Countershading

Countershading, sometimes called Thayer's law, is a form of camouflage in which an animal's coloration is darker on its upper or top side and lighter on its underside, a pattern found across mammals, reptiles, birds, fish, and insects. It works by counteracting the natural effect of overhead light, which normally makes an object's upper surfaces look lighter and its underside look darker; by reversing this pattern, countershading cancels out an animal's own shadowing, flattening its apparent three-dimensional shape and making it harder for predators or prey to detect it. Animals that display countershading include grey reef sharks, blue sharks, herring, mackerel, dolphins, deer, ibex, and various rodents, insects such as caterpillars and grasshoppers, and, in the fossil record, Mesozoic marine reptiles such as ichthyosaurs and the dinosaur Psittacosaurus.