Uses of Colour

USES OF COLOUR It makes for clearness to distinguish (a) primary physiological significance in the everyday life of the organism, such as is pos sessed by chlorophyll and haemoglobin; (b) secondary physi ological significance, such as is illustrated when a pigment is in terpretable as a waste-product or the like in the normal meta bolism; and (c) ecological or bionomic value in the external activities and inter-relations of the organism.

In the same way structural details, such as are due to rhyth mically punctuated growth, as in shells and scales, must have a primary physiological or developmental explanation, but this is not inconsistent with a secondary utility in the struggle for ex istence. Many peculiarities that give rise to structural coloration are primarily the ripple-marks of growth, but these may furnish the raw material for secondary utilization as an ornament or a disguise.

In some cases a dark superficial colour, due to a deposition of melanin, may protect the animal from too much sunshine. The intensely black pigmentation of the peritoneal lining of the body-cavity of some fishes, amphibians and reptiles is not known to be of use, but in the choroid of the eye it helps to make a dark chamber ; and it is not easy, nor particularly profitable, to draw a hard and fast line between such an internal utilization and the external concealment that is afforded when a cuttlefish dis charges the melanin of its ink-sac, throwing dust in the eyes of its enemies.

For a warm-blooded animal in cold surroundings the dress that conserves most heat is a coat of white fur or white feathers, as in the ermine, Arctic fox, mountain hare, ptarmigan and snowy owl. But this is not inconsistent with trying to find a physi ological reason for the development of gas-vacuoles and the re moval or non-production of pigment. Nor is it inconsistent with proving, if it can be proved, that the whiteness is of protective value in making the ptarmigan, let us say, inconspicuous against a background of snow. It is possible that this secondary utility holds for one creature, such as the ptarmigan, yet not for another, such as the mountain hare, though the two agree in not being per manently white, but in changing to white when winter sets in. Thus it often happens that after heavy snowfall, when everything edible is buried deep, the mountain hare seeks the bare low grounds where it is staringly conspicuous.

Concealing Coloration.

The colour of an animal often makes it very inconspicuous in its natural environment. This is an obvious fact ; but it does not follow that the resemblance has survival value by concealing the animal, for that requires to be proved statistically. This has been done in the case of the praying mantis, for Cesnola demonstrated experimentally that the green variety of this quaint insect usually escaped the eyes of birds when tethered on green foliage, though not when the leaves were withered; and conversely with the brown variety. The degree of protection afforded must be measured in each case, and that is the more difficult since the cryptic coloration may hide the animal from the eyes of some of its enemies, but not from others. The same critical estimate of advantage must be made when the colour-resemblance is supposed to favour aggression, as when the white ermine steals over the snow on the crouching bird. The concealing coloration may be general, like the brown lizard in the desert, or it may be detailed, as when the disruptive pattern of the ground-bird breaks up the outline of the body. The resemblance is enhanced when the form of the body, or part of the body, as well as its colour, resembles some object in the ordinary environment, as in the case of leaf-insects or the squat spider, Ornithoscatoides, which is like a bird's dropping on a leaf. The colour resemblance is usually permanent, but it may be seasonal, as in the ptarmigan, or momentary as in the case of those flat-fishes that assimilate their colouring very rapidly to that of the adjacent sand or shingle. An obvious importance attaches to cases where the coloured animal appears to choose a particular spot on which it is inconspicuous ; thus Longstaff re ports cases of butterflies which alight on backgrounds which match their own coloration.

Warning Colours.

Since many animals are able to form associations rapidly, and since many can discriminate differences in the intensity of illumination as well as certain colours as such, there is no a priori reason for rejecting the theory that conspicuous coloration on the part of an unpalatable or troublesome animal may serve as a noli-me-tangere advertisement, impressing itself on the memory or reflex-registrations of an enemy, and thus con ducing to the greater security of the race. E. B. Poulton has given experimental evidence of the protective value of the warning coloration of various unpalatable insects; for frogs, lizards and birds to which they were offered learned after a little experience to leave them alone. Some of G. D. H. Carpenter's field observa tions in Africa point strongly in the same direction. Thus a large bloated grasshopper, Dictyophora laticincta, which emits when grasped copious bubbles of strongly smelling yellow froth, is in the habit of exposing its short scarlet wings when disturbed, but hardly troubles to move out of the way. It was pulled to pieces and tasted, but not eaten by a young monkey who would eat more ordinary grasshoppers with zest. It must not be inferred that startlingly conspicuous colouring necessarily implies un palatability, or that it necessarily serves as a warning advertise ment, but there is reason to accept the theory in some well investigated cases.

Recognition Colours.

It is often advantageous that mem bers of the same species should recognize their kindred rapidly, partly in relation to the possibility of pairing, and partly because kindred are not likely to be hostile. The recognition may be effected by smell or by touch or by characteristic sounds, but it is sometimes visual, especially among the higher animals, such as birds and mammals. Without including cases where the courting male displays signals which interest and excite the female, there is abundant evidence of immediate recognition by sight, and to this visual recognition the colouring of the animal may contribute. The cotton-tail of the rabbit may be a case in point.

