Biting mechanics and niche separation in a specialized clade of primate seed predators

Abstract

We analyzed feeding biomechanics in pitheciine monkeys (<i>Pithecia, Chiropotes, Cacajao</i>), a clade that specializes on hard-husked unripe fruit (sclerocarpy) and resistant seeds (seed predation). We tested the hypothesis that pitheciine crania are well-suited to <i>generate</i> and <i>withstand</i> forceful canine and molar biting, with the prediction that they generate bite forces more efficiently and better resist masticatory strains than the closely-related <i>Callicebus</i>, which does not specialize on unripe fruits and/or seeds. We also tested the hypothesis that <i>Callicebus-Pithecia-Chiropotes-Cacajao</i> represent a morphocline of increasing sclerocarpic specialization with respect to biting leverage and craniofacial strength, consistent with anterior dental morphology. We found that pitheciines have higher biting leverage than <i>Callicebus</i> and are generally more resistant to masticatory strain. However, <i>Cacajao</i> was found to experience high strain magnitudes in some facial regions. We therefore found limited support for the morphocline hypothesis, at least with respect to the mechanical performance metrics examined here. Biting leverage in <i>Cacajao</i> was nearly identical (or slightly less than) in <i>Chiropotes</i> and strain magnitudes during canine biting were more likely to follow a <i>Cacajao-Chiropotes-Pithecia</i> trend of increasing strength, in contrast to the proposed morphocline. These results could indicate that bite force efficiency and derived anterior teeth were selected for in pitheciines at the expense of increased strain magnitudes. However, our results for <i>Cacajao</i> potentially reflect reduced feeding competition offered by allopatry with other pitheciines, which allows <i>Cacajao</i> species to choose from a wider variety of fruits at various stages of ripeness, leading to reduction in the selection for robust facial features. We also found that feeding biomechanics in sympatric <i>Pithecia</i> and <i>Chiropotes</i> are consistent with data on food structural properties and observations of dietary niche separation, with the former being well-suited for the regular molar crushing of hard seeds and the latter better adapted for breaching hard fruits.