Mechanics of Arboreal Locomotion in Swinhoe’s Striped Squirrels: A Potential Model for Early Euarchontoglires

Jan Wölfer,Tina Aschenbach,Jenny Michel,John A Nyakatura

doi:10.3389/fevo.2021.636039

Abstract

Differences between arboreal and terrestrial supports likely pose less contrasting functional demands on the locomotor system at a small body size. For arboreal mammals of small body size, asymmetrical gaits have been demonstrated to be advantageous to increase dynamic stability. Many of the extant arboreal squirrel-related rodents display a small body size, claws on all digits, and limited prehensility, a combination that was proposed to have characterized the earliest Euarchontoglires. Thus, motion analysis of such a modern analog could shed light onto the early locomotor evolution of eurarchontoglirans. In this study, we investigated how Swinhoe’s striped squirrels (Tamiops swinhoei; Scuiromorpha) adjust their locomotion when faced with different orientations on broad supports and simulated small branches. We simultaneously recorded high-Hz videos (501 trials) and support reaction forces (451 trials) of squirrels running on two types of instrumented trackways installed at either a 45° incline (we recorded locomotion on inclines and declines) or with a horizontal orientation. The striped squirrels almost exclusively used asymmetrical gaits with a preference for full bounds. Locomotion on simulated branches did not differ substantially from locomotion on the flat trackway. We interpreted several of the quantified adjustments on declines and inclines (in comparison to horizontal supports) as mechanisms to increase stability (e.g., by minimizing toppling moments) and as adjustments to the differential loading of fore- and hind limbs on inclined supports. Our data, in addition to published comparative data and similarities to the locomotion of other small arboreal rodents, tree shrews, and primates as well as a likely small body size at the crown-group node of Euarchontoglires, render a preference for asymmetrical gaits in early members of the clade plausible. This contributes to our understanding of the ancestral lifestyle of this mammalian ‘superclade’.

Highlights

IntroductionEvidence from the fossil record suggests stem placentals (i.e., eutherian mammals) to have been small scansorial animals
Evidence from the fossil record suggests stem placentals to have been small scansorial animals
Out of 501 stride cycles used for kinematic analysis, 500 showed asymmetrical gaits; One stride cycle was characterized by an asymmetrical forelimb pattern and a symmetrical hind limb pattern

Summary

Introduction

Evidence from the fossil record suggests stem placentals (i.e., eutherian mammals) to have been small scansorial animals. Locomotor Mechanics of Tamiops swinhoei from China was characterized by very similar body proportions and overall small size (Ji et al, 2002). The ‘superclades’ or ‘grandorders’ Laurasiatheria and Euarchontoglires form well-accepted monophyletic sister taxa (Murphy et al, 2001; Asher et al, 2009; Zhou et al, 2015). Body size of early euarchontans (e.g., plesiadapiforms) was likely small, too (Silcox and López-Torres, 2017). A consensus regarding the phylogenetic affinities of the taxa forming the Euarchontoglires has not been reached despite considerable effort over recent years (reviewed in Nyakatura, 2019)

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Results

Discussion

Conclusion