The effect of exogenous testosterone on growth and secondary sexual character development in juveniles of Sceloporus virgatus

Closely related species often vary considerably in the degree of sexual siz e dimorphism and in the expression of secondary sexual characters. Relative ly little is known about the proximate mechanisms underlying among-species variation in sexually dimorphic traits. I examined the influence of exogeno usly administered testosterone on body growth and on the expression of two possible secondary sexual characters (blue ventral coloration and femoral p ore secretion) in the striped plateau lizard, Sceloporus virgatus. Male and female hatchlings were given testosterone-filled or empty (control) implan ts at approximately 40 days of age. Testosterone-treated females experience d a significant reduction in the rate of growth of body mass and body lengt h over the six-month period following implantation. A similar but non-signi ficant trend for reduced rate of body growth was detected in testosterone-i mplanted males. Testosterone had no effect on the final size or intensity o f the blue throat patch, but the testosterone-implanted males increased in patch size more slowly than control males. Both males and females greatly i ncreased their femoral pore secretions in apparent response to the dose of testosterone. Testosterone-implanted males had greatly enlarged tail bases relative to those of control males. In this species, then, the lack of exte nsive blue ventral coloration is apparently not due to a corresponding redu ction of testosterone secretion in early life.