Locomotor kinematics of tree squirrels (Sciurus carolinensis) in free‐ranging and laboratory environments: Implications for primate locomotion and evolution

Abstract

The grasping capabilities and gait kinematics characteristic of primates are often argued to be adaptations for safely moving on small terminal branches. The goal of this study was to identify whether Eastern gray squirrels (Sciurus carolinensis) -arboreal rodents that frequently move and forage on small branches, lack primate-like grasping and gait patterns, and arguably represent extant analogs of a stem primate ancestor -adjust gait kinematics to narrow and non-horizontal branches. We studied locomotor kinematics of free-ranging and laboratory-housed squirrels moving over various substrates. We used high-speed video to film 1) a population of free-ranging squirrels moving on natural substrates and 2) laboratory-housed squirrels moving on horizontal poles. Substrates were coded as small, medium, or large relative to squirrel trunk diameter, and as inclined, declined, or horizontal. Free-ranging squirrels used more gallops and half-bounds on small and medium sized substrates, and more high-impact bounds, with reduced limb lead durations, on declined substrates. Laboratory squirrels moved at higher speeds than free-ranging squirrels and responded to decreasing diameter by using more gallops and half-bounds, lowering speed, and -controlling for speed -increasing mean duty factor, mean number of supporting limbs, and relative forelimb lead duration. Our inability to detect substantial diameter or orientation-related gait adjustments in the wild may be due to a limited accounting of confounding influences (e.g., substrate compliance).Ultimately, studies assessing stability measures (e.g., center of mass fluctuations, peak vertical force) are required to assess whether primates' enhanced grasping and gait patterns engender performance advantages on narrow or oblique substrates.

Effects of substrate and phylogeny on quadrupedal gait in free‐ranging platyrrhines

“…Finally, we found that free‐ranging Callitrichinae primarily used asymmetrical running gaits. Asymmetrical gaits permit the two limbs of a girdle to work in concert to grasp substrates and are frequently used by other small‐bodied arboreal mammals (Dunham et al, ; Lammers & Zurcher, ; Schmidt, ; Shapiro, Kemp, & Young, ).…”

“…We used modified GoPro Hero 5 cameras (GoPro, Inc., San Mateo, CA) equipped with a Back‐Bone Ribcage H5Pro (Back‐Bone, Ottawa, Ontario, Canada) and C mount Fujinon HD 8–80 mm Vari‐Focal CCTV lenses (Fujifilm, Tokyo, Japan) (Dunham et al, ). We filmed at 1080 p resolution and 120 fps which provided a sufficient footage for recording limb touchdown and liftoff events (Dunham et al, ).…”

“…Relative stride length was then calculated by dividing stride length in pixels by body length in pixels. We then calculated relative speed by dividing relative stride length by stride duration in seconds, resulting in values with units of body lengths per second (Dunham et al, ). We used GaitKeeper, an open‐source MATLAB package developed by JWY, to digitize spatiotemporal events (Dunham et al, ; freely available for download at http://www.younglaboratory.org/GaitKeeper).…”

The effects of natural substrate discontinuities on the quadrupedal gait kinematics of free‐rangingSaimiri sciureus

“…Recently, researchers studying locomotion in both field and laboratory contexts have called for increased efforts in integrating research methods and theoretical frameworks to expand our understanding of primate gait plasticity and the relationship between gait and ecology (Bezanson, ; D'Août & Vereecke, ; Nyakatura & Heymann, ; Stevens & Carlson, ). Bringing laboratory methods to the field (including high‐speed video and the collection of 3D kinematic data) is now realistic given technological advances (Dunham et al, ; Dunham et al, ; Guillot, ; Shapiro et al, ; Thompson et al, ; Youlatos & Gasc, ). Therefore, primatologists now have the opportunity to precisely measure locomotor behaviors in natural habitats, and future research will further our understanding of the evolution of complex locomotor behaviors.…”

“…Further investigation of primate gait across complex substrates in the laboratory as well as in the wild is necessary to fully understand kinematic adjustments in response to environmental factors. Now that laboratory methods can be effectively implemented in field settings (Blanchard & Crompton, ; Cheyne, ; Dunbar & Badam, ; Dunham et al, ; Dunham et al, ; Guillot, ; Isler & Thorpe, ; Nyakatura & Heymann, ; Shapiro et al, ; Stevens, Ratsimbazafy, & Ralainasolo, ; Youlatos & Gasc, ), there is great potential for assessing adaptations to the substantial substrate variability that exists in the wild.…”