Pigments of Animals. That animals are often brightly coloured is evident. Some of the simplest, such as many Radiolarians, are brilliant; sponges are often suffused with pigment; sea-anemones and corals are justly compared to flowers; many marine worms have an iridescent sheen; the Echinoderms are almost always bright; many crustaceans have a jewel-like radiance; myriads of insects are lustrous; the shells of molluscs are rich in beauty; the fishes gleam in silver and gold and many hues; even the amphibians are sometimes gaily pigmented; some lizards and snakes seem like flashes of colour; birds are often brilliantly decorated; and mammals have a subdued but often rich colouring in their fur. But all the colours of animals are not due to pigments, for air-spaces in hairs and feathers make these structures white; crystals of guanin or line often produce a silvery glimmer; striation and other physical peculiarities of the surface cause iridescence.
The most important Pigments.—(a) Pigments called lipochromes are among the commonest, occurring in the skin of crustaceans, molluscs, fishes, and birds, in yolk of egg, in the vascular fluid of invertebrates, and also in flowers. Carotin, lutein, tetronerythrin, yellowish chlorophanes, and rosy rhodophanes are representative examples. The reddish pigment conspicuous on many crustaceans is probably a rhodophane, and into this or analogous lipochromes the bluish and greenish pigments of some lobsters and crabs seem to be converted when the animals are boiled or preserved in alcohol or treated with acids. (b) Melanoid and lipochromoid pigments, usually of a dark colour, sometimes derivable from (a) or from the substances which give rise to (a), occur, for instance, in Gorgonid corals, shells of molluscs, the ink of Sepia, the eyes of vertebrates, and in tumours. (c) Uranid pigments of a yellow colour, becoming brown or dark-violet in association with ferments, occur in Tunicates, Gastropods, insects, and sponges. (d) Hæmoglobin and its derivatives, of pre-eminent importance in connection with respiration, are of wide occurrence. Hæmoglobin itself, the red pigment of the blood which enters easily into a loose union with oxygen, is present in all vertebrates except Tunicates, Amphioxus, and two or three (perhaps anæmic) fishes. Among invertebrates it is known in some molluscs, crustaceans, and 'worms,' in the larva of the dipterous insect Chironomus, and in a few Echinoderms. It is not known to occur in any cœlenterate, sponge, protozoan, or plant. Among the important derivatives of hæmoglobin are hæmatoporphyrin in the oviduct of birds, giving some of the colour to the egg-shells, bilirubin in the bile, and biliverdin, another bile-pigment, which also occurs in the shells of some snails. As respiratory pigments ought also to be reckoned the hæmocyani in the vascular fluid of molluscs and Arthropods, various violet and purple floridine pigments from sponges and from the Polyzoan Bugula, and, according to Sorby, the aphidein of aphides. (e) Chlorophyll, the almost constant colouring matter of plants, apparently consisting of a mixture of two pigments (chlorophyll-green and chlorophyll-yellow), perhaps occurs in a few animals, green infusorians, the fresh-water sponge, the green hydra. It is not quite certain, however, that the green pigment of these animals is identical with that of plant-green; it may be a closely analogous substance. Chlorophyll-yellow is a lipochrome and occurs in many animals. (f) Indigo pigments are said to occur in the urine of mammals, and in the purple secretion of the whelk Purpura. The secretion of Murex, which turns violet in the light, has not been sufficiently investigated.
Physiology of Pigments.—Within recent years many of the pigments of animals have been analysed, and some facts about their relationships have been discovered. But in regard to the conditions of their formation, and the purposes which they may serve within the body, comparatively little is known. As the soil may influence the colouring of flowers, so the food given to birds may affect the brightness of their plumage. There are several facts of a similar nature. Cold seems to be one of the conditions which induce a winter change of colouring in a few birds and mammals, and in the butterflies Araschnia levana and A. prorsa, which seem to be diversely coloured varieties of one species. Light, which is so important a factor in the development of chlorophyll in plants, has also an influence on the pigmentation of animals. Thus, apart from the effects of keeping animals in darkness, it has been shown by E. B.
Poulton that surrounding colours modify those of some caterpillars, and J. T. Cunningham has demonstrated that young flat-fishes when illumined on the normally shaded and unpigmented side develop pigment-cells on that surface.
But in regard to the internal conditions of the formation of pigment we are more ignorant. Their relation to the general metabolism of the body is one of the unsolved problems of comparative physiology. Some seem to be of the nature of waste-products, a few are perhaps reserve substances, many may be called by-products of metabolism. On the other hand it is well known that many pigments are auxiliary to some of the important functions of the body. Thus, hæmoglobin and analogous substances are important in connection with respiration; the chlorophyll of plants is essentially associated with assimilation; the pigments found in the eyes of animals seem to aid in the visual function.
External Utility of Colouring.—In many different ways the colouring of animals is of importance to their well-being in the struggle for existence. Thus, the green insects and reptiles which live in the grass or on trees, the sandy-coloured beasts which are hardly to be detected against a background of similar colour, the white animals which are almost invisible among the snow illustrate protective colouring. This colour-resemblance between animals and their surroundings is sometimes marvellously exact, and doubtless of great advantage. Moreover, not a few animals, among crustaceans, fishes, amphibians, and reptiles, have the power of changing their colour, for the pigment-cells expand or contract under nervous stimulus, and this is in some cases advantageously protective. The conspicuous colours of some unpalatable or noxious animals may be an expression of their constitution, while, according to a theory of another order, they warn off possible molesters. It may also be that colours are sometimes useful in assisting mutual recognition between individuals, or in indicating attitudes and movements. Finally, in many animals the sexes differ markedly in colour, the males being almost always brighter than their mates. According to some, this depends on the constitutional peculiarities of maleness and femaleness, while Darwin has emphasised that the brightness of males has been enhanced by the selective taste of the females, and Wallace has urged that natural selection has retarded female butterflies and birds from attaining a brightness which would expose them during the weakness or preoccupation of the reproductive period to the hungry eyes of their enemies. But our recognition of the way in which variations of colouring are fostered or eliminated in the course of natural selection must not lead us to forget the primary problems of the origin of the pigments, and of the physiological conditions which determine their distribution.
See BILE, BLOOD, CATERPILLARS, CHLOROPHYLL, ENVIRONMENT, FLOWER, MIMICRY, SEX, and SEXUAL SELECTION. C. Fr. W. Krukenberg in his Grundzüge einer vergleichenden Physiologie der Farbstoffe und der Farben (Heidelberg, 1884) has given an admirable account of the animal pigments, with a bibliography of important researches, such as those of Kühne, MacMunn, Moseley, and Sorby. See also M'Kendrick, Text-book of Physiology (1888); Halliburton, Physiological and Pathological Chemistry (1891). For the relation between colour and the external conditions of life, see Semper's Animal Life (Inter. Sc. Series, 1881); and for the importance of colour in the external life of animals, Poulton's Colours of Animals (Inter. Sc. Series, 1890).