Another factor to be considered in comparing races and species is the degree of differentiation between adult males and females in a population. This is called sexual dimorphism. It varies greatly both in mammals and birds. Male and female cardinals have feathers of different colors; yet it is difficult for a nonorni-thologist to tell a male from a female robin. Among the primates, a male gorilla may be twice as large as any member of his harem, whereas the only visible difference in gibbons in the wild is the protrusion, through the fur, of nipples in the female that has borne offspring.
Sexual dimorphism serves two principal purposes. First, it may be part of the selective process in mating, as when male birds strut their plumage in the nuptial ceremony, and as when stags lock their horns in mortal combat in competition for a doe. Second, among some animals that inhabit distinct territories, as for example lions, or baboons living in a forest, the exaggerated size and fighting equipment of the males permit them to serve the function of a border patrol in human communities. The male keeps rivals off his feeding ground and away from his wife or wives. Neither the male lion nor the male baboon is any better at obtaining food than his womenfolk; in fact, among lions the female excels at hunting. These animals expend their biological capital for territorial defense, just as we spend the bulk of our tax money for atomic submarines and missiles.
In fossil man there is evidence of sexual dimorphism, but it is clouded by the paucity of material available for study. In living races a great variability can be seen. Australian aborigines and western Europeans are highly variable; Mongoloids little. As Tibetans dress and wear their hair alike, it is sometimes difficult to tell whether any one person is a man or a woman. This does not mean that sexual dimorphism is the same as pedomorphy, for some populations with little sexual dimorphism are in certain ways gerontomorphic. No one could call a Plains Indian infantile, and his women can be huge and craggy. It is difficult, then, to decide whether certain racial traits, like the absence of a beard in many Mongoloid males, are the result of pedomorphy, of a lack of sexual dimorphism, or of some other aspect of the endocrine story yet to be discovered.
In any case, the presence or absence of marked sexual dimorphism is an inherited racial trait that distinguishes some living populations from others. This trait may date back to remote antiquity since it was not involved in the complex of evolutionary changes that led from Homo erectus to Homo sapiens. Of this we may be fairly confident because the two races that have achieved the greatest cultural advancement, the Caucasoid and the Mongoloid, stand at opposite poles in this respect. At the other end of the cultural scale, so do the Australian aborigines, who show marked sexual dimorphism, and the African Bushmen, who show little of it.