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 female s are the larger sex. To understand which hormones may influence differenti al growth in this species, growth curves and hormone profiles of estradiol- 17 beta (E-2) and testosterone (T) were constructed in male and female neon ates. Growth was manipulated via implantation of exogenous hormones and hor mone antagonists. Female neonates are heavier or longer beginning at either 20 or 24 weeks of age, respectively. Although low circulating levels of E- 2 and T were present in males and females from birth through 15 weeks of ag e, these levels were not significantly different between the sexes. Differe nces in the growth curves of the treated and untreated snakes were signific ant after 24 weeks of age. Antiestrogen produced male-like growth in female s 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 E-2 reduced male growth. These results suggest that a basal level of either E-2 or T is sufficient in males to retain typical male growth patterns. Si milar endogenous levels of E-2 appear to have growth-promoting effects in f emales. Endogenous T does not appear to play a role in female growth. (C) 2 001 Academic Press.