The Courtship of Animals

by W. P. Pycraft

Photos copyright, D. Seth-Smith.

THE KAGU IN DISPLAY.

What is true of the Sun-Bittern is true also of the Kagu.

It is significant that whenever bright colours appear, they do so first in the males, the females and young retaining the dress common, up to this time, to the species at all ages. In the majority of instances, at any rate, it would seem that this accession of colour appears with the seasonal re-awakening of the reproductive activities: it forms a “nuptial” dress, and is discarded after the breeding season is over for a livery indistinguishable from that of the female, this forming the so-called “winter plumage.” But if all the available facts are taken into consideration there seems good reason to believe that the nuptial plumage tends to be assumed earlier and to be retained later, as this disposition to develop ornament gathers force, till finally only the head and neck go into “eclipse,” as in the case of the Black-cock, Jungle-fowl and Partridge.

In the Pheasant we have an instance—one of hundreds—where the resplendent dress is worn throughout the year. The next phase in the direction of the growth of colour occurs when the female, towards old age, develops a more or less well marked tendency to assume the hues of her lord, and this accession of colour makes its appearance earlier and earlier in succeeding generations, till finally the adults of both sexes are coloured alike, save that, as a rule, the female lacks the intensity of coloration which her mate displays. The original sombre dress is now only worn by the young. In due course the resplendent dress is assumed also by the young, as witness the numerous instances among the Kingfishers and among the Parrots, where adults and young are all habited in the same vivid hues. There are infinite variations of these changes which cannot be discussed here, for obvious reasons. All that matters now is the fact of such sequences, which inevitably raise the questions: Why, in so many cases, do the females show no disposition to assume resplendent colours? And to what factors can such coloration, when it occurs, be attributed? The second only of these questions is germane to the present discussion, and to this no very satisfactory answer can be returned.

To say that the development of brilliance in species hitherto sombrely clad is due to “changes in the metabolism” is only an affectation of wisdom. What we want to know is what induces the changes? Time was when no more than a guess could be hazarded as to this: a suggestion that ornament, of whatever kind, was one of the many modes of the expression of that instability of the organism which is characteristic of living things: that it was one of the outward and visible signs of that inward, intangible tendency to vary which is so familiar. Later research seemed to show, fairly conclusively, that ornament was one of those “secondary sexual characters” which was dependent on the stimulating juices, or “hormones,” emanating from the primary sexual glands. To-day it is manifest that this is only partly true, for it is certain that these glands are not alone concerned and they may only participate indirectly. It seems to have been clearly demonstrated that the thyroid and pituitary glands, or the “hormones” therefrom, play a large part in this matter of the “secondary sexual characters.”

Castration, it is true, profoundly affects these characters. In the case of Deer it inhibits the growth of antlers, in Cattle the horns are increased in length but reduced in thickness—they are longer than those of the female, but resemble them in appearance, and further, the whole stature is greatly increased, but it is at the same time conspicuously less massive, particularly at the neck and fore-quarters. In eunuchs it results in immense stature and the loss of the more characteristic male features, such as the beard and the bass voice. The removal of the testes in birds is always a difficult operation and is rarely successfully performed. Hence the accounts of changes in plumage consequent on this operation are inconclusive. It has generally been supposed that whenever, either by removal or by disease, the testes are rendered inoperative the plumage, when normally of a resplendent type, assumes the coloration of the female. This is probably an erroneous supposition, but what happens is a failure to secrete the more intense pigments and the more specialized forms of feathers, so that the resultant dress answers to the juvenile male dress. It is not a case of “reversion” to this livery, but a failure to assume the latest acquirements of the species. These, as has already been shown, are only very gradually developed. The intensity of pigmentation, or concentration of pigmentation, which results in sharply defined areas of colour, is a cumulative process. As it loses in intensity at any given moult, so the individual tends to reproduce the phases of the earlier and vanishing livery. Sooner or later, however, this earlier livery disappears more or less completely: is eliminated from the system, so to speak: and what is commonly called lack of “vigour” results, not in a return to the earlier, sombre dress, but in the later-acquired, resplendent plumage lacking intensity. The seasonal, temporary secondary sexual character has become, as some say, a “somatic” character. Highly probable as this view appears, it ought, it may be argued, to receive support from nestling plumages. Young gulls, for example, should occasionally revert from the mottled to the earlier striped livery. But we have no evidence of this; and it does not follow that this sequence of events should occur. The conditions of control are different.

What exactly are the factors which govern the evolution of resplendent plumage is not known. But they would seem to be more complex than was supposed. That the primary sexual glands play an important part, through the juices or “hormones” which they liberate, there can be no doubt but these are only partial factors. The “hormones” of the pituitary and thyroid glands are also necessary contributors, controlling as they do both fertility and the more superficial characters, such as colour and ornament. Evidence, indeed, is slowly accumulating to show that the problem of the behaviour of animals during the period of sexual activity, as well as the peculiarities of structure and coloration which they develop at this time, are all largely governed by the action of these secretions.

These, in their turn, are undoubtedly inhibited, or increased, by the control of the nervous system, though this control is of course involuntary. This much seems clear from the fact that birds will display when under the excitement of fear, though the character of that display is never the same as that in moments of sexual exaltation. If the nervous system, through the eye, by “suggestion,” played no part, there could be no use for display, but it is equally certain that for the realization of the sexual activities a number of other factors have to contribute.