Why Are There No Green Mammals?

The Answer

Mammals colour their fur with just two pigments, both forms of melanin: one black or brown and one reddish-yellow. Mixed in different amounts they give every mammal colour from black to ginger to cream, but they can't make green.

White fur has no pigment at all, and grey comes from mixing dark and white hairs. That's the whole toolkit. Green animals get their colour in other ways. A parrot's feathers contain tiny structures that reflect blue light, with a yellow pigment on top; blue and yellow together look green. Tree frogs do something similar with layers of cells in their skin. Mammal hair never evolved anything like that.

The one mammal that sometimes looks green is the sloth, and that's not its own colour. Its hair has grooves that hold damp, and green algae grow in them. A sloth moves so slowly that the algae have time to thrive, giving it a greenish coat that helps it hide in the trees.

Mammals may not have needed green anyway. Many of them, such as deer, see fewer colours than we do and can't easily tell orange from green. To a deer, a tiger's orange stripes probably blend into the leaves.

The principle: Pigments limit colours. Mammals colour their fur with just two forms of melanin, black-brown and red-yellow, so they can't make green; other animals use other pigments or microscopic structures.