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ABSTRACT 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. 

ABSTRACT Jumping is a crucial behavior in fitness-critical activities including locomotion, resource acquisition, courtship displays and predator avoidance. In primates, paleontological evidence suggests selection for enhanced jumping ability during their early evolution. However, our interpretation of the fossil record remains limited, as no studies have explicitly linked levels of jumping performance with interspecific skeletal variation. We used force platform analyses to generate biomechanical data on maximal jumping performance in three genera of callitrichine monkeys falling along a continuum of jumping propensity: Callimico (relatively high propensity jumper), Saguinus (intermediate jumping propensity) and Callithrix (relatively low propensity jumper). Individuals performed vertical jumps to perches of increasing height within a custom-built tower. We coupled performance data with high-resolution micro-CT data quantifying bony features thought to reflect jumping ability. Levels of maximal performance between species – e.g. maximal take-off velocity of the center of mass (CoM) – parallel established gradients of jumping propensity. Both biomechanical analysis of jumping performance determinants (e.g. CoM displacement, maximal force production and peak mechanical power during push-off) and multivariate analyses of bony hindlimb morphology highlight different mechanical strategies among taxa. For instance, Callimico, which has relatively long hindlimbs, followed a strategy of fully extending of the limbs to maximize CoM displacement – rather than force production – during push-off. In contrast, relatively shorter-limbed Callithrix depended mostly on relatively high push-off forces. Overall, these results suggest that leaping performance is at least partially associated with correlated anatomical and behavioral adaptations, suggesting the possibility of improving inferences about performance in the fossil record. 

Abstract Locomotion on the narrow and compliant supports of the arboreal environment is inherently precarious. Previous studies have identified a host of morphological and behavioral specializations in arboreal animals broadly thought to promote stability when on precarious substrates. Less well-studied is the role of the tail in maintaining balance. However, prior anatomical studies have found that arboreal taxa frequently have longer tails for their body size than their terrestrial counterparts, and prior laboratory studies of tail kinematics and the effects of tail reduction in focal taxa have broadly supported the hypothesis that the tail is functionally important for maintaining balance on narrow and mobile substrates. In this set of studies, we extend this work in two ways. First, we used a laboratory dataset on three-dimensional segmental kinematics and tail inertial properties in squirrel monkeys (Saimiri boliviensis) to investigate how tail angular momentum is modulated during steady-state locomotion on narrow supports. In the second study, we used a quantitative dataset on quadrupedal locomotion in wild platyrrhine monkeys to investigate how free-ranging arboreal animals adjust tail movements in response to substrate variation, focusing on kinematic measures validated in prior laboratory studies of tail mechanics (including the laboratory data presented). Our laboratory results show that S. boliviensis significantly increase average tail angular momentum magnitudes and amplitudes on narrow supports, and primarily regulate that momentum by adjusting the linear and angular velocity of the tail (rather than via changes in tail posture per se). We build on these findings in our second study by showing that wild platyrrhines responded to the precarity of narrow and mobile substrates by extending the tail and exaggerating tail displacements, providing ecological validity to the laboratory studies of tail mechanics presented here and elsewhere. In conclusion, our data support the hypothesis that the long and mobile tails of arboreal animals serve a biological role of enhancing stability when moving quadrupedally over narrow and mobile substrates. Tail angular momentum could be used to cancel out the angular momentum generated by other parts of the body during steady-state locomotion, thereby reducing whole-body angular momentum and promoting stability, and could also be used to mitigate the effects of destabilizing torques about the support should the animals encounter large, unexpected perturbations. Overall, these studies suggest that long and mobile tails should be considered among the fundamental suite of adaptations promoting safe and efficient arboreal locomotion. 

Primates' near exclusive use of diagonal sequence gaits has been hypothesized to enhance stability on arboreal substrates. To assess how primate gait kinematics vary in complex arboreal environments, we filmed eight species of free-ranging primates (Ateles, Lagothrix, Alouatta, Pithecia, Callicebus, Saimiri, Saguinus, and Cebuella) at the Tiputini Biodiversity Station, Ecuador, and quantified the diameter and orientation of locomotor substrates using remote sensors (n = 858 strides). Five of the species used primarily diagonal sequence, diagonal couplet (DSDC) gaits. Callicebus frequently used lateral sequence gaits (i.e., ~50% of strides). Saguinus and Cebuella most frequently used asymmetrical gaits. We examined the effects of substrate diameter and orientation on duty factor and interlimb phasing, controlling for speed via ANCOVA. Ateles increased limb phase on inclines (p=0.04), Lagothrix had greater duty factors on inclines (p=0.002), Callicebus exhibited greater duty factors (p=0.04) and lower limb phase values on declines (p=0.001), and both Saimiri and Saguinus displayed an inverse relationship between limb phase and substrate diameter (p=0.05, p=0.03, respectively). This study confirms the ubiquity of diagonal sequence gaits in free-ranging primates and at least partially supports predicted biomechanical adjustments to promote stability including: increased duty factor on nonhorizontal substrates, increased limb phase on inclines, and decreased limb phase on declines. Other species-specific kinematic adjustments to substrate variation are likely related to body size and ecological variation but require further investigation.