COLORATION CHANGES

Coloration changes Coloration changes in individual organisms Short-term changes Adaptive colour change: (top) at rest the common octopus (Octopus vulgaris) blends with its 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. Adaptive colour change: the gradual colour change of the green anole (Anolis carolinensis) 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 White-tailed ptarmigan (Lagopus leucurus), (top) as winter approaches, (bottom) in summer 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 (see photograph). Many songbirds adopt a bright, contrasting nuptial plumage during the breeding season, reverting to a drabber winter plumage during the postnuptial molt. Seasonal colour change in the varying hare (Lepus); (top) summer pelage and (bottom) winter 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. Control of coloration Genetic control Coloration is in large measure determined genetically. As mentioned earlier, the inheritance of colour in garden peas provided part of the basis for the pioneering studies of heredity by Mendel. These studies led Mendel to postulate the existence of discrete units of heredity that segregate independently of one another during the formation of reproductive cells. The studies also led to his discovery of the phenomenon of dominance. The basic units of heredity are now known as genes, and the variant forms of a given gene are termed alleles. Among species that reproduce sexually, an individual normally possesses a pair of alleles for any geneone inherited from the female parent and one from the male parent. These two alleles are situated at corresponding loci on the paired chromosomes found in diploid cellsi.e., cells containing two similar sets of complementary chromosomes. Segregation of the alleles occurs during formation of reproductive cells, with the result that only one of the pairs enters each cell, which is called a haploid cell. In his experiments Mendel crossed purple-flowered peas with white-flowered ones. The plants he used in these crosses were true-breeding for flower colour, meaning that the purple-flowered plants were descended for generations from only other purple-flowered plants, and that the white-flowered plants were likewise descended for generations from only other white-flowered plants. Because of these true-breeding characteristics, Mendel postulated that the original plants were homozygous for the trait of flower colourin other words, that each plant carried a pair of identical heredity units (i.e., alleles) for this trait. When he crossed purple-flowered peas with white-flowered ones, he obtained a first filial (F1) generation in which all the offspring had purple flowers. He therefore deduced that the unit for purple (usually designated R) was dominant over the unit for white (r). Thus in the parental generation the purple-flowered plants can be designated RR (indicating that they are homozygous for the dominant allele), and the white-flowered plants can be symbolized as rr. The F1 plants were heterozygous for flower colour (Rr), but they expressed purple colour because of the complete dominance of the allele R over r. Dominance may be incomplete, however; a crossing between homozygous red Japanese four-o'clocks (Mirabilis) and homozygous white ones yields heterozygous Rr offspring, which are all pink. A cross of the heterozygous pink generation of four-o'clocks with each other yields a second generation with the colour ratio of 25 percent red (RR), 50 percent pink (Rr), and 25 percent white (rr). This is because each of the parent (F1) plants produces equal numbers of R- and r-containing reproductive cells through segregation, and there is a random chance of either type of male haploid cell (gamete) fertilizing either of the two female types. For peas, on the other hand, the ratio resulting from a cross of parent (F1) plants is three purple (one RR and two Rr) to one white (rr) because of the dominance of R. Although the principle of inheritance of colour and coloration patterns in all organisms is like that for the two plants described above, it is usually far more complex. Within the species population, a particular gene may have multiple alleles instead of two; thus numerous combinations within any individual may be possible; in addition, the coloration may depend upon genes at several sites. In this case either all pairs may segregate simultaneously and more or less independently into the gametes, or the genes may be linked in their inheritance by location on the same chromosome. Such possibilities, together with different degrees of dominance, result in tremendously complex hereditary bases for the genetic control of colour and colour patterns within many species. For a fuller treatment of these principles, see Genetics And Heredity, The Principles Of: Mendelian genetics. Physiological control The development of coloration often depends upon regulatory substances (hormones) secreted by endocrine glands. In birds the level of the hormone thyroxine determines the coloration of feathers and bill, although specific seasonal biochromes are often laid down under the influence of sex hormones, as in the beak of the starling, which turns from black to yellow in early spring. The variability in control among