Animal coloration is the general appearance of an animal resulting from the reflection or emission of light from its surfaces. Some animals are brightly coloured, while others are hard to see. In some species, such as the peacock, the male has strong patterns, conspicuous colours and is iridescent, while the female is far less visible. There are several separate reasons why animals have evolved colours. Camouflage enables an animal to remain hidden from view. Signalling enables an animal to communicate information such as its ability to defend itself to other animals of the same or different species. Animals also use colour in advertising, signalling services such as cleaning to animals of other species; to signal sexual status to other members of the same species; and in mimicry, taking advantage of another species' warning coloration. Some animals use colour to divert attacks by startle, surprising a predator e.g. with eyespots or other flashes of colour, and by motion dazzle, confusing a predator's attack by moving a bold pattern (such as zebra stripes) rapidly. Some animals are coloured for physical protection, such as having pigments in the skin to protect against sunburn, while some frogs can lighten or darken their skin for temperature regulation. Finally, animals can be coloured incidentally. For example, blood is red because the haem pigment needed to carry oxygen is red. Animals coloured in these ways can have striking natural patterns. Animals produce colour in different ways. Pigments are particles of coloured material. Chromatophores are cells containing pigment, which can change their size to make their colour more or less visible. Some animals, including many butterflies and birds, have microscopic structures in scales, bristles or feathers which give them brilliant iridescent colours. Other animals including squid and some deep-sea fish can produce light, sometimes of different colours. Animals often use two or more of these mechanisms together to produce the colours and effects they need.
Sexual selection, a concept introduced by Charles Darwin in his 1859 book On the Origin of Species, is a significant element of his theory of natural selection. The sexual form of selection У ... depends, not on a struggle for existence, but on a struggle between the males for possession of the females; the result is not death to the unsuccessful competitor, but few or no offspring. Ф У ... when the males and females of any animal have the same general habits ... but differ in structure, colour, or ornament, such differences have been mainly caused by sexual selection. Ф His sexual selection examples include ornate peacock feathers, birds of Paradise, the antlers of stag (male deer), and the manes of lions. Darwin greatly expands his initial three-page treatment of Sexual Selection in the 1871 book The Descent of Man and Selection in Relation to Sex. This 900-page, two-volume work includes 70 pages on sexual selection in human evolution, and 500 pages on sexual selection in other animals. In summary, while natural selection results from the struggle to survive, sexual selection emerges from the struggle to reproduce. У The sexual struggle is of two kinds; in the one it is between individuals of the same sex, generally the males, in order to drive away or kill their rivals, the females remaining passive; whilst in the other, the struggle is likewise between the individuals of the same sex, in order to excite or charm those of the opposite sex, generally the females, which no longer remain passive, but select the more agreeable partners