The effects of natural substrate discontinuities on the quadrupedal gait kinematics of free‐ranging Saimiri sciureus.

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]