Bite-Force Generation and Feeding Biomechanics in the Loggerhead Musk Turtle, Sternotherus Minor

Ontogenetic growth can profoundly affect the ability of organisms to perform ecologically-relevant feeding tasks that ultimately impact survival. In particular, bite-force generation is exceedingly important for vertebrates that process and consume robust prey (i.e. durophagy). Consequently, bite-force generation used in durophagy is a suitable parameter to investigate the functional relationships between musculoskeletal biomechanics, feeding performance, and ecology. I studied the ontogeny of bite-force generation and feeding biomechanics in the durophagous turtle, Sternotherus minor. Across an ontogenetic series of 75 S. minor, craniofacial growth was characteristized by allometric increases (i.e. positive allometry) in the dimensions of the head and beak. Moreover, bite-force generation increased with positive allometry relative to body and head dimensions. These results indicate that ontogenetic modifications to the lever mechanics of the jaw system, and/or changes in the size (i.e. mass) and/or physiology (e.g., fiber lengths, degree of pennation) of the jaw adductor musculature have more explanatory power for bite-force generation than external measures. A detailed, quantitative examination of the musculoskeletal biomechanics was performed to elucidate how these animals are capable of generating disproportionately high bite forces throughout ontogeny. Mechanical levers, muscle masses, and muscle architecture (fiber lengths and pennation angles) were measured from an ontogenetic series of 30 S. minor. With these data, a biomechanical model of the feeding apparatus was developed that accurately predicts individual and ontogenetic scaling of bite forces. Increasing muscle masses and changing the muscle architecture resulted in an increase in total physiological cross-sectional area (PCSA) of the jaw adductor muscles that was proportional to changes in bite-force generation. These results indicate that the disproportionate increase in bite-force generation relative to skull length found in S. minor is explained by allometric changes in muscle size and architecture that collectively act to allometrically elevate the PCSA and muscle force. The alternative explanation of improving the mechanical leverage was not supported. Dietary data indicated that the positive allometry in bite-force generation observed in S. minor is tightly linked to the incorporation of exponentially larger snails into the diet and positive allometry of the forces required to fracture the largest dietary items. These forces were found to be greater than the observed and theoretical bite forces, which suggested that fatigue failure resulting from multiple bite-force loadings may allow S. minor to fracture snails at lower compressive forces and access large snails that are apparently outside the range of their bite-force capacity. Moreover, age-based growth patterns for bite-force generation fit a logistic growth curve and reveal a close relationship with the forces required to fracture snails found in the diet. The results of this study provide empirical evidence that ontogenetic changes to musculoskeletal morphology and ecology are inextricably linked by the performance of the feeding apparatus in S. minor. Such results are exceedingly important for establishing a baseline from which to explore the mechanistic and functional issues that underlie the evolution of phenotypic traits among vertebrates.