is a repeating structure at the microscopic level causing light to reflect a certain color. This is how most butterfly wings get their colors. And how some bird’s feathers get their colors. For example, a scarlet macaw doesn’t really have red feathers… but the feathers have a structure that interacts with light in such a way that only red light is reflected. And most birds that are green do not actually have green pigment, but instead they have structural features that reflect green light. This begs the question… if mammals don’t have any green pigment, couldn’t they evolve green structural coloration? Possibly. But, this fact also suggests that most mammals are better off with shades of brown. Even those mammals that climb trees a lot, like squirrels, are often seen against the tree bark rather than out at the tips of limbs surrounded only by green leaves.

So, maybe part of our answer to this question is that few mammals actually need to be green.

Below is a three-toed sloth in Costa Rica that has green algae living in its fur.

Photo Credits:
- Green sloth - Marimar Ortiz, CC BY 3.0, via Wikimedia Commons