1922 Encyclopædia Britannica/Colours of Animals
COLOURS OF ANIMALS (see 6.731).—Since 1910 the knowledge of animal coloration has been added to in many directions. Broadly speaking, however, the new facts confirm the views previously held, which are only modified in points of detail.
Cryptic Colouring.—As regards cryptic coloration, A. H. Thayer and his followers have shown that many arrangements of colour and pattern which had been previously considered to be revealing, are in truth concealing. A few biologists go so far as to express the view that all coloration is concealing and explain all cases of the mimicry of one animal by another, as due to a common cryptic (syncryptic) coloration, both animals having independently developed the same concealing coloration. Apart from mimicry, there are great difficulties in maintaining this thesis. The habits of many brilliantly coloured animals clearly prove that they do not seek to hide themselves but rather to show off their bright colours.
Mention may be made of the method of concealment by disruptive coloration first described by Thayer. This is for the concealment of animals likely to be seen against two or more backgrounds. An animal, for instance, coloured green and brown in large areas, when viewed against a green background will be visible in respect of its brown areas, and as these brown areas will not have the shape of the animal but will be like one of the many oddments of nature (stones, leaves, etc.) so the animal will be mistaken for one of these. It will be similarly concealed against a brown background: in this case, only the green areas will be noticed. In these cases concealment is effected by disrupting the characteristic outline of the animal. The white areas which many animals present, are considered to be for disruption against the background of the sky, as, for instance, when a partially white bird in a tree is viewed against the low horizon. Many experimental findings and field observations form the basis of these conclusions.
Another use of pattern is to give a blurred or indistinct appearance to an outline. It is common to find, along the margin of the wings of butterflies and moths, a very small black and white, or contrasted, pattern which is visible at short distance; at longer range the pattern blends and then the margin has an indistinct appearance, causing the insect to fade into its back- ground. The finely spotted, barred and striped patterns of many mammals and birds are similarly effective in concealing the outlines at distance the spots cannot be seen.
The solidity of an animal is concealed by what is now called counter-shading, namely, by the darkening of surfaces exposed to the light, and the whitening of those in shadow. Large dark spots or broad dark stripes on the back, gradually changing to small spots and narrow stripes on the under sides, has the same concealing effect and the added advantage of a blurred outline when the animal is viewed at a distance. The very remarkable striped pattern of the zebra has been considered to be of this nature. In some animals not only is their solidity thus concealed, but a false solidity or modelling is superimposed. A flat surface is often made to appear rough or uneven: this is effected by an arrangement of light or dark tones used in a manner precisely similar to that by means of which the artist produces the delusion of a solid object on a flat canvas.
The use of colour in animal coloration can at present only be very imperfectly understood because little is known of the colour vision of animals. There is some evidence that animals have a colour perception less sensitive than man; if such prove to be the case, then a ready explanation for several dimorphisms is available. For instance, among insects it is common to find that whilst some individuals of a species are green, others are brown; but towards a colour perception slightly less sensitive than man's, these colours will be indistinguishable, and thus against either green or brown backgrounds neither the brown nor green individual would have the advantage. A common defect in man's colour sense is an inability to distinguish red from green; it is possible that such a colour as the red of the robin's breast against green foliage may serve to conceal the bird from its enemies.
When bright colours are used for revealment, as shown by the animal's habits, and by the particular pattern and position which the colour occupies, then advantage appears to be taken of the fact that red is a very conspicuous colour at high illuminations and blue at low illuminations. The brilliant inhabitants of the forest present a preponderance of blue, whereas red is more often found among those living in the open. This also applies to flowers, the bluebell typifying the colour for woods and the poppy for the open.
Experiments have shown that it is possible to determine, in many cases, whether a given pattern is for concealment or revealment: for instance, it has already been mentioned that concealment may be effected by a pattern which breaks out along the animal's margin and thus tends to conceal its characteristic shape. Conversely, patterns which follow an animal's margin and tend to accentuate its characteristic shape and separate it from its surroundings, make for revealment. A pattern of this kind, commonly seen in butterflies, is a broad black band following the outer margins of both wings and often enclosing a brilliant yellow or blue central area. Experimental evidence of this kind, as well as that derived from a study of an animal's habits, is strongly against the view that all coloration is for concealment.
The concealment of cast shadow is commonly brought about by the crouching or squatting of either hunted or hunter. Among butterflies Marshall has pointed out two methods of avoiding cast shadow: Certain species when resting on the ground with closed wings will tilt over the wings, generally away from, sometimes towards the sun, thus reducing and hiding the shadow cast upon the ground. Other species will settle on the ground with wings spread and orient themselves so that either their head or more often their tail is pointed at the sun. Should a bird or other enemy come near they at once close the wings over the back and then only a line shadow of the wings is cast on the ground.
