Ontogenetic changes in jaw-muscle architecture facilitate durophagy in the turtle Sternotherus minor

Differential scaling of musculoskeletal traits leads to differences in performance across ontogeny and ultimately determines patterns of resource use during development. Because musculoskeletal growth of the feeding system facilitates high bite-force generation necessary to overcome the physical constraints of consuming more durable prey, durophagous taxa are well suited for investigations of the scaling relationships between musculoskeletal growth, bite-force generation and dietary ontogeny. To elucidate which biomechanical factors are responsible for allometric changes in bite force and durophagy, we developed and experimentally tested a static model of bite-force generation throughout development in the durophagous turtle Sternotherus minor . Moreover, we quantified the fracture properties of snails found in the diet to evaluate the relationship between bite force and the forces required to process durable prey. We found that (1) the static bite-force model accurately predicts the ontogenetic scaling of bite forces, (2) bite-force positive allometry is accomplished by augmenting muscle size and muscle pennation, and (3) the rupture forces of snails found in the diet show a similar scaling pattern to bite force across ontogeny. These results indicate the importance of muscle pennation for generating high bite forces while maintaining muscle size and provide empirical evidence that the allometric patterns of musculoskeletal growth in S. minor are strongly linked to the structural properties of their primary prey. * M. add. mand. : Musculus adductor mandibulae M. post. : Musculus posterior M. pseudo. : Musculus pseudotemporalis M. ptery. : Musculus pterygoideus P. prof. α : Pars profunda α P. prof. β : Pars profunda β P. prof. γ : Pars profunda γ P. sup. : Pars superficialis PCSA : physiological cross-sectional area SL : skull length θ : muscle pennation angle BF : bite force BFmed : bite force generated by the Pars media BFpost : bite force generated by the M. posterior BFpseudo,ptery : bite force generated by the M. pseudotemporalis and M. pterygoideus BFsup,pro : bite force generated by the Pars superficialis and Pars profunda (α, β, γ) F : muscle force F med : force generated by the Pars media F post : force generated by the M. posterior F pseudo,ptery : force generated by the M. pseudotemporalis and M.pterygoideus F sup,pro : force generated by the Pars superficialis and Pars profunda (α, β, γ) IL : in-lever length ILmed : in-lever length of the Pars media ILpost : in-lever length of the M. posterior ILpseudo,ptery : in-lever length of the M. pseudotemporalis and M. pterygoideus ILsup,pro : in-lever length of the Pars superficialis and Pars profunda (α, β, γ) J : jaw joint MA : mechanical advantage (RIL:OL) OL : out-lever length RBF : right-side bite force RIL : resolved in-lever length a : regression intercept b : regression slope (scaling coefficient) CI : confidence interval r 2 : coefficient of determination RMA : reduced major-axis regression α : significance level ARF : average force required to rupture the five largest snails found in the diet of each turtle I : isometry N : negative allometry OpL : snail operculum length P : positive allometry SnL : snail length