Hormones and the development of sex differences in behavior
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Birds exhibit striking diversity in behavioral sex differences. A necessary complement to the study of the ecology and evolution of these sex differences is discovering the proximate physiological mechanisms for their development (sexual differentiation) and adult expression. Experiments with Japanese quail (Coturnix japonica) have shown that sex differences in crowing, strutting, and sexual receptivity are produced partly or largely by hormonal dimorphism in adulthood (activational effects of sex steroid hormones), whereas the sex difference in copulatory mounting is produced by permanent actions of sex steroids occurring early in development, during the embryonic period in this precocial species (organizational effects). Experiments with zebra finches (Taeniopygia guttata), an altricial species, have revealed organizational effects on singing and mating that occur after hatching. In both species, sex differences in whether birds are interested in females vs. males are produced by organizational rather than activational effects. Results of experimental manipulations of sex steroid actions in juvenile zebra finches suggest hormonal regulation of the onset of interest in the opposite sex as sexual maturity is reached. Zebra finches are socially monogamous and permanently paired across breeding attempts. Experimental reduction of sex steroid actions had no effect on pairing success in either sex. The regulation of adult pair formation by sex hormones is more likely to occur in species that pair seasonally. The concepts of organization and activation and the results of these experiments raise a number of questions and are a potential source of hypotheses about developmental changes responsible for the evolution of species diversity in sex differences.