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

摘要

The grasping capabilities and gait kinematics characteristic of primates are oftenargued to be adaptations for safely moving on small terminal branches. The goal ofthis study was to identify whether Eastern gray squirrels (Sciurus carolinensis)—arboreal rodents that frequently move and forage on small branches, lack primatelikegrasping and gait patterns, and arguably represent extant analogs of a stemprimate ancestor—adjust gait kinematics to narrow and nonhorizontal branches. Westudied locomotor kinematics of free‐ranging and laboratory‐housed squirrels movingover various substrates. We used high‐speed video to film (a) a population of freerangingsquirrels moving on natural substrates and (b) laboratory‐housed squirrelsmoving on horizontal poles. Substrates were coded as small, medium, or large relativeto squirrel trunk diameter, and as inclined, declined, or horizontal. Free‐rangingsquirrels used more gallops and half‐bounds on small‐ and medium‐sized substrates,and more high‐impact bounds, with reduced limb‐lead durations, on declinedsubstrates. Laboratory squirrels moved at higher speeds than free‐ranging squirrelsand responded to decreasing diameter by using more gallops and half‐bounds,lowering speed, and—controlling for speed—increasing mean duty factor, meannumber of supporting limbs, and relative forelimb lead duration. Our inability todetect substantial diameter or orientation‐related gait adjustments in the wild may bedue to a limited accounting of confounding influences (e.g., substrate compliance).Ultimately, studies assessing stability measures (e.g., center of mass fluctuations andpeak vertical force) are required to assess whether primates’ enhanced grasping andgait patterns engender performance advantages on narrow or oblique substrates.