With Ruminants sexual differences of colour occur more commonly than in any other order. A difference of this kind is general with the Strepsicerene antelopes; thus the male nilghau (_Portax picta_) is bluish-grey and much darker than the female, with the square white patch on the throat, the white marks on the fetlocks, and the black spots on the ears, all much more distinct. We have seen that in this species the crests and tufts of hair are likewise more developed in the male than in the hornless female. The male, as I am informed by Mr. Blyth, without shedding his hair, periodically becomes darker during the breeding-season. Young males cannot be distinguished from young females until above twelve months old; and if the male is emasculated before this period, he never, according to the same authority, changes colour. The importance of this latter fact, as distinctive of sexual colouring, becomes obvious, when we hear[355] that neither the red summer-coat nor the blue winter-coat of the Virginian deer is at all affected by emasculation. With most or all of the highly-ornamented species of Tragelaphus the males are darker than the hornless females, and their crests of hair are more fully developed. In the male of that magnificent antelope, the _Derbyan Eland_, the body is redder, the whole neck much blacker, and the white band which separates these colours, broader, than in the female. In the Cape Eland also, the male is slightly darker than the female.[356]
In the Indian Black-buck (_A. bezoartica_), which belongs to another tribe of antelopes, the male is very dark, almost black; whilst the hornless female is fawn-coloured. We have in this species, as Mr. Blyth informs me, an exactly parallel series of facts, as with the _Portax picta_, namely in the male periodically changing colour during the breeding season, in the effects of emasculation on this change, and in the young of both sexes being undistinguishable from each other. In the _Antilope niger_ the male is black, the female as well as the young being brown; in _A. sing-sing_ the male is much brighter coloured than the hornless female, and his chest and belly are blacker; in the male _A. caama_, the marks and lines which occur on various parts of the body are black instead of as in the female brown; in the brindled gnu (_A. gorgon_) "the colours of the male are nearly the same as those of the female, only deeper and of a brighter hue."[357] Other analogous cases could be added.