The Courtship of Animals

by W. P. Pycraft

After the birds, probably the group most conspicuous for its splendour is that which contains the Scale-winged Insects or Lepidoptera, and it has always been allowed that any explanation of the one must apply also to the other. It seems impossible to avoid this conclusion. But before going further it would be well to take note of one or two interesting features in regard to coloration that have so far not been touched upon in these pages.

The Coloration of Animals is generally regarded as a by no means fortuitous feature, but one, on the contrary, controlled and determined by various factors. Hence are recognized various kinds of coloration: Obliterative or Protective-resemblance Coloration; Warning Coloration; Mimetic Coloration; and Epigamic Coloration, or the colours associated with courtship. These various types have been subdivided and accorded technical labels by Professor E. B. Poulton, in his “Colours of Animals,” but these need not be enlarged upon here. Suffice it to say that it is generally held that all forms of coloration can be explained, and all can be labelled, as to their origin, with more or less certainty. There are those who doubt the warranty for this classification. Commonly, it must be admitted, the arguments of these sceptics are not impressive; they are sometimes even stupid. That such coloration, however it be labelled, is subjected to some control seems to be shown in the case of the Lepidoptera, for, generally, in the Butterflies, the upper surface of the wings is much more vividly coloured than the under surface, and this, apparently, because when the creature is at rest the wings are brought up over the back like the leaves of a book, so that the brightly-tinted, and therefore conspicuous, area is concealed, as, for example, in the “Red Admiral.” With the Moths the wings, while the creature is at rest, are held horizontally, and it is the upper instead of the under surface which is exposed, but the hind-wing is covered by the fore-wing. The coloration is here very different; for while the exposed surfaces of the fore-wings are commonly soberly tinted, the hind-wings may be quite glaringly coloured. These bright colours are exposed only during flight, or during moments of unusual excitement, as in the case of the Eyed Hawk-moth. According to Weismann, this insect when alarmed raises the fore-wings so as to expose the “eye-spots “on the hind-wings, which, with the increased area of the wings, impart a terrifying appearance to the body to would-be assailants. This is as it may be, but for the moment the feature to be insisted upon is that the bright colours are almost invariably hidden when the insect is at rest, and by quite different means, determined, apparently, by the different carriage of the wings. Now, according to some, bright colours are begotten by strong light, but in the Moth and Butterfly the surface area of the wing which is most exposed is the surface turned to the light during rest, and this is the least coloured. The curious relation between this coloration and the resting position is strikingly illustrated by the case of one of the “small Blues” (Lyccenæ), cited by Weismann. Herein the male, which has the upper surface of the wings of a bright blue, rests in the position common to Butterflies—with the wings raised and concealing the bright colour—while the female, which has the upper surface of a dull brown, rests with the wings expanded. As, however, the concealed under surface is not brightly coloured, it is difficult to believe that these different postures and conspicuously different colours can have been brought into existence solely by the action of Natural Selection, which, it is generally contended, has brought about the extinction of those individuals which neglected, when resting, and therefore liable to be “caught napping,” to conceal their arresting colours. There is, indeed, no apparent reason why the female, which has nothing to conceal, should depart from the custom common to Butterflies, of resting with the wings closed and raised, this position effectively protecting the male. The facts seem to show that the coloration of the exposed surfaces of the wings is determined primarily by some physiological factor rather than by the incidence of Natural Selection directly through external agencies. 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. Selection imposes a bar only to certain types of coloration.

Some Butterflies and Moths, it has just been hinted, when resting exhibit bright colours. Our “Swallow-tail” the under surface of the wings is as brightly tinted as the upper. Among the Moths may be cited many of the gorgeous Atlas Moths, the Hawk Moths, the beautiful Indian Dysphania militaris—wherein the whole of the exposed surface is of a beautiful and vivid violet and yellow—and the tropical members of the Burnet Moths, belonging to the family Syntomidæ. In all these cases it is not the under but the upper surface of the fore-wings which has thus departed from the usual rule of the tribe. Not the least remarkable feature of these insects is the fact that while the Atlas and Hawk Moths are crepuscular in habits, the Dysphanias and Syntomids and Burnet Moths are diurnal, and revel in the sunlight.

