Peacocks and turkey-cocks alone can spread wheel-wise their fan-tails, as also the cock bustard; they alone are provided with great wattles. The menure hen lifts, as the cock, a lyre of feathers, but it is a tarnished and mediocre imitation of her master's, which glistens in all shades rising and curving with such paradoxical grace.
The dimorphism of birds of paradise is even more marked than in the preceding cases. Nape citron-yellow, throat green, forehead black, back in burnt chestnut, the cock's tail has two long plumes, his flanks two fine tapering feathers of yellow-orange marked in red, which he can spread branching or draw in at will; the dim female is without ornament. The sifilet, a bird related to the birds of paradise has, fixed between eye and ear a pair of fine plumes twice the length of his body, which float as he walks like white blue-shimmering streamers. It is a lover's paraphernalia, which the female in consequence does without, while the male loses his after mating.
The dissemblance of barnyard cock and hen are well enough known to give everyone a clear idea of dimorphism in birds and to show difference of characters parallel with difference of form.
The dimorphism of mammals is even less often favourable to the female than is that of birds. One can cite but the sole example of the American tapir where the male is smaller than the female.[2] The contrary is nearly always the case. Sometimes the two sexes have an identical appearance: cougars, cats, panthers, servals. If there is a rule, it is difficult to formulate, for side by side with these felines without sexual dimorphism, the sex of lions and tigers clearly determines their forms.
Among mammifers there are bizarre resemblances and baroque differences. The he and she mole, at first sight, appear the same even to their exterior sexual organs, the female's clitoris is, like the male's penis, perforated to let the ureter pass through it. But here, as we shall see later, the morphologic resemblance by no means indicates similarity of characters; the female mole is excessively female. There is baroque difference of sexes in the capped seal of Greenland and Terra Nova. The male can puff out his head-skin into an enormous helmet. To what purpose? Possibly to scare naïve enemies. True to her rôle of protégé the female can not throw this bluff, which is used by Chinese warriors, by certain insects like the mantis and by the cobra among serpents.