Thinking Inside the Box: Comparative Limb Bone Shape in Emydid Turtles

Limb bone morphology often correlates with functional demands placed on animals by the environment. Comparisons of limb bone allometry in functionally divergent turtle taxa indicate highly specialized lineages show extensive flattening of the humerus. In Sea Turtles this contributes to flipper-shaped forelimb morphology that facilitates lift-based swimming (i.e., underwater flight). In contrast robust humeri and femora in terrestrial tortoises may reflect specializations for resisting high torsional loads during terrestrial walking and digging. However, it is unknown whether allometric patterns of ecomorphological divergence can be detected among more closely related lineages within clades that encompass species with diverse ecological habits. To test whether limb bone size and shape vary among closely related taxa that live in divergent habitats, we used phylogenetic comparative methods to assess scaling patterns and overall morphology of the humerus and femur of 27 emydid turtle species representing four genera: Graptemys (semiaquatic), Pseudemys (semiaquatic), Trachemys (semiaquatic), and Terrapene (terrestrial). In general, we found that limb bones of emydid taxa scale isometrically for most length-diameter and length-mass relationships. However, terrestrial Terrapene (box turtles) species exhibit relatively short femora compared with species from the more aquatic genus Trachemys (sliders). The relatively small limb bones of box turtles may promote limb withdrawal into the shell, but without mechanical costs because of high safety factors of turtle limb bones. The comparatively robust forelimb and hindlimb dimensions of Crap temys (map turtles) relative to other emydid Glades may reflect habitat and foraging pressures in this group.

Automated summary

The morphology of limb bones in turtles is closely related to their functional demands in different environments. Specialized lineages show specific adaptations in their limb bone shapes depending on their habitats, such as flattened humeri in sea turtles for lift-based swimming and robust humeri and femora in terrestrial tortoises for resisting high torsional loads during walking and digging. However, the allometric patterns of ecomorphological divergence among closely related lineages within clades with diverse ecological habits remain unknown.