Coloration (zoology)

“ We have further the dusky hue of creatures that haunt the night, such as mice, moles, and bats; and the gorgeous tints of fishes that swim among the coral reefs. These local color adaptations of animals are of great use to them, either enabling them to escape the notice of their enemies or to come upon their prey with the least risk of being detected. Certain groups of animals have a local color-adaptation, and may be noticed under the heading of Protective Coloration ”

“ To those who believe that every spot or stripe or patch of colour on every animal is a beautiful illustration of the truth of the theory of protective coloration, this may seem a very fanciful idea. Yet I feel convinced that the influence of environment has played a greater part than is generally believed in the evolution of colour in living organisms. The weak point in the theory of protective coloration when applied to large mammals is the fact that all carnivorous animals are nocturnal and seek their prey habitually by night and by scent, and only occasionally by daylight and by sight. ”

“ Many very striking facts can be adduced in support of this theory, and no doubt it is of advantage to most species of mammals, birds, reptiles, and insects to harmonise in colour with their surroundings; but there are many instances in nature, especially amongst birds and insects, where a very striking and conspicuous coloration does not appear to have been prejudicial to the life of a species. ”

“ We need not repeat the fabulous stories told about these color-changes: the facts which can be strictly verified by direct observation are wonderful enough without the adornments of imagination. These changes of color range from whity-yellow, through yellow, bright and dark green, to dull black; and these diversities of coloration may affect the entire surface of the animal, or one or other of its sides, or may appear only in spots. When the chameleon is asleep, and not exposed to the direct rays of the sun, its color is a whity-yellow ”

“ These causes of coloration may be superposed and combined in a thousand ways. When birds are under the influence of physiological excitations like those of rage or love, the flow of blood contributes to enliven the color of the bare parts to the point of greatly modifying it. The bright, metallic tints of the peacock and the humming-bird are due to phenomena of interference and to the presence of a dark pigment combined; the green tint of the lizard to the association of a yellow pigment and blue-reflecting iridocytes. ”

“ We have seen in what infinitely varied ways the need of concealment has led to the modification of animal colours, whether among polar snows or sandy deserts, in tropical forests or in the abysses of the ocean. We next find these general adaptations giving way to more specialised types of coloration, by which each species has become more and more harmonised with its immediate surroundings, till we reach the most curiously minute resemblances to natural objects in the leaf and stick insects, and those which are so like flowers or moss or birds' droppings that they deceive the acutest eye. ”

“ Warning Coloration and Mimicry
The colors of a great many animals, including a considerable percentage of American butterflies and their larvae, have been commonly explained by the theory of warning colors. According to this theory animals which [Pg 34] were for any reason not edible by birds and mammals have developed various striking combinations of color such as black and yellow, red and black, or black and white, in order to advertise to their foes their inedible qualities. ”

“ It is, I believe, the general opinion of modern naturalists that, putting aside cases where brilliant colours may have been produced amongst birds and insects by the action of the law of sexual selection, the coloration of all living organisms is protective, "serving," as that distinguished naturalist Mr. Alfred Russel Wallace puts it, when discussing the [Pg 3] subject of the coloration of mammals, "to conceal herbivorous species from their enemies, and enabling carnivorous animals to approach their prey unperceived." ”

“ Protective coloration, in some of its varied forms, has not improbably modified the appearance of one-half of the animals living on the globe. The white of arctic animals, the yellowish tints of the desert forms, the dusky hues of crepuscular and nocturnal species, the transparent or bluish tints of oceanic creatures, represent a vast host in themselves; but we have an equally numerous body whose tints are adapted to tropical foliage, to the bark of trees, or to the soil or dead leaves on or among which they habitually live. ”

“ The examples given above show how natural selection may lead to the evolution of distinctive forms of coloration which are advantageous to the Fish either for concealment, aggression, or protection, and in conclusion it may be pointed out that by the same cause colour may be eliminated or its development checked if in any way harmful to the animal ”

“ In the hair is contained the pigment to which the colour of mammals is chiefly due. Tracts of brightly-coloured skin may exist, as in the Apes of certain genera; but such structures are not general. The pigment of the hair seems to consist of those pigmentary substances known as melanins. It is remarkable to find such a uniform cause of coloration, when we consider the great variety of feather-pigments found in birds. The variations of colour of the hair of mammals are due to the unequal distribution of these brown pigments. There are very few mammals which can be called brightly coloured. ”

“ It is when we come to the essential parts of plants on which their perpetuation and distribution depends, that we find colour largely utilised for a distinct purpose in flowers and fruits. In the former we find attractive colours and guiding marks to secure cross-fertilisation by insects; in the latter attractive or protective coloration, the first to attract birds or other animals when the fruits are intended to be eaten, the second to enable them to escape being eaten when it would be injurious to the species. ”

“ I have never met with a more variable species. Specimens taken from each end of the scale of variation would undoubtedly be regarded as distinct species were it not for the connecting links. Some have the dorsal ridge distinctly serrated, while in others it is quite smooth. Again, as regards coloration, many of them have a row of oblong-ovate spots of a light French-grey colour (quite distinct from one another) on each side of the dorsal ridge, while in others these spots are so connected as to form one straight wide band. Another noticeable character is change of colour by heat. ”

