Physiological Variation and Allometry in Western Whiptail Lizards (Cnemidophorus tigris) from a Transect across a Persistent Hybrid Zone

A hybrid zone involving Cnemidophmus tigris punctilinealis (formerly gracilis) and C. tigris marnoratus in southwestern New Mexico and adjacent Arizona is narrow and characterized by abrupt and concordant change in both morphological characters and allele frequencies studied by protein electrophoresis. We compared adult C. tigris sampled from three locations that span the hybrid zone. Body mass was positively associated with both treadmill endurance at 1.0 km/h and maximal sprint running speed on a high-speed treadmill, although the largest individuals were not the fastest sprinters. Males and females differed significantly for maximal sprint running speed, liver mass, and kidney mass (ANCOVA with body mass as covariate). We found no statistically significant population differences for body mass, maximal sprint running speed, standard metabolic rate at 40C, blood hematocrit levels, or heart mass. Hybrids tended to have lower treadmill endurance running capacities as compared with the pure forms, but the difference was not statistically significant. Cnemidophorus t ip ' s punctilinealis and the hybrids both had significantly heavier kidneys, relative to body mass, than did C. tigris marnorah. Hybrid individuals also had significantly heavier livers as compared with either pure population. However, the present data cannot rule out the possibility that the observed differences in organ masses were related to reproductive status as opposed to being genetically based population differences. Thus, our results do not suggest that hybrid individuals differ from nonhybrids with respect to Darwinian fitness.