Finite element analysis of performance in the skulls of marmosets and tamarins.

Reliance on plant exudates is a relatively rare dietary specialization among mammals. One well-studied example of closely related exudate feeders is the New World marmosets and tamarins. Whereas marmosets actively gouge tree bark with their incisors to stimulate the flow of sap, tamarins are opportunistic exudate feeders that do not gouge bark. Several studies of the dentaries and jaw adductors indicate that marmosets exhibit specializations for increased gape at the expense of bite force. Few studies, however, have looked to the cranium of marmosets for evidence of functional specializations. Using 3D finite element models of the marmoset Callithrix jacchus and the tamarin Saguinus fuscicollis, we investigated the performance of the cranium under loading regimes that mimicked unilateral molar biting and bark-gouging. We investigated three measures of performance: the efficiency with which muscle force is transferred to bite force, the extent to which the models are stressed (a predictor of failure), and the work expended by muscles as they deform the skull (total strain energy). We found that during molar biting the two models exhibited similar levels of performance, though the Saguinus model had slightly higher mechanical efficiency, a slightly lower state of stress, and expended more energy on deformation. In contrast, under the bark-gouging load, Callithrix exhibited much higher mechanical efficiency than Saguinas, but did so at the expense of more work and higher levels of von Mises stress. This analysis illustrates that differences in the shapes of the skulls of Callithrix and Saguinus confer differences in performance. Whether these aspects of performance are targets of selection awaits broader comparative analyses.