Meeting Abstract
It is often assumed that natural selection minimizes energetic expenditure during cyclical feeding and locomotion so that energy might be allocated to growth, reproduction, predator avoidance, or mate competition. While this assumption has been supported in the locomotor system, the feeding system is small, and the muscles responsible for jaw opening and closing represent a mere fraction of total body weight. Because of this size disparity, we hypothesized that selection does not to minimize the metabolic costs of feeding. To test this hypothesis we measured respirometry-based metabolic energy expenditure during cyclic chewing and walking in tufted capuchin monkeys (Sapajus sp.) ranging in body size and metabolic costs. Mass-specific metabolic cost and cost per cycle during walking reached values up to ~15 times higher than cyclical chewing. During locomotion, we observed a negative relationship between the body size of the animal and the mass-specific metabolic cost, as well as the mass-specific metabolic cost per cycle. In contrast, both mass-specific metabolic cost and mass-specific metabolic cost per cycle increased along with body size during cyclical chewing. This result likely stems from anatomical and mechanical patterns in the feeding system inconsistent with energetic minimization, such as: a high proportion of myosin heavy chain masticatory isoforms known for their high rates of ATP consumption; relatively low joint angular excursions that correspond to higher than expected costs of movement; and a relatively stiff system that does not utilize passive mechanisms. Taken together, these features suggest that in the feeding system, minimizing energy costs may be less important than the control of force and displacement.
