SEXUAL SIZE DIMORPHISM, DIET, AND REPRODUCTION IN THE MEXICAN GARTER SNAKE, THAMNOPHIS EQUES

Although adult T. eques females had a longer SVL and HL than males, the sexes did not differ in their diet, and we found that larger females had more offspring. These results suggest that sexual size dimorphism in natural populations of T. eques may be determined by fecundity selection rather than by the ecological factors associated with the spectrum of available prey sizes. The lack of intersexual differences in the slope of the regression between HL and SVL, and the upward shift in the intercept of the regression line for females together with the shift along the common slope toward larger females (data for larger females do not overlap with the data for larger males), showed that the larger heads of the females are probably an outcome of their larger bodies (SVL) and not of the allometric relationship between these traits.In this study we examined diet, reproduction, and sexual dimorphism in body size of adult Thamnophis eques. We measured 307 adult snakes within the hydrologic system of High Lerma. Prey was obtained by forced regurgitation when snakes were collected, and gravid females were housed in the laboratory until parturition. Females had a longer snout-vent length and shorter tails than males, but there was no difference in total length between the two sexes. We found no evidence to support the hypothesis that sexual dimorphism is due to differences in prey size for this species because the sexes did not differ in the type and mass of prey they consumed; however, the size of gravid females was positively related to the number, but not size and weight, of their offspring. Our results suggested that sexual size dimorphism in natural populations of T. eques may be determined by fecundity selection rather than the ecological factors associated with the spectrum of available prey sizes. We also discuss possible reasons why the sexes did not differ in total lengthUniversidad Autónoma del Estado de México 2077/2005U, 2365/2006, 2865/2010U Consejo Nacional de Ciencia y Tecnología (33710)