Sex differences in the nervous system of reptiles.

1. The study of sex differences in the brain and behavior of reptiles presents an excellent opportunity both to discern general principles of sexual differentiation in the nervous system and to explore the evolutionary history of this process in amniote vertebrates. 2. Findings in several reptiles suggest that some sex differences found in mammals and birds are conserved while others are not. Conserved features include areas in the limbic forebrain involved in the regulation of social and sexual behaviors. As in mammals and birds, it is rare to find differences in the distribution of sex steroid concentrating neurons in reptiles but common to find differences in the distribution of the various steroid hormone receptors and in their regulation. 3. This research has revealed that differences in social and sexual behavior are reflected better by the activity, not by the size, of hormone-sensitive limbic areas. 4. Finally, species differences in plasma levels of sex hormones are paralleled by differences in behavioral sensitivity to these hormones as well as by differences in the regulation of genes coding for steroid hormone receptors.