Assessing the Determinants of Platyrrhine Quadrupedal Gait Kinematics in an Ecological and Phylogenetic Framework

Laboratory studies have broadened our understanding of primate arboreal locomotor biomechanics and adaptation but are necessarily limited in species availability and substrate complexity. In this field study, we filmed the locomotion of 11 species of platyrrhines (Ecuador and Costa Rica; n = 1234 strides) and remotely measured substrate diameter and orientation. We then explored ecological and phylogenetic influences on quadrupedal kinematics in multivariate space using redundancy analysis combined with variation partitioning. Among all species, phylogenetic relatedness more strongly influenced quadrupedal kinematics than variation in substrate. Callitrichines were maximally divergent from other taxa, driven by their preferred use of higher speed asymmetrical gaits. Pitheciids were also distinctive in their use of lower limb phases, including lateral sequence gaits. The biomechanical implications of interspecific differences in body mass and limb proportions account for a substantial portion of the phylogenetic-based variation. Body mass and kinematic variation were inversely related-whereas the larger taxa (atelids) were relatively restricted in kinematic space, and preferred more stable, symmetrical gaits, the smallest species (callitrichines) used faster, more asymmetrical and less cautious gaits along with symmetrical gaits. Intermembral index had a positive relationship with limb phase, consistent with higher limb phases in atelines compared to pitheciids. Substrate alone accounted for only 2% of kinematic variation among all taxa, with substrate orientation influencing kinematics more than diameter. Substrate effects, though weak, were generally consistent with predictions and with previous laboratory and field-based research. Excluding callitrichines and asymmetrical gaits, the influence of substrate alone remained low (2%), and the phylogenetic signal dropped from 31% to 8%. The substantial residual kinematic variation may be attributable to substrate or morphological variables not measured here, but could also reflect basic biomechanical patterns shared by all taxa that serve them well when moving arboreally, regardless of the challenges provided by any particular substrate.