Optimal egg size theory predicts that natural selection optimizes egg size within populations and most of the variation in reproductive output is attributable to clutch size variation driven by body size, available resources, and age. For small-bodied turtles, morphological (pelvic) constraint on egg size has been considered the main explanation when populations exhibit considerable variation in egg size, because the pelvis could be under selection for other functions besides reproduction. Kinosternids, a small-bodied and semiterrestrial lineage of turtles, show evidence for both pelvic and nonpelvic constraint on egg size. In order to test if small species show a tendency toward pelvic constraint on egg size, we examined possible pelvic constraints in 1 population of the small-bodied Kinosternon chimalhuaca from western Mexico and in 3 populations of medium- to large-bodied Kinosternon integrum from central Mexico. Gravid females were X-rayed to measure both pelvic aperture and egg width. To test for pelvic constraint on each population we compared the slopes of pelvic aperture and egg width (mean and maximum) to body size (plastron length) with analysis of covariance (ANCOVA); we also compared egg elongation with fresh egg measurements among populations using ANCOVA (with body size as covariate), and we conducted an allometric analysis in order to test for egg size optimization. We found evidence of pelvic constraint in 1 population of K. integrum, and evidence of nonpelvic constraint in the K. chimalhuaca population and in the other 2 populations of K. integrum. Our data did not support the supposed adaptive compromise and pelvic constraints on egg size in small-bodied turtles reported in other studies. Environmental factors such local pressure on egg size (stability of the environment) could explain this pelvic constraint discrepancy in kinosternids.
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