Sex Steroids and the Differentiation of Avian Reproductive Behavior

Experiments in which avian embryos are treated with sex steroids or steroid antagonists suggest that sexual differentiation of reproductive behavior (and thus differentiation of the brain mechanisms for such behavior) is controlled by steroids produced by the embryonic gonads. In chickens and Japanese quail, males hatched from eggs treated with estradiol or testosterone during incubation are feminized (demasculinized); they fail to exhibit masculine sexual behavior as adults, and no longer are behaviorally distinguishable from females. Some evidence suggests that testosterone may mimic the feminizing action of estradiol by being converted to an estrogen in the embryonic brain. Genetic female quail exposed to an antiestrogen during embryonic development are masculinized; they exhibit an increased ability to display the masculine copulatory pattern. Thus the behavior of these species is feminized by embryonic exposure to sex steroids, the anhormonal (neutral) sex for behavioral differentiation appears to be the male, and females appear to result from estrogen produced by the embryonic ovaries. In contrast, sex steroid treatment of mammals early in development masculinizes behavior, the female is the neutral sex, and males result from fetal androgen secretion. These opposite patterns of psychosexual differentiation in birds and mammals are correlated with a major difference between the avian and mammalian sex-determining mechanism. Implications for other vertebrates are discussed.