In certain breeds and varieties of the domestic fowl,
e. g
., the Brown Leghorn, the adult plumage presents a sexual dimorphism in the kind and distribution of colour in the individual feathers, and in one or more of the seven areas in which plumage colour differences may distinguish breed from breed, and variety from variety, and also in the structure of the feathers of the neck and saddle hackles, and by the presence of the large tail sickles of the male. In other cases, such as the White Leghorn, the sexes are to be distinguished only by the structural differences in the hackle feathers and by the large tail sickles of the male. In the case of certain other breeds, the Campines and the Sebrights, for example, the plumage of the male is identical with that of the female both in colouration and in structure. Cocky-feathering in the case of such varieties as the Brown and the White Leghorns can be regarded as a trustworthy indication that within the body there is, or was at the time when the plumage was developed, active functional testicular tissue; henny-feathering as an indication that there is, or was when the plumage was developed, active functional ovarian tissue. Gonadectomy in both sexes is followed after a moult by the assumption of a plumage which has the colouration of that of the male of the variety to which the bird belongs, whilst the barbules in the distal portions of the feathers of the hackle regions are absent, also a characteristic of the cocky-feathered male, but the plumage is much looser and far more luxuriant in its growth ; the plumage characters of the capon and of the poularde are exactly alike. Since this is the case, it is commonly argued that the gonads exhibit an endocrine function, the ovarian hormone possessing the faculty of inhibiting the development of cocky-feathering. The fact that in the Campine and the Sebright the cock is also hennyfeathered is explained on the assumption that in their functioning the testes of such a male are equivalent endocrinologically to the ovary of a hen. Such an interpretation is supported by the facts that castration of the normally henny-feathered cock is followed by the assumption of typical cocky-feathering ; that castration of the normally cocky-feathered male and subsequent successful ovarian implantation is followed by the development of henny-feathering; and that, in such cases as have so far been recorded, successful testicular implantation in an ovariotomised hen has been followed by the development of a plumage of the cock (as opposed to that of the capon or poularde). There is undoubtedly a considerable body of evidence, secured from careful experimentation, that supports the endocrine interpretation of the relations between gonad structure and plumage characterisation, and most biologists have accepted this interpretation. However, there are several facts concerning the fowl that cannot easily be reconciled to this hypothesis. The case about to be discussed in our opinion, severely questions its validity.
Courtship behaviour influences social partner choice in male guppies