Adjustments of Limb Mechanics in Cotton-top Tamarins to Moderate and Steep Support Orientations: Significance for the Understanding of Early Primate Evolution

Abstract

Early primate evolution is connected to the efficient exploitation of the terminal branch habitat. Mammals that forage in this habitat constantly encounter flexible thin branches that bend under the weight of the animals and thus form steeply inclined and declined supports. This study was aimed to gain insight into how cotton-top tamarins – a previously proposed modern analogue for a hypothetical stage in early primate evolution with prehensile autopodia – meet the specific functional demands when navigating thin, branch-like supports of different orientation. X-ray motion analysis was combined with synchronous single limb substrate reaction force measurements to discern limb mechanical adjustments. Previously reported gait parameter adjustments were confirmed for moderate support orientations, but on the steepest inclines and declines kinematic discontinuities were observed. These are interpreted to emphasize the functional roles of the forelimbs (net-braking role) and hind limbs (net-propulsive role) already established for level and moderately inclined supports. Tensile forces were exerted by the forelimbs on the steepest inclines and by the hind limbs on the steepest declines (head-first descents). Even though non-specialized small mammals have also been shown to successfully negotiate similar supports, prehensility offers advantages for foraging on thin, steeply inclined and declined terminal branches. Thus, the evolution of prehensile autopodia in small early primates likely has enhanced the exploitation of the terminal branch habitat.