The Courtship of Animals

by W. P. Pycraft

In this new light, the gaily-bedizened individuals of the Insect world may be surveyed afresh. The explanation of such of their features as are commonly attributed to Sexual Selection in terms of female choice, whereby only the most favoured from among a crowd of suitors could hope to succeed, may now be replaced by that which obtains also in the case of the higher animals. It seems to fit the facts better. One cannot understand, for example, how, on the interpretation of Sexual Selection, the extraordinary disparity in numbers between the sexes of some species of Butterflies came about. Thus in that marvellously beautiful genus Ornithoptera there is one species (O. brookiana) in which the females are excessively rare; so much so that the collector Kunstler could only obtain fifteen females to one thousand males. Though the males, among the Butterflies, are commonly much more numerous than the females, the disparity is rarely so great as with this species; but there are many in which the proportion of males to females is as fifty to one. As with the higher vertebrates selection affords no explanation of this curious disproportion. Though according to Weismann it fulfills “the first postulate in ‘Sexual Selection’ namely, that there be an unequal number of individuals in the two sexes.” But Sexual Selection here has a little over-reached itself, for surely one hundred suitors seems an embarrassing number for an inexperienced female to have to choose from! To say nothing of the ninety-nine males doomed to perish without leaving offspring.

That the beauty of colour and form which the Lepidoptera, and especially the diurnal Lepidoptera, or Butterflies, exhibit is due to the choice by the females—albeit an unconscious choice—of the most resplendent of her suitors, that is, in other words, that she yields at last to the most ravishing member of the crowd—there is no evidence to show. There would seem to be no possibility of a differential selection from among a number of males, for there is no “display” comparable to that, say, of birds. And what is more, it is unlikely that, if there were, she would find anything to choose between them, for the range of variation in, say, one hundred males of any given species is very slight. Finally we have no trustworthy evidence to show that the eyes of Butterflies and Moths are sufficiently good to enable them to make nice distinctions between slightly different males. We have no evidence that the eyes of Insects are capable of discriminating the details of the often intricate patterns which their own wings, and those of their suitors, exhibit.

In the matter of “Secondary Sexual Characters,” indeed, the Lepidoptera exhibit very little difference between the sexes. As a rule the females are larger, often strikingly so, but in the matter of coloration they show far less disparity. But there are exceptions to every rule. A striking illustration of this is afforded by the genus Ornithoptera. The butterflies of this superb group are of huge size, and the females are larger than their consorts, and commonly are extremely different therefrom both in coloration and habits. In Ornithoptera paradisea this disparity attains its maximum. The female, remarks Mr. David Sharp, “is a large, sombre creature of black, white and grey colours, but the male is brilliant with gold and green, and is made additionally remarkable by a long tail of unusual form on each wing.” But a glance at the two sexes will show that the female, though less gorgeously arrayed, still disports a livery which is of a highly specialized or elaborated character. How are we to account for her differences in shape, size and coloration on the older interpretation of Sexual Selection? The perceptual powers, the mentality, of a Butterfly are surely of a far lower grade than those of a bird, or even a fish. Here, therefore, we cannot attribute the same possibilities of response to form and colour which we can ascribe with tolerable safety to the vertebrates. Yet the Sexual Selection theory as generally understood demands this.

So far so good. And now as to the part played by Sexual Selection among the Lepidoptera. Darwin, in formulating this, found its application to the Lepidoptera a very disconcerting problem, being naturally disposed to regard the extraordinary wealth of colour which these insects exhibit as the outcome of a process of female selection, in every way comparable to that which he held to obtain among the birds. He did not postulate a conscious, deliberate, selection; but a final abandonment on the part of the female to the male which, by his beauty and demonstrativeness, pleased her most. He assumed that at this critical time she would always be surrounded by rival suitors, offering varying if slight degrees of difference: and, indeed, in many cases she is thus surrounded. He remarks, in discussing the case of Butterflies: “The males sometimes fight together in rivalry, and many may be seen pursuing or crowding round the same female.” But in the case of the Silk-moths—and here is another illustration of the merciless criticism to which he submitted his own theories—he remarks: “The females appear not to evince the least choice in regard to their partners.” This fact, which is certainly true in the case of both Butterflies and Moths, and these gorgeous hues, disconcerted him, as is shown in the passage: “Unless the female prefer one male to another, the pairing must be left to mere chance, and this does not appear probable.” The facts which have come to light in regard to the “Courtship” of Butterflies since Darwin wrote are meagre enough, but such as have been recorded give no support to the supposition that the females are really influenced by, or even perceive the colours of, their mates. Just on five-and-twenty years ago the naturalist Skertchly published some observations on the Courtship of that magnificent Bornean Butterfly Ornithoptera brookiana. He one day came on a male sipping honey from the flowers of a tree, vibrating its wings with the rapidity of a Hawk-moth, and the vivid green of the wings flashing in the sunlight, though the crimson areas thereof were invisible. The female came “and did all the wooing.” They circled about in flight with the female above and somewhat behind, so that she could see, we are told, the emerald markings; but there was no real evidence here that she was really influenced by his coloration, and if this really were the case then the coloration of the female equally demands an explanation, for this, though less gorgeous than that of the male, is far from a primitive type; on the contrary, it is of a highly differentiated character. Furthermore, in this genus, as has already been remarked, the males outnumber the females by, roughly, one hundred to one. Again, Moseley, the naturalist on the Memorable Voyage of the Challenger in 1872, when in the Aru Islands, was once “lucky enough to find a flock of about a dozen males fluttering round and mobbing a single female. They were then hovering slowly, quite close to the ground, and were easily caught.” But he was by no means convinced that any choice was exerted. And he suggests “a series of experiments, in which, in the case of highly-coloured and decorated Butterflies, the colours should be rubbed off the wings of a few among a number of males, or painted over of a black or brown colour. It might be tested whether the females would always prefer the highly-coloured ones.” Such experiments are foredoomed to failure, for the removal of the scales would remove the only source of communication between the sexes.

