Sexual Dimorphisms in Avian and Reptilian Courtship: Two Systems that Do Not Play by Mammalian Rules

Abstract

Sexual dimorphisms in the central nervous system exist in numerous vertebrate species, and in many cases these structural differences between males and females parallel differences in the display of reproductive behaviors. Often both the behavioral and anatomical differences are controlled by exposure to gonadal steroid hormones, either during ontogeny or in adulthood. This article reviews some of the evidence supporting the hypothesis that in mammals, testosterone or its metabolites regulate the structure and function of neural and muscle systems involved in the control of masculine sexual behaviors. It then describes data suggesting that the mechanisms regulating sexually dimorphic courtship systems in zebra finches and green anole lizards are not completely parallel to the mammalian systems. Finally, some directions for future study are suggested, with the hope that they will stimulate thought about the nature of comparisons made across vertebrate models when investigators are attempting to determine both which morphological sex differences are important to the control of the reproductive behaviors, and which mechanisms regulating both structure and function are widely employed or are unique.

Zebra finch sexual differentiation: The aromatization hypothesis revisited

“…The volumes of RA and HVC and the size of neuron somata within those brain regions completely paralleled genetic sex, regardless of when the Fadrozole was administered (on embryonic day 3, 5, 8, 12 or 13; on days 4 and 7; or on days 1-12 or 1-13) (Gong et al, 1999;Springer and Wade, 1997;Wade, 1999;Wade et al, , 1999, with one exception that we could not replicate, a small but significant decrease in RA volume in adulthood in males that had received Fadrozole on embryonic day 5 . Behavior was tested in adult birds treated on embryonic day 5 (Springer and Wade, 1997) and 12 or 13 , and the aromatase inhibition and/or presence of testicular tissue did not sex-reverse the development of courtship or copulatory behavior in either males or females (in fact the earlier treatment appeared to enhance, or hypermasculinize, singing in males).…”

Parental Responsiveness Is Feminized after Neonatal Castration in Virgin Male Prairie Voles, but Is Not Masculinized by Perinatal Testosterone in Virgin Females

“…The inability of perinatal TP to masculinize and defeminize parental behavior in female prairie voles is reminiscent of the inability of perinatal hormone manipulations to equate the sexual behavior potentials of males and females in some nonrodent species such as rabbits (Campbell, 1965) and ferrets (Baum, 1976;Baum, Gallagher, Martin, and Damassa, 1982). In addition, a conceptually opposite phenomenon, such that the brain and behavior of females can be masculinized by perinatal administration of gonadal hormones but that males cannot be demasculinized or feminized by perinatal blockade of gonadal hormone activity, occurs in some species of birds (Arnold and Schlinger, 1993;Wade, 1999).…”

The Orthodox View of Brain Sexual Differentiation

“…Castration shortly after hatching has no effect, and anti-estrogen treatment seems to hypermasculinize male zebra finch brains. Juli Wade discusses these problems in the birdsong system and offers a new and exciting explanation for these discrepancies in her paper in this issue [Wade, 1999].…”