COLORATION

in biology, the general appearance of an organism as determined by the quality and quantity of light that is reflected or emitted from its surfaces. Coloration depends upon several factors: the colour and distribution of the organism's biochromes (pigments), particularly the relative location of differently coloured areas; the shape, posture, position, and movement of the organism; and the quality and quantity of light striking the organism. The perceived coloration depends also on the visual capabilities of the viewer. Coloration is a dynamic and complex characteristic and must be clearly distinguished from the concept of colour, which refers only to the spectral qualities of emitted or reflected light. Many evolutionary functions have been suggested for the effects of coloration on optical signaling. An organism with conspicuous coloration draws attention to itself, with some sort of adaptive interaction the frequent result. Such advertising coloration may serve to repel or attract other animals. While conspicuous coloration emphasizes optical signals and thereby enhances communication, coloration may, conversely, suppress optical signals or create incorrect signals and thereby reduce communication. This deceptive coloration serves to lessen detrimental or maladaptive interactions with other organisms. Coloration may also affect an organism in ways other than its interaction with other organisms. Such nonoptical functions of coloration include physiological roles that depend on the molecular properties (e.g., strength and type of chemical bonds) of the chemicals that create colour. For example, dark hair is mechanically stronger than light hair, and dark feathers resist abrasion better than light feathers. Coloration may also play a part in the organism's energy budget, because biochromes create colour by the differential reflection and absorption of solar engery. Energy absorbed as a result of coloration may be used in biochemical reactions, such as photosynthesis, or it may contribute to the thermal equilibrium of the organism. Nonoptical functions of coloration also include visual functions in which coloration or its pattern affects an animal's own vision. Surfaces near the eye may be darkly coloured, for instance, to reduce reflectance that interferes with vision. Emitted light, the product of bioluminescence, forms a portion of the coloration of some organisms. Bioluminescence may reveal an organism to nearby animals, but it may also serve as a light source in nocturnal species or in deepwater marine animals such as the pinecone fishes (Monocentris). These fishes feed at night and have bright photophores, or bioluminescent organs, at the tips of their lower jaws; they appear to use these organs much like tiny searchlights as they feed on planktonic (minute floating) organisms. Because many pigments are formed as the natural or only slightly modified by-products of metabolic processes, some coloration may be without adaptive function. Nonfunctional coloration can, for example, be an incidental effect of a pleiotropic gene (a gene that has multiple effects), or it can result from pharmacological reaction (as when the skin of a Caucasian person turns blue in cold water) or from pure chance. It seems unlikely, however, that any apparently fortuitous coloration could long escape the process of natural selection and thus remain totally without function. Regardless of its adaptive advantages, a particular coloration or pattern of coloration cannot evolve unless it is within the species' natural pool of genetic variability. Thus a species may lack a seemingly adaptive coloration because genetic variability has not included that coloration or pattern in its hereditary repertoire. Because humans are highly visual animals, we are naturally interested in and attentive to biological coloration. Human attention to coloration ranges from the purely aesthetic to the rigidly pragmatic. Soft, pastel colorations aid in increasing work efficiency and contribute to tranquil moods; bright, strongly contrasting colours seem to contribute to excitement and enthusiasm. These phenomena may be extensions of the basic human response to the soft blue, green, and brown backgrounds of the environment as opposed to sharply contrasting warning colorations found on many dangerous organisms. It is possible that much of the aesthetic value humans attach to coloration is closely related to its broad biological functions. Human interest in coloration has led to biological studies. The classical work by the Moravian abbot Gregor Mendel on inherited characteristics, based largely on plant coloration, formed the foundation for modern genetics. Coloration also aids in the identification of organisms. It is an easily perceived, described, and compared characteristic. Related species living in different habitats, however, frequently have strikingly different colorations. Since coloration is susceptible to alteration in various functional contexts, it usually lacks value as a conservative characteristic for determining systematic relationships between all but the most closely related species. Related articles of interest include animal behaviour; mimicry. George S. Losey Edward Howland Burtt, Jr. in biology, appearance of an organism as determined by the quality and quantity of light reflected or emitted from its surface. Coloration is achieved both by pigments and by structural features, and it serves a variety of ecological and physiological functions. The pigments of biological tissues that reflect or