To revert for a moment to the factors to which these and other bright and often conspicuous hues are due. That all highly-coloured animals are descendants of dull-coloured ancestors there can be no room for doubt. The vivid tints they now display are to be regarded as due to some change in the metabolism, some clarifying process of the organism whereby the various pigments became segregated, concentrated and intensified. But many of the most vivid hues are not due to pigment at all, but to changes in the surface structure of the coloured areas. Such are the wonderful metallic colours which all kinds of animals display. The iridescence is due to the breaking up of the light by reflection from finely-grooved surfaces.

Whatever their nature, one still asks what is their origin, what brought them into being. They cannot be regarded simply as adaptations which have arisen to meet the demands of the environment, as are the structural peculiarities of the skeleton for example; for in this case both sexes, and all stages of growth, should display the same hues, and this is rarely the case. Furthermore, we should not in this case be left with a vast assemblage of forms which certainly cannot be “pigeon-holed” as to the nature of their coloration. Such, for example, as the marine types of birds.

The metallic and iridescent tints to which reference has just been made, occur among animals to which they can be of but doubtful value, as in the Golden Mole, for example, or the inside of the Oyster shell. Their existence in such places well illustrates what we may call the fortuitous, or apparently fortuitous, beginning of colour of whatever kind, regarded from an analytical point of view. That is to say, we are not concerned with the fact that animals are coloured—that is inseparable from their existence; but with why this coloration should, in some cases, assume so conspicuous a brilliancy and vividness—a coloration varying in its character with every species, but apparently unchanging among the individuals of that species.

No answer to this, likely to find general acceptance, seems to be forthcoming at present. But it is significant to remark that all coloration of the kind now under consideration has its origin, as have most other structural characters, in the male. It is as true of coloration as of, say, skeletal characters. One turns to the male for what is new in the history of a species, to the female and young for indications of past history. It is equally true that in their coloration one finds the same sequence of development—the male first, then the female, then the young, till both sexes, and all stages, are once more alike in hue. And this rule seems to apply to coloration of all kinds—Protective—Warning—Epigamic.

The tendency to develop brilliant colours is associated with some physiological diathesis with which we are not yet acquainted. But once having started, this tendency gathers force with each succeeding generation and continues to exhibit an almost kaleidoscopic capacity for change, unless, and until, checked by Natural Selection, whereby its further progress in any given direction may be barred, or some other element or aspect of the coloration may be introduced.

Given this controlling factor, all the various types of coloration would seem to be interpretable. By almost common consent, however, the resplendent coloration of the males among many species of birds, a coloration often apparent only during the reproductive period, and the more conspicuous ornamentation of the males of many other groups, higher and lower in the scale of organization, are supposed to be governed by an entirely different factor—female choice, or preference. The exercise of this, it is contended, has gone on for countless generations, and the tendency has ever been to heighten the intensity of the ornament by the rejection of the less favoured suitors in favour of their more resplendent rivals. Birds and Butterflies alike are supposed to be swayed by the same irresistible desire to mate, and mate only with what we may call the smartest and best—groomed of their many suitors; and these, of course, being the most vigorous, most virile, sustain the stamina of the race and so attain Nature’s end.

So long as attention was focused alone, or mainly, on birds conspicuous for the highly ornamental character of their plumage, this theory seemed reasonable and probable enough, for one may admit in their courtships an element, at least, of intelligence and keenness of perception. But it has now been abundantly demonstrated that the animated displays so characteristic of these gaily-bedecked gallants, are enacted with no less persistence and vim by species which exhibit a Quaker-like soberness of dress. Thus, then, the champions of the Sexual Selection theory have been dazzled by the tinsel, and have missed the essential elements—the physical and psychological side of the display—the contortions, prancings, and so on, and they have missed the even more important element, the preliminary struggle for territory.