Ontogenetic scaling of cranial morphology and bite-force generation in the loggerhead musk turtle

The feeding systems of durophagous vertebrates are well suited for studying how the performance of feeding structures is affected by growth. For these animals, feeding structures that deviate from isometric growth (i.e. allometry) may be biologically meaningful in terms of disproportionate increases in bite-force generation across ontogeny. We measured body size, cranial morphology and bite-force generation in an ontogenetic series of loggerhead musk turtles Sternotherus minor and compared the scaling coefficients with predictions based on isometry. We found that morphological growth in S. minor is characterized by positive allometry in the dimensions of the head and beak (rhamphotheca) relative to body and head size. Because negative allometry or isometry in head size relative to body size is a nearly universal trait among vertebrates, S. minor appears to be unique in this regard. In addition, we found that bite forces scaled with positive allometry relative to nearly all morphological measurements. These results suggest that modified lever mechanics, and/or increased physiological cross-sectional area through changes in muscle architecture (i.e. fiber lengths, degree of pennation) of the jaw adductor musculature may have more explanatory power for bite-force generation than external head measures in this taxon. Lastly, we found that bite force scaled with negative allometry relative to lower beak depth and symphyseal length, indicating that the development of high bite forces requires a disproportionately more robust mandible. These results indicate how deviations from isometric growth may make it possible for durophagous vertebrates to generate, transfer and dissipate mechanical forces during growth.