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

Abstract

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.

Evolutionary history of quadrupedal walking gaits shows mammalian release from locomotor constraint

“…and tree shrews (Tupaia sp.) use LSDC gaits that are more typical of generalized terrestrial locomotion and recent studies have highlighted that many small-bodied (less than 200 g) mammals do not consistently use symmetric gaits on arboreal substrates [73,74]. Together, these findings suggest that DSDC gaits are only beneficial when body size is larger than support size, or for taxa that use the visual system to guide forelimb placement during walking [23].…”

Are they arboreal? Climbing abilities and mechanics in the red‐backed salamander (Plethodon cinereus)

“…Several hypotheses about the benefits of climbing behaviors within plethodontids have been proposed, including increased prey quality or abundance (Jaeger, 1978), increased detection of olfactory cues (Madison & Shoop, 1970), and avoidance of predators (Roberts & Liebgold, 2008). While the climbing ability of the red‐backed salamander has been well‐documented (Jaeger, 1978; McEntire, 2016), it remains unclear whether this anatomically unspecialized species truly demonstrates arboreal capabilities [sensu stricto Preuschoft (2002)] and adopts spatiotemporal gait characteristics observed in other species (Table 1) that have been shown to increase stability on arboreal substrates (Clemente et al, 2013; Endlein et al, 2017; Granatosky et al, 2019, 2021; Hill et al, 2018).…”

“…*Definitions, functions, and interpretations derived from Granatosky and Fitzsimons (2017), Granatosky et al (2019, 2021). …”

“…Specifically, individuals will move slower with significantly longer stance phases both as substrate size decreases and on vertical substrates. They will adopt gaits patterns that maximize the portion of the stride where the limbs are positioned as a wide polygon of support on thin and vertically‐oriented supports (Granatosky et al, 2019, 2021). Additionally, we hypothesize that velocity is regulated by stride frequency when on terrestrial substrates, but animals will have a greater reliance on stride length when on arboreal supports (Granatosky et al, 2021; Karantanis et al, 2015).…”

“…They will adopt gaits patterns that maximize the portion of the stride where the limbs are positioned as a wide polygon of support on thin and vertically‐oriented supports (Granatosky et al, 2019, 2021). Additionally, we hypothesize that velocity is regulated by stride frequency when on terrestrial substrates, but animals will have a greater reliance on stride length when on arboreal supports (Granatosky et al, 2021; Karantanis et al, 2015). However, as the red‐backed salamander likely utilizes mucous adhesion to cling onto arboreal supports rather than other mechanisms (e.g., grasping hands and feet, claws, setae, suction), it is unclear whether this species utilizes any of the spatiotemporal gait characteristics traditionally associated with arboreal locomotion.…”