JOHN-ALDER, H.B.*; COX, R.M.; Rutgers Univ., New Brunswick; Rutgers Univ., New Brunswick: Growth Effects of Testosterone and Castration in Lizards (Sceloporus spp.) with Female- Vs. Male-Larger Sexual Size Dimorphism

Adults of most Sceloporus spp. (fence and spiny lizards) are characterized by either male- or female-larger sexual size dimorphism (SSD). In all described species, maturational sex differences in growth rate before first reproduction lead to adult SSD, but mechanisms of differential growth regulation are not understood. Testosterone (T) can inhibit growth in both male and female S. undulatus, and castration promotes growth in males of this species (female-larger SSD). Here, we report effects of castration alone (C) and castration plus T replacement (CT) on growth in yearling, male S. virgatus (female-larger) and S. jarrovii (male-larger). In a field experiment on S. virgatus, growth was inhibited in CT relative to C at all body sizes, but treatment effects were size-dependent relative to controls (CON). In small males, growth rate did not differ between C and CON but was significantly slower in CT. In larger males, growth rate did not differ between CT and CON but was significantly faster in C. In intact males, plasma T was positively correlated and growth rate was negatively correlated with body size. Thus, T inhibits growth in S. virgatus, and T-mediated growth inhibition may increase with body size in CON. In a laboratory experiment on S. jarrovii, growth rate was not influenced by surgical castration or T implantation, even though the effectiveness of both treatments was verified by independent response variables. Further experimental work is required to clarify whether T can explain sex differences in growth and to resolve apparent discrepancies among species, but our present results collectively suggest that testosterone can influence growth differently in species characterized by female-larger versus male-larger SSD. Funded by NSF IBN-0135167.