Sources of variance in temporal and spatial aspects of jaw kinematics in two species of primates feeding on foods of different properties.

Chewing kinematics reflects interactions between centrally generated motor signals and peripheral sensory feedback from the constantly changing oral environment. Chewing is a strongly modulated behavior that responds to differences in material properties among different type of foods and to changes in the external physical properties of the food as the bolus gets processed. Feeding, as any complex biological behavior, presents variation at multiple hierarchical levels, from among species or higher-order levels to variation among chewing cycles within a single feeding sequence. Thus, to understand the mechanics and evolution of feeding systems requires estimation of how this variation is distributed across each of these hierarchical levels, which in turn requires large sample sizes. The development of affordable, high-resolution, three-dimensional kinematic recording systems has increased our ability to collect large amounts of data on complete or near-complete feeding sequences that can be used to shed light on the mechanisms of control in vertebrate feeding. In this study, we present data on the nature and sources of variation (from species to chewing cycle levels) in kinematics of chewing in two species of primates, Cebus and Macaca, while they feed on foods of known material properties. Variation in chewing kinematics was not evenly distributed among hierarchical levels. Most of the variation was observed among chewing cycles, most likely in response to changes in the external properties of the food bolus throughout the feeding sequence. Species differences were found in duration and vertical displacement during slow-close phase suggesting that each species exhibits different power stroke dynamics. Cebus exhibited more variable gape cycles than did Macaca, in particular when eating low-toughness foods. This increased ability to temporally and spatially modulate the gape cycle may reflect increased efficiency in processing food because Cebus monkeys use fewer, but longer cycles, than does Macaca when feeding on low-toughness foods. This is due to an increase in duration of the jaw-opening phases of the gape cycle, when the tongue repositions the food bolus in the oral cavity.