transmit light are known as biochromes. Colours produced by the structural features of an organism, created by submicroscopic structures such as striations or facets, are called schemochromes. Biological coloration is genetically determined and functions largely to attract or repel other organisms or to conceal an organism from predators or prey. Coloration also plays a vital role in such biochemical reactions as photosynthesis, as well as contributing to an organism's thermal balance by absorbing or reflecting solar energy. Schemochromes result from the reflection, fractionation, or scattering of incident light. Reflection of the entire light spectrum, which imparts whiteness to flower petals, feathers, fur, and hair, for example, is often produced by minute air spaces lying between finely divided materials. Interference, the fractionation of light into its constituent colours, is produced by repeated reflection through ultrathin films and results in striking iridescence, as seen in peacock feathers, some insect wings, and pearls. The scattering of light to produce blue colours of structural origin, such as those of eyes or of many feathers, occurs when small air vesicles or suspended particles scatter the shorter wavelengths of blue light while allowing the remaining colours to be absorbed by an underlying layer of dark pigments. The structural production of colours is often reinforced by pigments lying above or below the structures, and the two may act in combination; the greens of fishes, amphibians, and reptiles, for example, often arise from blue scattered through a layer of yellow pigment. Biochromes, organic compounds often classified according to the presence or absence of nitrogen, produce colour by the selective absorption of light. The molecules of pigment absorb a limited range of wavelengths; the light that is not absorbed is reflected, and its dominant wavelength determines the pigment's colour. Many biochromes also function as catalysts for biochemical reactions, and others play a pivotal role in such metabolic processes as photosynthesis and the production of vitamin A. Nonnitrogenous pigments include carotenoids, quinones, and flavonoids, many of which are present in animals only through the ingestion of synthesizing plants. Carotenoidsyellow, orange, and red pigmentsare almost universally distributed in living things. Quinones, including vitamin K, cause yellow, orange, red, purple, or green coloration in plants and the animals that feed on them. Flavonoids, including the pigments that impart the yellow found in many flower petals, the purple-red of autumn leaves, and the red of beets, are important as indicators of mineral deficiencies in various crops. Nitrogen-bearing pigments include the porphyrins, indoles, and flavins. Two of the most important porphyrins are chlorophyll, a dark green pigment vital as the energy transformer necessary to photosynthesis in plants, and hemoglobin, the red pigment found in the blood cells of many animals that makes possible the transfer of oxygen to tissues. Melanins and indigoids make up the indole pigments. Melanin, a brownish or black pigment, is present in all human skin, most heavily in dark-skinned races, and in various tissues of many animals, and it helps protect underlying tissues from ultraviolet radiation. The indigoids include indigo, found in many plants and used to make a blue dye, and Tyrian purple, which is extracted from certain marine snails to produce a red-violet dye. Adaptive mechanisms that involve coloration have developed in a variety of species. Concealing coloration can be imitative, as when an insect resembles a twig or a marine animal mimics a rock or coral, or cryptic (background resemblance), as seen in the chameleon's colour changes or the camouflaging coloration of the leopard and lion. In some cases, especially among fishes, the pattern of colour can disrupt the outline of an organism, either masking its true shape or making it difficult for a predator to visually resolve it from a colourful or disruptive background. The use of coloration to make an organism highly visible, either to attract members of its own or another species or to warn or repel predators or challengers, is known as advertisement. Flowering plants that rely on animals, frequently insects, for the transmission of pollen necessary to sexual reproduction exhibit bright colours and intricate patterns, some visible only in ultraviolet light, to attract pollinators. Some fish and insects use bioluminescence to attract mates and differentiate species. Many vertebrates have developed spectacular courtship colorations to attract mates. Bright colours, frequently yellow, red, or black, often warn that an apparently defenseless organism is poisonous, equipped with spines, or in some other way dangerous. Short-term coloration changes are often mediated by chromatophores, structures that act to control coloration by collecting and dispersing pigments. Chromatophores are found in many fishes, crustaceans, lizards, mollusks, amphibians, and insects. The octopus, squid, and cuttlefish are capable of the most rapid colour changes, and many marine animals can emit pigmented fluids (inks) as protection when excited or threatened. Many organisms also undergo seasonal coloration changes. These may serve to ensure cryptic matching to a seasonally changing background, as in the varying hare, which goes from brown pelage in summer to white in winter. Or they may be tied to seasonal reproduction and the establishment of breeding territories, as in