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.
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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.more » « less
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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.more » « less
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Laboratory investigations have provided important insight into the functional underpinnings of primate locomotor performance; however, it is unclear to what extent gait patterns in the laboratory reflect those of primates moving in natural settings. We filmed quadrupedal loco-motor activity in eight platyrrhine species at the Tiputini Biodiversity Station, Ecuador, and three additional platyrrhine species at La Suerte Biological Field Station, Costa Rica, and also quantified the diameter and orientation of locomotor substrates using remote sensors (N = 1,233 strides). We compared overall arboreal quadrupedal gait kinematic patterns in free-ranging individuals to those of laboratory platyrrhine congenerics. As expected, gait kinematics of free-ranging individuals were more variable than laboratory counterparts. Within the free-ranging dataset, Ateles and Alouatta increased limb phase on inclines (p=0.04; p=0.002, respectively), Lagothrix increased duty factors on inclines (p=0.002), Cebus increased duty factors on declines (p=0.02), 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 preference for diagonal sequence gaits in free-ranging primates (i.e., 87.9% of all recorded symmetrical strides) and that in both settings primates tend to adjust gait patterns to promote security through longer contact times on non-horizontal substrates and increased limb phase on inclined substrates. We show that laboratory and field investigations of primate locomotion yield consistent patterns but that field studies can capture additional aspects of gait variability and flexibility in response to the increased substrate complexity of natural environments.more » « less
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Abstract Objectives We tested whether patterns of intraspecific variation in catarrhine vertebral shape are consistent with developmental or functional predictions. Intraspecific variation was compared across column regions, morphological features, and species. Transitional regions and later ossifying morphological features were predicted to exhibit increased variation. The lumbosacral region, biomechanically important morphological features, and species with high locomotor demand and/or dedicated pronogrady were predicted to exhibit decreased variation.
Materials and Methods We used a modified Levene's test to compare intraspecific variation in dimensions of the neural canal, vertebral bodies, and spinous and transverse processes in lower thoracic to proximal sacral vertebrae. The sample included all hominoid genera and one cercopithecoid (
Chlorocebus ).Results We found little difference in variation across regions of the vertebral column. In hominoids, vertebral body dimensions were the least variable, neural canal dimensions the most variable, with spinous and transverse processes generally intermediate. Among species, there was a general though not always significant pattern for
Chlorocebus to exhibit the least variation, followed byHomo orHylobates. Discussion Patterns of variation across morphological features may reflect the complex interaction of functional constraints, developmental timing, and/or variable biomechanical forces.
Pongo 's elevated variation in spinous process length suggests a release from functional constraint, consistent with its suspensory locomotion and reduced spinous processes. Interspecific differences in vertebral variation based on locomotor demand or posture are generally consistent with patterns previously reported for vertebral formula and other aspects of morphology. Future research would benefit from an expanded taxonomic sample and more detailed analyses of vertebral modularity and developmental timing. -
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Abstract Wild primates encounter complex matrices of substrates that differ in size, orientation, height, and compliance, and often move on multiple, discontinuous substrates within a single bout of locomotion. Our current understanding of primate gait is limited by artificial laboratory settings in which primate quadrupedal gait has primarily been studied. This study analyzes wild
Saimiri sciureus (common squirrel monkey) gait on discontinuous substrates to capture the realistic effects of the complex arboreal habitat on walking kinematics. We collected high‐speed video footage at Tiputini Biodiversity Station, Ecuador between August and October 2017. Overall, the squirrel monkeys used more asymmetrical walking gaits than symmetrical gaits, and specifically asymmetrical lateral sequence walking gaits when moving across discontinuous substrates. When individuals used symmetrical gaits, they used diagonal sequence gaits more than lateral sequence gaits. In addition, individuals were more likely to change their footfall sequence during strides on discontinuous substrates. Squirrel monkeys increased the time lag between touchdowns both of ipsilaterally paired limbs (pair lag) and of the paired forelimbs (forelimb lag) when walking across discontinuous substrates compared to continuous substrates. Results indicate that gait flexibility and the ability to alter footfall patterns during quadrupedal walking may be critical for primates to safely move in their complex arboreal habitats. Notably, wild squirrel monkey quadrupedalism is diverse and flexible with high proportions of asymmetrical walking. Studying kinematics in the wild is critical for understanding the complexity of primate quadrupedalism. -
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Abstract Objectives Primate diagonal sequence (DS) gaits are often argued to be an adaptation for moving and foraging in the fine‐branch niche; however, existing data have come predominantly from laboratory studies that are limited in taxonomic breadth and fail to account for the structural and ecological variation of natural substrates. We test the extent to which substrate diameter and orientation influence gait sequence type and limb phase in free‐ranging primates, as well as how phylogenetic relatedness might condition response patterns.
Materials and methods We filmed quadrupedal locomotion in 11 platyrrhine species at field sites in Ecuador and Costa Rica and measured the diameter and orientation of locomotor substrates using remote sensors. We quantified limb phase values and classified strides by gait sequence type (
N = 988 strides).Results Our results show that most of the species in our sample consistently used DS gaits, regardless of substrate diameter or orientation; however, all taxa also used asymmetrical and/or lateral sequence gaits. By incorporating phylogenetic eigenvectors into our models, we found significant differences in gait sequence patterns and limb phase values among the major platyrrhine clades, suggesting that phylogeny may be a better predictor of gait than substrate diameter or orientation.
Discussion Our field data generally corroborate locomotor patterns from laboratory studies but capture additional aspects of gait variability and flexibility in response to the complexity of natural environments. Overall, our results suggest that DS gaits are not exclusively tailored to narrow or oblique substrates but are used on arboreal substrates in general.
