Physical apertures as constraints on egg size and shape in the Common Musk Turtle, Sternotherus odoratus

1, Egg size in turtles often increases with female size, contrary to expectations of optimality. Functional constraints on egg width imposed by the pelvic aperture or the gap between the carapace and plastron (the caudal gap) have been inferred for a few populations but appear inapplicable in others. 2, For Sternotherus odoratus (the Common Musk Turtle), the pelvic aperture was always wider than the width of the female's largest egg by at least 3.7 mm. The caudal gap was narrower than the widest egg for 25.7% of the females. 3, Egg width increased, and elongation (length/width) decreased, as female size and clutch size increased. 4, Females at three ecologically contrasting sites differed appreciably in size but produced eggs of the same mean shape and size, despite the strong within-site changes in both egg size and shape with female size. As the younger females at all sites were of similar age and produced eggs of similar size and shape (again, despite differences in body size), egg size and shape may be age-specific. 5, No optimal egg size prevailed but the scaled residuals of egg size with female mass were less variable than were those for clutch size.