many species of fish and birds. Other animals undergo age-related changes in coloration. The juvenile plumage of birds, for example, gives way to adult coloration as the bird ages. Biological coloration has had great importance to humans throughout history: commercial dyes have been manufactured from plants and animals since ancient times; many of the first clues to understanding genetics and evolution came from coloration and its properties; the diagnosis of crop disorders through coloration has increased agricultural yields; the development of military camouflage owes its roots to animal models. There also is evidence that human aesthetic and emotional responses to colour have broad biological bases, thus tying a large part of art and design to biological coloration. Additional reading Structural and biochemical bases for colour Denis L. Fox, Animal Biochromes and Structural Colours, 2nd ed. (1976); and H. Munro Fox and Gwyne Vevers, The Nature of Animal Colours (1960), are technical but readable works on pigments and schemochromes; Arthur E. Needham, The Significance of Zoochromes (1974), is a technical analysis of the chemistry, control, and function of biochromes. Denis L. Fox, Biochromy: Natural Coloration of Living Things (1979), is a study of chemical and physical aspects of the coloration of flora and fauna; and J.N. Lythgoe, The Ecology of Vision (1979), is a summary of research in the influence of colour chemistry on the life of marine organisms. Erston V. Miller, The Chemistry of Plants (1957); T.J. Mabry, K.R. Markham, and M.B. Thomas, The Systematic Identification of Flavonoids (1970); T.W. Goodwin (ed.), Chemistry and Biochemistry of Plant Pigments, 2nd ed., 2 vol. (1976); and Theodore A. Geissman, Anthocyanins, Chalcones, Aurones, Flavones and Related Water-Soluble Plant Pigments, in Karl Paech and M.V. Tracey (eds.), Modern Methods of Plant Analysis, vol. 3 (1955), are technical dissertations on plant pigments. See also Theodore A. Geissman, The Chemistry of Flavonoid Compounds (1962). F. Blank, Anthocyanins, Flavones, Xanthones, in Wilhelm Ruhland (ed.), Encyclopedia of Plant Physiology, vol. 10 (1958), provides insight into the formative processes of plant pigments. Sylvia Frank, Carotenoids, Scientific American, 194:8086 (1956); and Sarah Clevenger, Flower Pigments, Scientific American, 210:8492 (1964), are well-illustrated articles for the lay reader. See also Otto Isler, Hugo Gutmann, and Ulrich Solms (eds.), Carotenoids (1971); and John Proctor and Susan Proctor, Color in Plants and Flowers (1978). Control of coloration M. Fingerman, The Control of Chromatophores (1963), is a good, readable account of the knowledge of physiological colour change. Chromatophores and Color Changes, American Zoologist, 23(3):461592 (1983), is a symposium of papers on hormonal and neural control of colour change. Frank B. Smithe, Naturalist's Color Guide (1975), is a pocket-size, loose-leaf book containing 182 named colours. A.H. Sturtevant, A History of Genetics (1965), is a vivid description of the beginnings and development of classical genetics. C. Donnell Turner and Joseph T. Bagnara, General Endocrinology, 6th ed. (1976), contains a treatment of hormonal regulation of animal coloration, with a selected bibliography. See also Joseph T. Bagnara and Mac E. Hadley, Chromatophores and Color Change: The Comparative Physiology of Animal Pigmentation (1973); Paul A. Johnsgard, The Hummingbirds of North America (1983), which explains the physics of changing plumage colour; and Willys K. Silvers, The Coat Colors of Mice: A Model for Mammalian Gene Action and Interaction (1979), a study of the genetic phenomena of interaction of colour factors. The adaptive value of biological coloration Hugh B. Cott, Adaptive Coloration in Animals (1940, reprinted 1966), is a detailed and scholarly treatment; Edward H. Burtt, Jr. (ed.), The Behavioral Significance of Color (1979), is a technical but readable treatment of nonoptical and optical functions of coloration, with discussion of visual psychology and the physics of light; Edward H. Burtt, Jr., An Analysis of Physical, Physiological, and Optical Aspects of Avian Coloration with Emphasis on Wood-Warblers (1986), looks at the evolution of colour and pattern in a single subfamily of birds; Jack P. Hailman, Optical Signals: Animal Communication and Light (1977), is a thought-provoking analysis of colour and behaviour as they affect optical signaling; Bernard Kettlewell, The Evolution of Melanism: The Study of a Recurring Necessity (1973), presents an analysis of the role of colour in natural selection; Sally Foy, The Grand Design: Form and Colour in Animals (1983), is a study of animal anatomy and morphology, with excellent illustrations; William J. Hamilton III, Life's Color Code (1973), is a popular account of some functions of coloration; and Gerald H. Thayer, Concealing Coloration in the Animal Kingdom, new ed. (1918), is a classic work on the theories of camouflage. Other works of interest for the general reader include Michael Fogden and Patricia Fogden, Animals and Their Colours: Camouflage, Warning Coloration, Courtship and Territorial Display, Mimicry (1974), a comprehensive scientific treatment with excellent photographs; Denis Owen, Camouflage and Mimicry (1980), a popular account full of interesting anecdotes and photographs; and Wolfgang Wickler, Mimicry in Plants and Animals (1968; originally published in German, 1968), a thorough and reliable survey. Denis Llewellyn Fox Frank A. Brown, Jr. George S. Losey Edward Howland Burtt, Jr.