“ Thus, for example, the action of light on the surface of the wings when in the resting posture may well inhibit the production of vivid pigment owing to some inherent physiological idiosyncrasy. But any individuals which lack this inhibiting factor—as some species which, though resting, are brightly coloured, appear to do—will be eliminated, if they live in an environment harbouring eliminating factors, which the exceptions to the rule we must suppose do not. But on this interpretation the fundamental factor in the determination of the coloration is the action of light. ”

“ Coloration. — General color uniform brown or blackish (the head sometimes rufescent) above, dusky, grayish, or brownish below, sometimes with chest rufescent or tawny, the under tail-coverts usually rufescent and throat black (sometimes margined posteriorly with a narrow band of cinnamon or chestnut) ; inner webs of remiges with basal portion (abruptly) buff, ochraceous, or tawny, the under wing- coverts similar but tipped with black; sexes alike; young similar but throat usually more or less variegated with white. ”

“ The great variety in coloration has given rise to the establishment of many races, varieties, and sub-species. In the typical forms the upper parts are brown or {558}greyish, with blackish spots or streaks, sometimes with a bronzy greenish sheen. The under parts are white, yellow, pink, or red, either uniform or, especially in the males, with large black spots. The lateral rows of ventral shields are frequently blue. The colour-varieties are almost endless. ”

“ Now green is a very rare colour among mammals, and there ought therefore to be some special reason for its development in the Sloths. And, as a matter of fact, the means by which this coloration is produced is one of the most marvellous phenomena in the whole animal kingdom—so marvellous, indeed, that it is at first almost impossible to believe that it is true. ”

“ Darwin, who says: "Each of the endless variations which we see in the plumage of our fowls must have had some efficient cause; and if the same causes were to act uniformly during the long series of generations on many individuals, all probably would be modified in the same manner." And in relation to the fact that there is a general constancy of coloration in the wild birds, he remarks that this uniformity of coloration is preserved by free intercrossing, and where this is prevented by isolation or migration, variations of color very frequently take place. ”

“ These changes depend chiefly upon the surroundings and the locality, in which certain styles of coloration seem to be the fashion, not necessarily to the absolute exclusion of others. Some specimens are of a rich brown colour, with or without dark brown spots and patches, and these are sometimes confluent, forming irregular, longitudinal bands. The ground-colour of other individuals is olive grey, with or without darker patches, and these paler tones prevail in toads which live on light-coloured soil, for instance on chalk. ”

“ But, on the principles here suggested, the divergence itself is useful, and must have been produced pari passu with the structural differences on which the differentiation of species depends; and thus we have explained the curious fact that prominent differences of colour often distinguish species otherwise very closely allied to each other. Among insects, the principle of distinctive coloration for recognition has probably been at work in the production of the wonderful diversity of colour and marking we find everywhere, more especially among the butterflies and moths ”

“ To see this bird carefully seeking the green species in a garden, and deliberately avoiding the multitudes of highly coloured but nauseous larvæ on the currant bushes, arduously examining every leaf and twig for the protected brown and green larvæ which the keen eye of the naturalist detects only by close observation; hardly deigning to look at the speckled beauties that are feeding in decorated safety before his eyes, while his callow brood are clamouring for food—to see this is to be assured for ever that birds can, and do, discriminate colour perfectly. ”

“ Many kinds of fish similarly alter their colour to suit their background by forcing forward or backward certain special pigment-cells known as chromatophores, whose various combinations produce at will almost any required tone or shade. Almost all reptiles and amphibians possess the power of changing their hue in accordance with their environment in a very high degree; and among certain tree-toads and frogs it is difficult to say what is the normal colouring, as they vary indefinitely from buff and dove-colour to chocolate-brown, rose, and even lilac. ”

“ Their common and nearly constant features of coloration are, uniform soft dark greenish body, with blue tinge on belly, and bluish-green tail-coverts and tail, violet head, more blue anteriorly and more bronzy on the foreneck, and with this color abruptly defined posteriorly against the peculiar uniform blackish dull green of the body; the wing-coverts usually tipped with vivid violet and green spots. One male is a typical example of the var. ”

“ Bird enemies come from above rather than below, and it will be noticed that the modifications referred to all have reference to the upper plumage. Protective colouring, having for its object the preservation of the species which adopt it, will be found to enter more or less into the economy of every animal and bird and insect in a state of nature; and therefore it will be seen that there is a general harmony between the colours of an animal and those of its habitation, of which fact almost every living natural object furnishes evidence. ”

“ But we cannot say that the hue which we describe as white is the same as that apprehended by the insects. Distinct it must undoubtedly be to secure the permanent selection of their visits, but we can say no more. Because an insect invariably selects and visits a flower of one particular colour, we can only record the observation, but certainly not assume that what we see as white is seen by them in the same hue. ”

“ It therefore seems possible that assimilative colouration may have been a first and a very general consequent in animal development; and that the subsequent protective resemblance acquired by numerous living creatures through the process of natural selection, when life had advanced to the competitive stage, is far too frequently used as an explanation for whole series of uniform phenomena in colouration, which have probably survived unaltered from remote antiquity. ”

“ Now in domesticated animals there is great variation in colouring, but not in the majority of wild species. What the causes are that operate in the painting of the skin of an animal no one can say, any more than one can say how particular spots are arranged on the petal of a flower or the wing of a butterfly. ”

“ But of late years these cases have been greatly increased both in number and variety, especially in regard to those which closely imitate special objects among which they live; and there are other kinds of coloration which long appeared to have no use. Large numbers of animals, more especially insects, are gaudily coloured, either with vivid hues or with striking patterns, so as to be very easily seen. ”