The Courtship of Animals

by W. P. Pycraft

The hermaphrodite conditions of these animals, as with other Mollusca in like case, present some knotty points for consideration, and especially in regard to the problem of sex-attraction. Where each individual is as much male as female, which is the dominating factor in desire, the maleness or the femaleness? Though each individual contains both ova and sperm cells, probably these ripen at different times, to avoid danger of self-fertilization. In this case the sex impulses are on the same footing as in the case of animals wherein the sexes are not thus combined. That is to say, the individual which is for the moment only potentially male mates with another for the moment only potentially female. But this being so, how does each discover the condition of the other?

Many of the Snails, like Helix nemoralis, are gaily coloured. Are these hues, these bands of black and yellow, the product of “sexual selection”—the outcome of a process of selection from among the most conspicuously coloured individuals as postulated by the Darwinian theory of Sexual Selection? If so, then this choice must be regarded as a periodic recurrence coinciding with the period during which the individual is dominated by its female attributes. In due course it becomes, for the time, a male, and may find itself rejected, owing to a lack of intensity in its coloration, or, on the other hand, it may vanquish a rival by its very splendour. Each, in short, would help materially in this process of beautification. If the choice of mating for it is this rather than a choice of mates—proceeds on these lines, the bright coloration of the members of this species becomes easy to understand. But does it? It is more than doubtful whether the eyes of Snails are sufficiently good to distinguish the coloration of their neighbours’ shells, or for the matter of that of their own, for their eyes being carried on long mobile stalks, they should have no difficulty in contemplating their own charms. And what of Snails of more sober hues? It seems highly probable that here, as in so many cases, scent is the selecting factor, and the coloration is an “accidental” feature. That the colour of the shell plays no such part as that just postulated may be gathered from the evidence afforded by many marine species, whose shells, though conspicuously marked, are, during life, completely enveloped and concealed by the all-investing, fleshy mantle. In like manner the exquisite beauty in the form and sculpturing of the shell which so many species exhibit, are characters which cannot be regarded as due to sexual selection.

As touching the danger of self-fertilization to which reference has been made. That this is real is shown by the fact that the ova and spermatozoa are rarely ripe in one individual at the same time. However, among the pulmonata, or air-breathing gastropods, it seems to have been established that self-fertilization can, and does, occur. That in some species, at any rate, where cross-fertilization, for some reason, is impossible, the individual thus isolated can store up its own spermatozoa to be used in fertilizing its own eggs. But the fact that this rarely happens is testimony enough that such occurrences are inimical to well-being.

The Lamellibranch, or bivalve Mollusca, e.g., Oyster, Mussel, and Cockle, afford valuable evidence as to excrescences and extravagances of growth which appeal to our eyes as ornamental, and therefore likely to be due to the influence of sexual selection. And this because such ornamentation is a very conspicuous feature among these animals. Yet, save in a few cases, locomotion is impossible, and sight is wanting. Light-distinguishing organs, and therefore eyes, are possessed by some, but in no case probably are they strong enough to appreciate form. Even if they did, such revelations of beauty would play no part in mate selection from among the most ornamental; for these creatures are commonly fixed throughout life in one position, often, indeed, buried in mud or sand. Some move laboriously: a few, like the Cockles and Pectens, swim by rapidly opening and closing the shell. The Pectens are brilliantly coloured, not only as regards the shell, which is also beautifully sculptured, but the foot also is of a vivid scarlet, and the Pecten have numerous minute eyes. But the Cockles and Mussels possess like attributes as to colour and sculpture, yet they are blind. More to the point is the fact that these animals do not mate after the fashion of higher animals, but the males, where the sexes are distinct, discharge immense quantities of spermatozoa into the water, and these find their way to the ova of the female through the action of the inhalent currents set up by the animal for the purpose of drawing in fresh supplies of water containing food and oxygen. There are no “secondary sexual characters,” that is to say, that even where the sexes are separate, and many, like the Oysters, are hermaphrodite, they are externally indistinguishable. Nevertheless, many, as has been already remarked, have shells of great beauty. As, for example, the giant Tridacna and the strangely spinous valves of the “Thorny Oysters” (Spondylidæ).

The fact that the Lamellibranch, or bivalve molluscs, are far less numerous in point of species than the univalve tribes is accounted for by the fact that in the first place they are of necessity aquatic, and in the second their means of locomotion is extremely limited. Some few species swim spasmodically: some crawl: many are incapable of movement when once the motile larva settles down and the shell-bearing adult stage is attained. Such species can extend their range only by means of larval wanderings. Enormous numbers, millions, of young have to be produced and set adrift each year by every adult in the community, and yet but a few of each brood can ever attain to maturity. Life, for such species, must be a dull, monotonous business: the only opportunity for excitement is that which is preliminary to being eaten, and the only purpose in life is to be eaten. But happily Oysters don’t think. They and their kind are simply semi-conscious living things, responding mechanically to stimuli. Any approach, then, to beauty, either of form or coloration, or both, must be regarded as due to innate, inherent changes in the germ-plasm affecting the parts so made conspicuous: the only form of selection to which such “ornaments” can be subjected is Natural Selection. If, and when, such ornaments penalize their possessor either by their cumbrousness or their conspicuous characters, or by increasing the difficulty of feeding or distributing offspring, then the further development of such excrescences is checked by the death of all individuals which have passed the bounds of endurance in this respect.

Sex, and all that appertains thereto, in short, is in these creatures reduced to its lowest terms. There are not wanting, to-day, both men and women, who affect to believe that all would be well for the human race could a similar slowing-down, or strangulation, of the sexual instincts be brought about. Such blind leaders might profitably contemplate the Oyster: but such contemplation, to be profitable, requires intelligence of a higher order than these protagonists of folly appear to possess.

In justice to Darwin it should be remarked that he himself fully realized, and carefully points out, the inconceivability of the application of the Sexual Selection theory to the Mollusca. In commenting on the beauty of colour and shape which many species display, he remarks: “The colours do not appear in most cases, to be of any use as a protection; they are probably the direct result, as in the lowest classes, of the nature of the tissues[1]: the patterns and the sculpture of the shell depending on the manner of growth.” Just so: and this is surely the fundamental explanation of ornament, using this term in its widest sense, everywhere in the Animal Kingdom. The peculiarities and eccentricities of behaviour, which occur among the higher groups, act as “aphrodisiacs” to hasten reproduction because this confers an advantage, the earliest to produce offspring—so soon as the conditions for their nurture are favourable—having the best chance of survival. Premature sexual activity is checked by the death of the offspring.

[1] Italics mine.

It has been contended that the hermaphrodite condition represents the primitive mode of reproduction among the multicellular animals—that is to say, all animals above the level of those whose bodies are composed of but a single cell, or particle, of protoplasm—but this view is probably erroneous, and the hermaphrodite state must be regarded as a secondary condition, a later innovation.

More remarkable are the facts concerned with that singular form of reproduction known as parthenogenesis, or the production of offspring by virgin females. This is undoubtedly a degenerate sexual condition occurring as a normal mode of reproduction, among the microscopic “Rotifers,” e.g. the “Wheel-animalcule,” Crustacea, and Insects, and in varying degrees of intensity.