JOHN-ALDER, Henry B.; COX, Robert M.; LEO, Angela; SKELLY, Stephanie: Growth Inhibition by Testosterone in Eastern Fence Lizards (Sceloporus undulatus)

In most populations of Sceloporus undulatus, adult females are about 10% larger than adult males, even when controlling for age. In our population, SSD arises because females grow faster than males prior to sexual maturation. However, the causes of this sexual growth dimorphism are not well understood. Here, we investigate the effects of testosterone (T) on growth in juvenile Sceloporus undulatus in the laboratory and in an enclosed area of the natural habitat. In the laboratory, sham-operated and castrated males and intact females were implanted with T-filled or placebo Silastic capsules, and growth was measured for about 2 months. In the field, only males were used as in the laboratory experiment. Plasma T was not significantly higher in sham-operated than in castrated males, suggesting that the juveniles’ testes had not attained high levels of T production. Experimental levels of plasma T were well within physiologically relevant limits established by field-active males. In the laboratory, growth rate did not differ between males and females, but T inhibited growth in both sexes and caused females to develop male-like pigmentation. In the field, T supplementation inhibited growth in castrated males relative to castrated and sham operated males receiving placebo implants. T-implanted males in the field moved greater distances and exhibited increased activity, thus suggesting potential behavioral and energetic pathways through which the effects of T on growth could be mediated. These results indicate that sexual differences in growth rate, which lead to SSD in S. undulatus, may be caused by sexual differences in testosterone production during development.