Melanism is merely the dark pigmentation of skin, fur–or in this case, feathers. The unique trait derives from increased melanin in the body. Genes may play a role, but so might other factors. While melanism is common in many different animal species (e.g. Washington, D.C. is famous for its melanistic squirrels), the trait is extremely rare in penguins. All-black penguins are so rare there is practically no research on the subject–biologists guess that perhaps one in every quarter million of penguins shows evidence of at least partial melanism, whereas the penguin pictured above appears to be almost entirely (if not entirely) melanistic.