The influence of sex steroids on the sexual size dimorphism in the red-spotted garter snake, Thamnophis sirtalis concinnus.

The red-spotted garter snake exhibits adult size dimorphism in which females are the larger sex. To understand which hormones may influence differential growth in this species, growth curves and hormone profiles of estradiol-17beta (E2) and testosterone (T) were constructed in male and female neonates. Growth was manipulated via implantation of exogenous hormones and hormone antagonists. Female neonates are heavier or longer beginning at either 20 or 24 weeks of age, respectively. Although low circulating levels of E2 and T were present in males and females from birth through 15 weeks of age, these levels were not significantly different between the sexes. Differences in the growth curves of the treated and untreated snakes were significant after 24 weeks of age. Antiestrogen produced male-like growth in females but had no effect on males. Antiandrogen had no effect on either males or females. Exogenous T reduced female growth to that observed in males, and E2 reduced male growth. These results suggest that a basal level of either E2 or T is sufficient in males to retain typical male growth patterns. Similar endogenous levels of E2 appear to have growth-promoting effects in females. Endogenous T does not appear to play a role in female growth.