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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 Arboreal locomotion is precarious and places multiple challenges upon stability. Studies have shown that captive primates respond to narrower and steeper supports by flexing limb joints and adopting a compliant gait. We tested whether these same kinematic responses are adopted by wild primates freely ranging over a variety of supports in their natural habitats. We recorded five species of platyrrhines, five species of catarrhines, and four species of strepsirrhines with modified GoPro cameras and used remote measurement to quantify substrate characteristics. Video images were imported into ImageJ to measure the angular kinematics of limb joints during quadrupedal locomotion on a variety of arboreal supports. We statistically tested for associations between joint posture and substrate characteristics, and then disentangled the influence of phylogeny and substrate on limb joint kinematics using variation partitioning and redundancy analysis. Our results partially confirm previous kinematic studies and suggest variation in support orientation, more than diameter or compliance, influences quadrupedal gait kinematics. Phylogenetic relatedness explained more variation in the data than substrate properties. This suggests primates either prospectively choose relatively ‘safe’ substrates for locomotion, or that they possess locomotor adaptations independent of limb joint kinematics per se to overcome the challenges of the precarious arboreal environment. 

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. 

Abstract ObjectivesLaboratory studies have yielded important insights into primate locomotor mechanics. Nevertheless, laboratory studies fail to capture the range of ecological and structural variation encountered by free‐ranging primates. We present techniques for collecting kinematic data on wild primates using consumer grade high‐speed cameras and demonstrate novel methods for quantifying metric variation in arboreal substrates. Materials and methodsThese methods were developed and applied to our research examining platyrrhine substrate use and locomotion at the Tiputini Biodiversity Station, Ecuador. Modified GoPro cameras equipped with varifocal zoom lenses provided high‐resolution footage (1080 p.; 120 fps) suitable for digitizing gait events. We tested two methods for remotely measuring branch diameter: the parallel laser method and the distance meter photogrammetric method. A forestry‐grade laser rangefinder was used to quantify substrate angle and a force gauge was used to measure substrate compliance. We also introduce GaitKeeper, a graphical user interface for MATLAB, designed for coding quadrupedal gait. ResultsParallel laser and distance meter methods provided accurate estimations of substrate diameter (percent error: 3.1–4.5%). The laser rangefinder yielded accurate estimations of substrate orientation (mean error = 2.5°). Compliance values varied tremendously among substrates but were largely explained by substrate diameter, substrate length, and distance of measurement point from trunk. On average, larger primates used relatively small substrates and traveled higher in the canopy. DiscussionUltimately, these methods will help researchers identify more precisely how primate gait kinematics respond to the complexity of arboreal habitats, furthering our understanding of the adaptive context in which primate quadrupedalism evolved.