In the course of enumerating the various loci where hormones affect brain, endocrine, and exocrine function, I have often noted that these loci were not only activated in adulthood, but also organized by gonadal hormones around the time of birth. At this point, it may be useful to recapitulate these various organizing effects, in order to show how they determine the male and female differences in reproductive states
The presence of androgens around the time of birth organizes a number of neural mechanisms in male muroid rodents. These include the male sex motivational mechanism (brain site 4), an olfactory filter that is tuned to androgen-dependent pheromones and that facilitates offense and inhibits exploration/marking (brain site 7), another olfactory filter that is tuned to estrogen-dependent pheromones and that facilitates exploration/marking while it inhibits offense (brain site 8), and motor patterning mechanisms for the spinal reflexes of male sexual behavior (brain site 10). Although, as far as I know, data are not available, one may suppose that perinatal androgen, also organizes the tissue which will later secrete androgen-dependent pheromones. These organizing effects predispose an animal to have male patterns of sexual and offense behaviors and to show exploration/marking in response to female s rather than males.
The presence of androgen around the time of birth suppresses a number of neural mechanisms that would otherwise develop as they do in females. These include the female s ex motivational mechanism (brain site 3), the parental motivational mechanism (brain site 5), and an olfactory filter that is tuned to androgen-dependent pheromones and that facilitates the exploration/marking motivational mechanism (brain site 9). If these mechanisms are not suppressed by perinatal androgen then the animal develops the female patterns of sexual and parental behavior and shows exploration/marking behaviors in response to males rather than to females.
These effects oŁ androgen or androgen absence at birth predetermine and specialize the lives of females for child-rearing and the lives of males for territoriality and fighting. Why should there be such a strong degree of specialization? My favorite explanation has been offered by Orians (1969) in terms of the differential metabolic cost of reproduction for males and females among vertebrates that live on the land.
In fish there is little need for specialization. In reptiles, birds and mammals, however, the female must specialize to provide nourishment to her young. Since the young of these land vertebrates, unlike fish and amphibia, cannot recapitulate the early life history of their phylogenetic ancestry by growing as small larvae in the water, they must depend for growth upon egg yolk or placental and parental feeding that are provided primarily by the mother. As a result female mammals must double their metabolic intake during pregnancy and lactation, while the male has little additional metabolic demand a t this time. The laws of supply and demand come to operate as a result of the asymmetry of "value" of male and female gametes. The female egg is very "valuable"; thus the males compete actively for it and the female 1s "choosy" about the male who should be allowed to fertilize it. The male's sperm is "cheap," and the female need not compete for it; nor does the male need to be choosy about the females with whom he copulates. Thus, while the female specializes for child-bearing, the male specializes for those aspects of behavior that are related to social competition, namely territoriality and fighting.