Courtship Colours.—Even among Invertebrate animals there are many instances of colour-display on the male's part before mating (see COURTSHIP OF ANIMALS). The fiddler-crab (Gelasi mus) waves his luridly coloured and extraordinarily exaggerated great claw when a female hoves in sight. Many observers have satisfied themselves that brightly-coloured male spiders show off their good points before the coy females at the breeding season. Colour-display reaches a climax in birds, where the male is often gorgeous in comparison with the female, as in birds of paradise, pheasants and peacocks. It is necessary, of course, to be careful in attaching too much importance to the colours as colours, for many birds are relatively colour-blind. Thus various experiments, apart from courtship altogether, indicate that many birds are unable to discriminate blue as such. Allowance must be made for the form of the decorations, their suggestive movements, the varying reflections of the plumage, and so forth, but in many in stances colour as colour seems to be a factor in the display which excites the interest and attention of the female bird, and may eventually evoke a sympathetic sexual echo.

Besides the uses of colour that have been mentioned, there are others. Thus there are the difficult phenomena of mimetic coloration, sometimes protecting the "mimic" from its enemies, sometimes enabling the "mimic" to be aggressive on the strength of its resemblance to the "model." A palatable butterfly may be protected from the attacks of birds by its mimetic resemblance to an unrelated unpalatable butterfly with which it consorts, or a spider may do certain aggressive things under the shield of its close resemblance to its companion ants (see MIMICRY).

As an instance of the more occasional uses of colouring we may refer to the bright colours inside the mouth of some nestlings. W. P. Pycraft has pointed out that the conspicuousness of the colour when the young birds gape, may enable the parents to supply the food with greater rapidity and precision.

Ecological Classification of Animal Colours.—Excluding colours of direct physiological value, Poulton classified the ecolog ically "significant" colours in the following scheme, which is very useful, though its technical Greek terms are somewhat forbidding.

I.

Apatetic colours, resembling the environment or some part of it.

A. Cryptic colours, concealing-resemblances.

I. Procryptic colours, resemblances for protection. 2. Anticryptic colours, resemblances for aggression.

B. Pseudosematic colours, false warning and signalling.

I. Pseudo aposematic colours, protective mimicry.

2. Pseudo episematic colours, aggressive mimicry and alluring coloration.

II. Sematic colours, warning and signalling colours. I. Aposematic colours, warning colours.

2.

Episematic colours, recognition markings.

III. Epigamic colours, displayed in courtship.Iii. Epigamic colours, displayed in courtship.

The subject of animal coloration is obviously fascinating and is apt to induce a natural enthusiasm that interferes with the sceptical mood of the scientific inquirer. Hence several cautions must be kept in mind—(a) No matter how apparently useful a particular type of coloration may appear in the observer's eyes, its utility to the animal must be proved experimentally or statis tically. This must not indeed be pressed too hard, for if the field naturalist repeatedly observes a predatory animal pass close beside a cryptically coloured brooding bird, that is in itself a very convincing fact. (b) When a certain utility in the coloration is highly probable and is illustrated by many analogous cases, it may be provisionally accepted; and yet there may be some other greater utility that is left undiscovered. For many years naturalists were more or less content to say that the whiteness of certain animals that frequent snow-covered areas was an adapta tion for concealment—giving them a cloak of invisibility against the white background. But in some cases the protective value is very dubious, and in most cases the primary utility is to be looked for in the conservation of the animal heat. Similarly, it seems reasonable to believe that many desert animals are pro tectively concealed by their sandy colouring, and yet there is probably some other value in the remarkable frequency of this brownish colouring in this particular environment. (c) Care must be taken to distinguish differences in the intensity of illumin ation of the surface of the animal from differences in colour as colour. (d) Whenever it is assumed that an animal is apprecia tive of colour as colour, as in a courtship display of blackcock, an enquiry must be made into the presence of a colour-sense in this type of animal. More than that, the enquiry must probe into the question whether the animal can discriminate the colours that are most prominent in the courting display or in the warn ing signals. Of recent years numerous reliable data have been accumulated in reference to colour-sense in different kinds of animals, and in regard to the not infrequent occurrence of partial colour-blindness. (e) In many cases it is also well to raise the previous question, whether the coloration has any ecological util ity at all. The colouring of many deep sea animals may have no more external significance than the colouring of the withering leaves in autumn. In all cases the first question should be into the biochemical nature of the pigment, or into the developmental reason for the structural details on which the coloration depends; after that, the secondary significance of the coloration, if it has any, may be more profitably investigated. (See also FISHES, S. V. Coloration, Vol. 9, p. 3o8c.) BIBLIOGRAPHY.--K. Semper, Animal Life (1881) ; C. F. W. KrukenBibliography.--K. Semper, Animal Life (1881) ; C. F. W. Kruken- berg, Grundziige einer vergleichenden Physiologie der Farbstoffe and der Farben (Heidelberg, 1884) ; E. B. Poulton, Colours of Animals (189o) ; F. E. Beddard, Animal Colouration (1892) ; M. I. Newbigin, Colour in Nature (1898) ; G. Bohn, L'evolution du Pigment (1901) ; P. Klingksiegk and I. Valette, Code des Couleurs (1908) ; G. H. Thayer, Concealing-coloration in the Animal Kingdom (1909) ; A. S. Pearse, Animal Ecology (1926) . A. TH.)