Sematic Colouring.—Passing on to the consideration of sematic or signaling coloration, certain advances have been made. Feeding experiments have shown that the preyed-upon can be arranged in a series, for any given preyer, from the most to the least palatable. The former will be eaten when the preyer's hunger is almost satisfied; the latter, only when the preyer is starved. The colours of this series are then found to be arranged from cryptic coloration corresponding to the very palatable, to a revealing (warning) coloration for the very unpalatable. Further, it has been observed that in many cases, whilst concealment by a cryptic coloration is the usual form of protection, nevertheless, a revealing coloration is exhibited to the preyer, when concealment has failed. This revealing coloration, such as the hind wing of a moth, is as a rule hidden by a cryptically coloured fore wing, and is only revealed at the last moment to advertise a relative unpalatability. There is also evidence obtained from watching feeding birds, that revealing coloration is used for deception in the manner of a conjuring trick. Thus, the Leaf butterfly (Kallina) when flying appears blue and yellow, but directly it settles with closed wings it becomes like a dead leaf: the pursuing bird will continue to look for the blue and yellow insect among the dead leaves in which the insect has taken refuge. Many grasshoppers and moths look red and blue when flying, but on settling, these bright colours are at once hidden.
Swynnerton has suggested the substitution of “distinctive coloration” for “warning coloration”; the assumption is that the preyer will remember the distinctive colours and patterns, and associate them with their varying palatability. In support of this there is much evidence to show that many animals have a good memory in this respect.
Mimicry.—In view of the fact that insects can be graded in respect of their palatability, the distinction between Mullerian and Batesian mimicry appears difficurt to maintain. A set of animals presenting common warning coloration (Mullerian mimicry) are never equally unpalatable, and therefore it may be said that the relatively palatable of this set are of the nature of Batesian mimics. Although in extreme cases a distinction may be drawn, nevertheless intermediate cases occur which it is impossible to classify in this way. In the study of mimicry many notable advances have been made, chiefly among butterflies. Several insects which were thought to be different species, or varieties, have lately been shown by breeding-experiments to be polymorphic forms: the same species mimicking sometimes one species and sometimes another, both forms being bred from the same mother. Also it has been shown that, in situations where models are scarce, the mimicking species presents transitions between its various polymorphic forms, and this fact is considered to indicate that natural selection is required to maintain a mimicry. At one time it was thought that butterflies had few enemies, and that therefore their remarkable mimicry could bear no relation to natural selection; however, evidence that they are eaten by birds to a considerable extent has been brought forward by several observers.
Sexual Coloration.—Secondary sexual coloration still gives rise to much speculation. The Darwinian view that it represents selection by the female is still held by some observers. Others consider that it serves the purpose of stimulating the sexual instinct of the female; or that it is related to the different habits of the male and female, as, for instance, the incubation of eggs; or that it represents a difference in value to the species between male and female, making the conspicuous but less valuable male more likely to be destroyed by enemies than the inconspicuous and valuable female. These various theories are mentioned to show that no general law to explain these colorations has been accepted. Much valuable field work has been done in which many new facts as regards sexual displays have been collected.
Chemistry of Animal Colours.—It has been shown that, in the case of the lobster and salmon, the colours which the males assume at the breeding-season are due to the laying down of a coloured waste product in the scales and shell. This waste product is finally got rid of when the scales become worn and the shell cast. In the case of the female the waste product is discharged in quite a different manner; it is deposited in the eggs and disposed of when they are laid. In certain parts of England and Germany, chiefly in the neighbourhood of large towns, many species of moths have developed melanotic forms, or these dark forms have greatly increased in number. Much work has been done in an endeavour to discover the cause of this change, as it was at one time thought to be a case of the acquirement of a dark coloration for concealment against sooty surroundings, and thus to be an example of the rapid action of natural selection. Melanism, however, occurs in other districts, distant from large towns, more particularly near the sea. Recently evidence has been brought forward that this change is due to the particular feeding of the caterpillar; that, in fact, a deposit occurs on the leaves near large towns and near the sea, which causes this change.
Physics of Animal Coloration.—Several eminent physicists have taken an interest in, and attempted to explain on a physical basis, the brilliant metallic and iridescent colours of many insects and birds. Although a physical explanation of the coloration of most objects is available the brilliant colours of these animals remain a mystery. A recent summary by the late Lord Rayleigh in the Philosophical Transactions may be quoted:—“These colours are probably structural rather than pigment, but still much remains to be effected towards a complete demonstration of the origin of these effects. Even if we admit an interference character questions arise as to the particular manner and there are perhaps possibilities not hitherto contemplated.”
It has been suggested that fluorescence plays a part, and to test this insects have recently been examined in a beam of ultraviolet light. It was found that the brilliantly coloured species are not fluorescent. A few Lepidoptera were found to be fluorescent and this character has been found to be of some service in classification, as the property appears to be limited to closely allied species when it occurs in a group.
Presidential Address Ent. Soc. (1911); Howard, British Warblers; Longstaff, Butterfly Hunting in Many Lands (1912); Marshall, Trans. Ent. Soc. (1909); Mottram, Proc. Zool. Soc. (1915, 1916, 1917) and Controlled Natural Selection (1914); Poulton, Essays on Evolution (1908); Punnett, Mimicry of Butterflies; Pycraft, The Courtship of Animals (1913); Swynnerton, Journ. Linn. Soc. (1919); Thayer, Concealing Colouration in the Animal Kingdom (1909). See also thearticle Camouflage.