The origin of our Order-Primates-is believed to be associated with a fundamental change in locomotor behavior in response to the mechanical requirements of moving and foraging on thin flexible terminal branches of the forest canopy. The quadrupedal walking of primates differs from other mammals in footfall sequence, limb and joint excursions, muscle recruitment, and peak load distribution. Although these data already suggest that primates are unusual in their walking patterns, recent research has suggested a more fundamental way in which the walking of primates differs from nonprimate mammals. During walking in most animals, the center of mass (COM) moves up and down as if it on the end of a stiff upside-down pendulum. By moving their center of mass up and down with appropriate timing during walking, most animals can conserve muscular energy through the exchange of potential and kinetic energy. In contrast, during running the COM moves like a bouncing ball and energy storage and recovery occurs in tendons and muscles. Virtually nothing is known about the movements of the COM in quadrupedal primates, but some research has suggested that primates use unusually compliant walking gaits. Such walking gaits involve deep elbow and knee yield, suggesting that the COM follows a flatter path with minimal exchange of potential and kinetic energy. If this is the case then this would represent a fundamental shift in the locomotor mechanics of primates compared to other mammals. However, no direct empirical data exist describing the movement of the COM in primates. In this study five primate species (two prosimians, one New World monkey, and two Old World monkeys) and one nonprimate species (domestic cat) will be studied walking on terrestrial and, for the primates, arboreal supports across a force platform. Force data will be used to determine vertical and fore-aft accelerations, velocities, and displacements of the COM of the whole body and the forequarters and hindquarters. From these values the oscillations of kinetic and potential energy as well as the percentage of energy recovery can be calculated. This study tests the hypothesis that walking gaits of primates are unusual among mammals in that they do not conform to an inverted pendulum model. This potential difference is not a trivial mathematical distinction nor is it simply a difference of degree but rather an abrupt adaptive shift. Understanding the mechanics of walking in primates will shed light on the origins of primate locomotion and the origin of specialized locomotor behaviors such as arm-swinging and bipedalism. Aside from improving our understanding of primate locomotor evolution, this project benefits scientific education at two levels. This project will directly involve undergraduate and graduate students with a strong emphasis on female and minority student participation. In addition, the research techniques, data, and results developed from this project will be incorporated into general anthropology courses and advanced graduate and undergraduate courses.
