Meeting Abstract
Decades of comparative research have established that the evolution of primate quadrupedalism is intimately tied to arboreality. However, attempts to explain primate synapomorphies as “arboreal adaptations” are hampered by the existence of other mammals that lack such features, but are nonetheless committed arborealists. Here, we test the prediction that primate-like grasping and locomotor kinematics confer a performance advantage relative to other arboreal mammals. We collected high-speed video of gray squirrels crossing broad (5cm) and narrow (2.5cm) diameter poles instrumented with force/torque transducers, and compared these data to matching datasets from similarly-sized New World monkeys – marmosets (Callithrix jacchus) and squirrel monkeys (Saimiri boliviensis). Squirrels consistently used high-speed bounding and galloping gaits across both substrates, moving significantly faster than either of the primates (p ≤ 0.018). Given that rolling plane angular momentum was negatively correlated with speed across species (r = -0.687, p < 0.001), squirrels likely moved quickly to increase lateral stability. When transitioning from the broad to the narrow pole, squirrels consistently required the greatest magnitudes of adjustment (i.e., changes in speed, duty factor, and overall substrate contact duration; squirrel effect sizes: 0.245-0.726; primate effect sizes: 0.026-0.422). Our findings suggest that primate-specific morphology and gait patterns increase stability on narrow substrates, providing empirical evidence for the hypothesis that functional aspects of primate anatomy and gait mechanics evolved in response to selective pressures to increase locomotor performance on narrow arboreal supports. Supported by NSF BCS-1126790.