Wallace, always a strenuous opponent of the Sexual Selection theory, found in the behaviour of Butterflies and Moths when mate-hunting a particularly powerful countervailing weapon. He assumes that Darwin postulated a conscious selection on the part of the female, and with some show of reason, though it is probable that Wallace was mistaken in this. “The weakness of the evidence for conscious selection among these insects,” he remarks, “is so palpable, that Mr. Darwin is obliged to supplement it by the singularly inconclusive argument, ‘Unless the female prefer one male to another the pairing must be left to mere chance, and this does not appear probable’ But he has just said, ‘The males sometimes fight together in rivalry, and many may be seen pursuing or crowding round the same female’ While in the case of the Silk-moths—‘the females appear not to evince the least choice in regard to their partners.’ Surely the plain inference from all this is, that the males fight and struggle for the almost passive female, and that the most vigorous and energetic, the strongest-winged or the most persevering wins her. How can there be chance in this? Natural Selection would here act, as in birds, in perpetuating the strongest and most vigorous males; and as these would usually be the more highly coloured of their race, the same results would be produced as regards the intensification and variation of colour in the one case as the other.”

Commenting on Darwin’s interpretation of those cases wherein the females are more brilliantly coloured than the males, he insists that on his (Darwin’s) theory “throughout the whole animal kingdom the males are usually so ardent that they will accept any female, while the females are coy, and choose the handsomest males, whence it is believed the general brilliancy of males as compared with females has arisen.”

“Mr. Darwin admits,” he continues, “that these bright colours have been acquired for protection [because they resemble those of species which from their disagreeable taste are avoided by birds and other insect-eating enemies]; but as there is no apparent cause for the strict limitation of the colour to the female, he believes that it has been kept down in the male by its being unattractive to her. This appears to me to be a supposition opposed to the whole theory of Sexual Selection itself. For this theory is, that minute variations of colour in the male are attractive to the female, have always been selected, and that thus the brilliant male colours have been produced. But in this case he thinks that the female Butterfly had a constant aversion to every trace of colour, even when we must suppose it was constantly recurring during the successive variations which resulted in such a marvellous change in herself. But the case admits of a much more simple interpretation. For if we consider the fact that the females frequent the forests where the Heliconidæ abound [the distasteful species already referred to] while the males fly much in the open and assemble in great numbers with other white and yellow Butterflies on the banks of rivers, may it not be possible that the appearance of orange-stripes or patches would be as injurious to the male as it was useful to the female, by making him a more easy mark for insectivorous birds among his white companions? This seems a more probable supposition than the altogether hypothetical choice of the female, sometimes exercised in favour of, and sometimes against, every new variety of colour in her partner.”

Wallace’s arguments are not so crushing as he supposed them to be, and they contribute nothing towards the solution of the problem to be faced. But if colour played the part which Darwin believed, and colour alone be concerned, it is curious that the males should recognize their mates in a guise so unlike their own. How is it that they do not pass them by as members of the totally different distasteful species? Whenever, indeed, the female is more or less brightly liveried than the male, how do the sexes recognize one another, and how, when they live in environments so different as those referred to by Wallace, do they find one another when possessed by the insistent demands of the “sex-hunger” which is the all-essential stimulant to secure the continuation of the race?

The factors which assure the satisfaction of this hunger differ in some important features from those which obtain among the higher animals—birds, for example. In the first place there is no necessity to find and hold territory, which is an imperative necessity where there are eggs to be brooded and young to be fed. In the second, the males, as has just been remarked, must search for the females, often indeed, in the case of many Moths, because they are wingless.

This search is conducted by the sense of smell. This fact, familiar enough to-day to the entomologist and the student of Evolution, was unknown to the earlier naturalists. Neither Darwin nor Wallace suspected it. It would have been wonderful if they had, for there is nothing in the general appearance of these insects which suggests an organ of smell, nor is there anything in the structure of the nervous system which would indicate this subtle sense. During recent years, however, the number of workers engaged on the investigation of the senses of animals has increased immensely, and great strides have been made in perfecting instruments of research. To the efforts of these workers we owe the discovery of the seat of the scent-detecting organs and the source of the scent. The former are furnished by the antennæ, which lodge also the senses of taste and touch.