TRACY, C.R.: Geographic variation in sexual dimorphism of chuckwallas (Sauromalus obesus: Iguanidae): natural selection vs. sexual selection?

Chuckwallas (Sauromalus obesus: Iguanidae) show a considerable geographic variation in adult body size and dimensions, with adults of some populations nearly twice the mass and 130% the snout-vent length of other populations. Additionally, chuckwallas show sexual dimorphism in general morphology. However geographic variation in the proportional size of meristic characters has not been explored. I compared several morphological characteristics to determine the degree of geographic variation and sexual dimorphism in chuckwalla populations from several sites where individual lizards differ greatly in body size. I also used data from a common garden study of lizards from the same populations to determine the ontogeny of the geographic variation and size dimorphism. Male chuckwallas appear to be proportionally larger than females in all characters measured, even after adjusting for the overall larger body size of males. Lizards in the common garden study showed significant differences between sexes and among populations in growth rate of the measured characters. These patterns were apparent in common garden lizards even before size at sexual maturity. Variation in body proportions and sexual dimorphism does not appear to follow the same pattern as variation in body size, which can be predicted from an environmental gradient in temperature and rainfall. This variation may represent variation in the relative intensity of natural selection and sexual selection among populations that experience different environmental conditions.