The ducks furnish us with another very good example of the apparently haphazard nature of sexual dimorphism. In the Common Mallard or Wild Duck (Anas boscas) the cock is far more showily coloured than the hen, but in all the species most nearly allied to it the males are as inconspicuous as the females, e.g. in the Indian Spotted-bill (Anas pœcilorhyncha), the Australian Grey Duck (A. superciliosa), the African Yellow Bill (Anas undulata), and the American Dusky Duck (A. obscura). As the dusky duck inhabits North America, where the mallard is also found, the case is particularly striking.
Among mammals the lion and the tiger and the sable and roan antelopes (Hippotragus niger and H. equinus) furnish familiar examples of nearly-related species, in one of which the sexes are alike and in the other dissimilar in appearance.
Hormones
Another important point to be borne in mind is the intimate correlation that exists between the reproductive organs and the general appearance of the organism, more especially of the secondary sexual characters. These last, in most cases, do not show themselves until the maturity of the sexual organs. The well-known effects of castration illustrate this connection. Again, females in which the reproductive organs have ceased to be functional often assume male characters.
It has lately been proved by experiment that, in many cases at any rate, the development of the ornaments, etc., characteristic of the sexes is due to the secretion by the sexual cells of what are known as hormones—that is to say, secretions which excite development of the secondary sexual characters. The tendency to produce the external characteristics of the sex to which an organism belongs is inherited, but the actual development thereof is in many cases dependent on the secretion of these hormones. Accordingly, if a male individual be completely castrated it ceases to develop the external characters of its sex. The evidence upon which the doctrine of hormones is based is admirably summarised in the above-quoted paper by Cunningham. Into this evidence we cannot go. It must suffice that the doctrine is quite in accordance with all the observed results of castration.
It is worthy of notice that the various features which characterise the sexes in sexually dimorphic animals are not associated with any particular organ or parts of the body, nor do they necessarily affect the same part in allied species. “We cannot say,” writes J. T. Cunningham, “that any part of the soma (i.e. the body tissue) is specially sexual more than another part, except that such differences between the sexes are usually external. They usually affect the skin, and especially epidermic appendages, and the superficial parts of the skeleton, or whole limbs and appendages; or the difference may be one of size of the whole soma. In mammals and birds the male is often the larger, sometimes very much so, but there are cases in which the female is larger. There is no general rule.”
Another important point is, that females, although they themselves show no trace of the male character, are capable of transmitting it to their progeny. This can be proved by crossing a hen pheasant with a cock barn-door-fowl; the male offspring of the union display the plumes so characteristic of the cock pheasant. These cannot have been derived from the barn-door-fowl father; they must have come from the dull-coloured hen pheasant.
In this connection we may mention the curious fact recorded by Bonhote, on page 245 of the Proceedings of the Fourth International Ornithological Congress, that in the case of ducks descended from crosses between the pintail, the mallard, and the spotbill, the drakes in full breeding plumage showed a mixture of pintail and mallard characteristics, while, in their non-breeding plumage, the colouring of the spotbill is predominant.
Eye-colour, Comb, and Spurs
An important point, and one which does not seem to have been pointed out by any zoologist, is that eye-colour, comb, and spurs in birds and horns in mammals do not stand in the same relation to the sexual organs as do the other external characteristics. For example, the castrated Nilgai (Boselaphus tragocamelus) acquires horns, but not the characteristic male colour. In the common Indian Francolin Partridge (Francolinus pondicerianius), the cock differs from the hen only in the possession of spurs. The same applies to the various species of Snow Cock (Tetraogallus). There is a breed of game-cocks which display plumage like that of the hen, but such birds have the comb and spurs developed as in normally feathered cocks.