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Nature often comes in bright colours - think of the vibrant plumage displayed by male birds, the explosion of colour in a coral reef or the magnificent shades of leaves in autumn. However, these colours do not exist to please man's eye but are in fact vital for their wearers.
Colours make very obvious visual signals and are widely used in, for example, courtship, defence and camouflage. Courtship colouration is particularly striking in birds, where the males of many species display their bright plumage and elaborate ornaments to attract females and mate with them. Females often prefer the most colourful and ornamented males, possibly because these traits are considered to be costly and might thus reflect good health and/or high quality. Plumage colouration in birds can be due to pigments or structural colouration and both types show convergent evolution across taxa (while the collagen arrays providing skin colours in birds are identical to structures that have evolved convergently in mammalian skin).
Colours are often crucial for defence, such as in aposematism, where bright colours (e.g. yellow or red) signal unpalatability or toxicity to a predator, and mimicry, where harmless species mimic the appearance of aposematic ones or aposematic species mimic each other. Mimicry has evolved in many different groups of animals, e.g. fish, amphibians, insects, spiders and cephalopods, and curiously also in some plants. In cephalopods, even sexual mimicry occurs, where a subordinate male mimics a female to sneak copulations. In cryptic colouration or camouflage, animals melt into their environment to escape from predators or become invisible for prey. Some taxa (e.g. octopus, cuttlefish and chameleons) are able to rapidly change colour through concentrating or dispersing pigment molecules within pigment-containing cells called chromatophores. These chromatophores are not only used for camouflage but also in communication.
Surprisingly, phenomena such as aposematism and camouflage can (most likely) even be found in plants. Autumn leaf colouration, which has evolved multiple times independently in gymnosperms as well as angiosperms, has been hypothesised to signal to aphids and other pest insects that the tree is unsuitable as a host to lay eggs on. And indeed, aphids appear to avoid trees with red leaves, which could serve as a warning of chemical defence compounds. Furthermore, the "living stones" in the family Aizoaceae and the members of the derived cactus genus Ariocarpus both mimic inedible rocks to deter herbivores.
Body colour can have additional functions in thermoregulation. This is particularly important for ectothermic ("cold-blooded") animals that cannot generate their own body heat. Exposure of dark body parts to sunlight in order to warm up more quickly can be observed in many groups of animals, from insects to reptiles.