Not so fast: speed effects on forelimb kinematics in cercopithecine monkeys and implications for digitigrade postures in primates.

Terrestrial mammals are characterized by their digitigrade limb postures, which are proposed to increase effective limb length (ELL) to achieve preferred or higher locomotor speeds more efficiently. Accordingly, digitigrade postures are associated with cursorial locomotion. Unlike most medium- to large-sized terrestrial mammals, terrestrial cercopithecine monkeys lack most cursorial adaptations, but still adopt digitigrade hand postures. This study investigates when and why terrestrial cercopithecine monkeys adopt digitigrade hand postures during quadrupedal locomotion. Three cercopithecine species (Papio anubis, Macaca mulatta, Erythrocebus patas) were videotaped moving unrestrained along a horizontal runway at a range of speeds (0.4-3.4 m/s). Three-dimensional forelimb kinematic data were recorded during forelimb support. Hand posture was measured as the angle between the metacarpal segments and the ground (MGA). As predicted, a larger MGA was correlated with a longer ELL. At slower speeds, subjects used digitigrade postures (larger MGA), however, contrary to expectations, all subjects used more palmigrade hand postures (smaller MGA) at faster speeds. Digitigrade postures at slower speeds may lower cost of transport by increasing ELL and step lengths. At higher speeds, palmigrade postures may be better suited to spread out high ground reaction forces across a larger portion of the hand thereby potentially decreasing stresses in hand bones. It is concluded that a digitigrade forelimb posture in primates is not an adaptation for high speed locomotion. Accordingly, digitigrady may have evolved for different reasons in primates compared to other mammalian lineages.