Coloration changes

Short-term changes

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Most rapid colour changes are chromatophoric ones that alter the colour of the organism through the dispersion or concentration of biochromes. Emotion plays a role in such changes among some cephalopods, fishes, and horned lizards (Phrynosoma). When excited, certain fishes and horned lizards undergo a transient blanching that probably results from the secretion of adrenaline (epinephrine), a hormone known to concentrate the dark biochrome of vertebrates. Excited cephalopods exhibit spectacular displays of colour, with waves of colour rippling across the body. Chromatophoric colour change is slower in vertebrates than in cephalopods. Although some fish may complete a colour change within a minute (compared to half a second or less for cephalopods), most vertebrates require several minutes to several hours.

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Colour changes extending over several hours are often entrained to external cycles. Fiddler crabs (Uca) that live in the intertidal zone show a complex pattern of cyclic chromatophoric colour change that is entrained not only to the local tidal cycle but also to the lunar and solar cycles. So important is this cyclic colour change that the response is innate to every part of the integument. The legs of a fiddler crab can be removed and sustained for a few days in saline solution; during this time melanophores in the legs continue to disperse and concentrate their melanin according to the cycle at the time they were removed from the body.

Changes in colour that extend over periods of several months may involve the synthesis or destruction of chromatophores or biochromes. The quantities of deposited guanine in some fishes vary in proportion to the relative lightness in colour of the background upon which they are living. Greenfish, or opaleye (Girella nigricans), kept in white-walled aquariums became very pale during a four-month period, storing about four times the quantity of integumentary guanine as was recoverable from the skins of individuals living in black-walled aquariums but receiving the same kind and amounts of food and the same overhead illumination.

Some chromatophores respond directly to relevant environmental stimuli, independent of the nervous system. Such response occurs in the young of some fish and of the clawed frog (Xenopus); but in older individuals the nervous system, which is by this time fully developed, controls responsiveness. More typically the chromatophore response is mediated by the sensorimotor system from the start. The eye plays a major role in cephalopods and most vertebrates, particularly in animals capable of matching complex backgrounds, but the pineal organ (a light-sensitive organ on top of the brain) and a generalized dermal light sense may also mediate the chromatophore response.

Seasonal changes

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Seasonal changes of fields and forests include the annual colour changes involving foliage, flowers, fruits, and seeds of plants. Many birds and mammals undergo seasonal molts, replacing their plumage or pelage with differently coloured feathers or hair. Winter whitening of the willow ptarmigan (Lagopus lagopus) and varying hare (Lepus) are examples of a shift in camouflage coincident with a change in the background coloration. Many songbirds adopt a bright, contrasting nuptial plumage during the breeding season, reverting to a drabber winter plumage during the postnuptial molt.

Seasonal colour changes are usually regulated by light (mediated by the visual or pineal systems) or by temperature. Decreasing day lengths initiate whitening in the willow ptarmigan, whereas falling temperatures initiate whitening in the weasel (Mustela erminea). The spring molt of the varying hare is stimulated by the lengthening day, but the rate of molt depends on temperature. Seasonal changes in coloration may occur without a molt as a result of bleaching or wear, for example, the bleaching of human hair in the summer sun and birds that have bright colours based on carotenoids.

Age-related changes

Colour changes during the life of an individual are common. Graying hair is a familiar badge of the elderly, both in humans and, to varying degrees, in other mammals. Among primate groups, particularly gorillas and chimpanzees, silver hairs indicate both age and dominance. Young birds of many species have a juvenile plumage that gives way to either an adult plumage in short-lived birds or a series of immature plumages in longer-lived species. Most gulls, for example, are deep gray or brown during their first year and become increasingly white thereafter. Changes of colour are also associated with age and size in many fish; for example, the blue parrot fish changes from a vertically barred pattern to all blue in association with increasing age and size.