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Gait mechanics of a blind echolocating rodent: Implications for the locomotion of small arboreal mammals and proto-bats.

Arboreal mammals have evolved a range of biomechanical adaptations that allow them to navigate trees effectively. One such feature that has received considerable attention is the importance of vision that helps arboreal animals assess gap distances, assure proper foot placement, and inspect potential risks. While there is considerable debate about the relative importance of the visual system specifics, there is little doubt that the ability to at least see the environment must confer some level of safety when navigating arboreal substrates. In this study, we explore spatiotemporal and kinematic patterns of arboreal locomotion in the Vietnamese pygmy dormouse (Typhlomys chapensis), a blind rodent that uses ultrasonic echolocation to navigate in tree canopies. We compare these data with five other species of arboreal rodents and primates. Spatiotemporal gait characteristics are largely similar between the Vietnamese pygmy dormouse and other small-bodied arboreal species analyzed. Most notable is the tendency for relatively high-speed asymmetrical gaits on large-diameter substrates and slower symmetrical lateral-sequence gaits on small-diameter substrates. Furthermore, for all species speed is primarily regulated by increasing stride frequency rather than length. Kinematics of the Vietnamese pygmy dormouse changed little in response substrate size and were primarily driven by speed. These findings suggest that the information gathered during ultrasonic scanning is sufficient to allow effective quadrupedal locomotion while moving on arboreal supports. The Vietnamese pygmy dormouse may serve as a model for the quadrupedal nocturnal ancestor of bats, which had started developing ultrasonic echolocation and reducing vision while likely occupying an arboreal niche.