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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. Research Highlights: Free‐ranging Saimiri sciureus quadrupedal gait is more variable than has previously been shown in the laboratory setting and includes a combination of asymmetrical and symmetrical walking gaits, lateral sequence gaits, and diagonal sequence gaits when moving across discontinuous substrates.Primates utilize gait flexibility to navigate their complex, arboreal habitats.Saimiri increases the delay in forelimb touchdowns when moving across discontinuous substrates, supporting the hypothesis that forelimbs may be used to explore the stability of a new substrate. [ABSTRACT FROM AUTHOR]

Given that most species of primates are predominantly arboreal, maintaining the ability to move among branches of varying sizes has presumably been a common selective force in primate evolution. However, empirical evaluations of the relationships between morphological variation and characteristics of substrate geometry, such as substrate diameter relative to an animal's body mass, have been limited by the lack of quantified substrate usage in the wild. Here we use recently published quantitative data to assess the relationships between relative substrate size and talar morphology in nine New World monkey species at the Tiputini Biodiversity Station, Ecuador. Within this sample, both fibular facet angle (the angle between the fibular facet and the trochlear rims) and body-mass-standardized area of the medial tibial facet decrease as average and maximum relative substrate size increases. Correlations between medial tibial facet area and relative substrate size are driven by the inclusion of callitrichids in this sample. Nevertheless, these findings strengthen the hypothesis that variation in fibular facet orientation and medial tibial facet area are functionally correlated with habitual degrees of pedal inversion. They also strengthen the notion that evolutionarily changing body mass could impact habitat geometry experienced by a lineage and thereby substantially impact major trends in primate morphological evolution. This study highlights the importance of empirical data on substrate use in living primates for inferring functional and evolutionary implications of morphological variation. [ABSTRACT FROM AUTHOR]

Arboreal environments present considerable biomechanical challenges for animals moving and foraging among substrates varying in diameter, orientation and compliance. Most studies of quadrupedal gait kinematics in primates and other arboreal mammals have focused on symmetrical walking gaits and the significance of diagonal sequence gaits. Considerably less research has examined asymmetrical gaits, despite their prevalence in small-bodied arboreal taxa. Here, we examined whether and how free-ranging callitrichine primates adjust asymmetrical gait kinematics to changes in substrate diameter and orientation, as well as how variation in gait kinematics affects substrate displacement. We used high-speed video to film free-ranging Saguinus tripartitus and Cebuella pygmaea inhabiting the Tiputini Biodiversity Station, Ecuador. We found that S. tripartitus used bounding and half-bounding gaits on larger substrates versus gallops and symmetrical gaits on smaller substrates, and also shifted several kinematic parameters consistent with attenuating forces transferred from the animal to the substrate. Similarly, C. pygmaea shifted from high-impact bounding gaits on larger substrates to using more half-bounding gaits on smaller substrates; however, kinematic adjustments to substrate diameter were not as profound as in S. tripartitus Both species adjusted gait kinematics to changes in substrate orientation; however, gait kinematics did not significantly affect empirical measures of substrate displacement in either species. Because of their small body size, claw-like nails and reduced grasping capabilities, callitrichines arguably represent extant biomechanical analogs for an early stage in primate evolution. As such, greater attention should be placed on understanding asymmetrical gait dynamics for insight into hypotheses concerning early primate locomotor evolution. ., Competing Interests: Competing interestsThe authors declare no competing or financial interests., (© 2020. Published by The Company of Biologists Ltd.)

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., (© 2019 Wiley Periodicals, Inc.)

Objectives: Laboratory 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 Methods: These 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., Results: Parallel 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., Discussion: Ultimately, 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., (© 2018 Wiley Periodicals, Inc.)