Dear Biologists,

Why is it that reptiles often have green skin, whereas mammals usually seem to have pink/brown/black skin?

Lots of love,
Emmy (age 26)

Reptiles usually have bare skin and so need to have colours that help keep them camouflaged and cryptic in their habitat. They may also be dark to help absorb heat from the sun and warm up for the day. Reptiles will often be green as they live in a very green/brown environment.

Many mammals have hairy skin and their hair is often coloured in a way that keeps them hidden in their habitat. This may be a different colour compared to where reptiles lives. They may also be coloured in a way that helps attract a mate. Those that do have bare skin eg whales and dolphins often have a dark back and pale belly. This is called counter-shading and means that they are camouflaged against the lighter sea and sky from predators swimming below them and hidden against the dark sea bottom from a predator looming above.

Brown and black skin contains a dark pigment called melanin that helps protect the skin from dangerous ultraviolet rays from the sun as well as helping to keep animas camouflaged in their environment. Animals with pink skin have less melanin in their skin and may often live in areas where they are less exposed to the sun.

Ed Drewitt

Also don't be fooled into thinking reptiles are just green and brown, look at the bright colours of agamids, coral snakes, gila monsters, chameleons and plenty more. Coulurs generally depend on what signal you want to give out.

Greens are good for camoflage, blues and yellows are good for showing off (especially if contrasted against black or white) and reds and yellows are commonly used as warning colours if an animal is toxic.

Mammals can't make so many colours being limited to what they can include in their hair, so thye tend to be white, black, brown or a mixture of those. Of course we do see reds in foxes and red pandas, the okapi is almost purple in colour and sloths grow algae in their hair to make them green.