bird species is so great, however, that generalizations are impossible. Hormonally controlled colour changes also occur in mammals; for example, swellings in the genital areas that become pink due to vascularization during the reproductive season. The species specificity of coloration patterns, however, always depends on a genetically determined responsiveness of various target tissues to certain hormones. Chromatophores occur in cephalopods, crustaceans, insects, fishes, amphibians, and lizards and are responsible for the most rapid colour changes. They allow conspicuous display of a biochrome by dispersing it in the chromatophore-bearing surface, or they conceal the biochrome by concentrating it into small areas. Chromatophores are of three kinds. The chromatophoric organs of cephalopods consist of an elastic sac filled with biochrome and controlled by a ring of radiating muscle fibres. These fibres contract in response to neural stimulation, thereby stretching the sac into a broad, thin disk. Chromatophoric syncytia occur in crustaceans, the movement of biochrome being due to the ebb and flow of cytoplasm through fixed tubular spaces that collapse when the cell is contracted and fill when the cell expands. Chromatophoric syncytia are hormonally controlled. Cellular chromatophores, the third kind, are found in vertebrates. In these cells melanic granules flow in stable cellular processes that maintain a fixed position, unlike the contracting and expanding processes of the syncytia. Control among vertebrates is varied: chromatophores of bony fishes are controlled by the autonomic nervous system; those of elasmobranch fishes (sharks and rays) and lizards are controlled by hormones and nerves; those of amphibians are regulated by hormones alone. One animal may contain biochromes of several colours, commonly red, yellow, black, and reflecting white; prawns also have a blue biochrome. By appropriate migrations of biochromes, an animal can achieve substantial alterations in colour or shade for varying periods of time. In prawns, dispersion of blue and yellow yields green; unequal dispersion of biochromes over parts of the body produces patterns of coloration. Rapid physiological colour changes are supplemented by morphological ones, the animal either gradually synthesizing or destroying biochromes, usually in an adaptive manner (see the section Coloration changes). Frank A. Brown, Jr. Edward Howland Burtt, Jr. Structural and biochemical bases for colour Organisms produce colour physically, by submicroscopic structures that fractionate incident light into its component colours (schemochromes); or chemically, by natural pigments (biochromes) that reflect or transmit (or both) portions of the solar spectrum. Pigmentary colours, being of molecular origin, may be expressed independently of structural colour and are not altered by crushing, grinding, or compression. Structural colours are often reinforced by the presence of biochromes and are altered or destroyed by crushing, grinding, or compression. Structural colours (schemochromes) The physical principles of total reflection, spectral interference, scattering, and, to some extent, polychromatic diffraction, all familiar in reference to inanimate objects, are also encountered among tissues of living forms, most commonly in animals. In plants these physical principles are exemplified only by the total reflection of white light by some fungi and bacteria and by the petals of some flowers and barks, and by some spectral interference in certain sea plants. The adaptive value of biological coloration Coloration and the pattern of coloration play a central role in the lives of plants and animalseven those species in which vision is lacking or not the dominant sense. For example, cryptic coloration often goes hand in hand with cryptic behaviour; nonreflective colours occur on the faces of birds that forage in bright sunlight; and abrasion-resistant coloration occurs more often among species that inhabit abrasive habitats than among species that inhabit nonabrasive habitats. The functions of biological coloration fall into three broad categories: (1) optical functions, in which coloration affects the animal's or plant's visibility to other animals; (2) visual functions, in which coloration affects the animal's own vision; and (3) physiological functions, in which the molecular properties of biochromes play a role unrelated to either optical signaling or vision. Optical functions: deceptive coloration Deceptive coloration depends on four factors: the coloured organism, hereafter referred to as the organism; its model, which may be the background against which it is concealed; the spectral quality of the illumination; and the visual sensitivity and behaviour of the animal or animals that the organism is deceiving. To some extent the following discussion considers the relationships among the four factors separately; but in reality the deceptive, opitcal effect results from the interaction of all four factors. There are two basic types of deceptive coloration: (1) concealing coloration, or camouflage, in which the organism blends into its surroundings; and (2) mimicry, in which the organism is not hidden but rather presents a false identity by its resemblance to another species.