The Evolution of Diagonal-sequence Gaits in Primates: A New Approach to the Problem

Duke University, Durham NC

Investigators

Abstract

When walking, most quadrupeds typically put down each forefoot just after the hindfoot on the same side (that is, the right hind footfall is followed by the right fore footfall, and so on). Quadrupedal primates (lemurs, monkeys, apes, etc.) are atypical in placing down the forefoot from the opposite side just after each hind footfall (that is, the right hind footfall is followed by the left fore footfall, and so on). Although these so-called diagonal-sequence (DS) gaits of primates are well documented, their functional significance has eluded biologists for over a century. Preliminary data show that DS gaits are also characteristic of arboreal marsupials, which resemble primates in being tree-dwelling animals with grasping hind feet. DS gaits allow all these animals to support themselves with a protracted hind foot, placed on a proven support under the body's center of mass, before putting down a forefoot on a potentially precarious support. Theoretical considerations imply that the phase relationship ("diagonality") between the movement cycles of the fore and hind limbs should vary directly with the duty factor of the hindlimb relative to that of the forelimb, that diagonality should vary directly with support slope and inversely with support diameter, and that the correlation between duty factor and diagonality has undergone a historical transformation from negative (as in typical mammals) to positive (as in higher primates). High-speed camcorders will be used to record footfall sequences on four support types for a suite of 15 species selected to control for the relevant variables. The data collected will be used to test the empirical predictions derived from the theory. The results are expected to shed new light on controversies concerning primate origins and the early stages of the human lineage.