Meeting Abstract

S8-2.1  Jan. 6  Proximate Developmental Mediators of Sexual Size Dimorphism: Case Studies from Squamate Reptiles JOHN-ALDER, H.B.*; COX, R.M.; TAYLOR, E.N.; Rutgers Univ.; Ohio State Univ.; California Polytechnic State Univ., San Luis Obispo

Sexual size dimorphism (SSD) is nearly ubiquitous in squamate reptiles, but we know very little about the relative importance of sex differences in proximate genetic and environmental mediators of the development of body size. Recent studies from our laboratories suggest that males and females share similar genetic growth potentials but express different growth phenotypes and adult body size in the wild because of inherent ecological differences and potential epigenetic effects of sex-specific growth regulators. In several species of free-living Sceloporus lizards and in the rattlesnake Crotalus atrox, both male-and female-larger SSD develop because of sex differences in age-specific growth. Growth is responsive to food availability in both sexes, but sex differences in growth and body size are nearly absent in the laboratory in both Sceloporus and Crotalus, indicating a sex difference in sensitivity to environmental conditions. In field-active Sceloporus, sex differences in growth rate are associated with sexual divergence in plasma testosterone (T). Experiments confirm that T inhibits growth in female-larger species (S. undulatus and S. virgatus) but stimulates growth in male-larger S. jarrovii. However, both surgical castration and T replacement are without effect in captive S. jarrovii, possibly because growth effects of T are superseded by an ad libitum diet. Our results indicate that resource limits on growth differ between sexes in free-living squamates and that T can serve as a sex-specific, bipotential growth regulator, leading to SSD. Discrepancies between field and laboratory experiments suggest that epigenetic effects of T interact with energy balance to influence growth.