Vertical Locomotion in Micromys minutus (Rodentia: Muridae): Insights into the Evolution of Eutherian Climbing

Abstract

Climbing is integral to scansorial and arboreal lifestyles as it enables access to and vertical ranging within the arboreal strata. As early eutherian mammals exhibit osteological correlates for arboreality, it is important to assess the behavioral mechanisms that are related to competent vertical climbing. In this context, we examined climbing gaits in one of the smallest extant rodents, the Eurasian harvest mouse. For these purposes, we filmed six adult Micromys minutus at 240 fps moving on four different substrate sizes (2 mm, 5 mm, 10 mm, 25 mm), during both vertical ascents and descents. All climbing cycles were lateral sequence slow gaits. Upward climbing was characterized by a higher contribution of the hind limbs, longer swing phases, and a significant involvement of stride frequency in velocity modulation. On the other hand, downward climbing was promoted by employing gaits of even lower diagonality, an increased contact with the substrate, enhanced role of the forelimbs, and a subtler modulation of velocity by stride frequency. Eurasian harvest mice effectively negotiate the finest substrates, but their effectiveness decreased significantly on the largest ones. The morphofunctional similarities of M. minutus to Juramaia sinensis and Eomaia scansoria imply analogous behaviors in early eutherians, which apparently contributed to the successful access and exploitation of the fine-branch arboreal milieu. In this way, extant small arboreal mammals can constitute good models for elucidating and comprehending the adaptive significance of behavioral mechanisms that are related to the evolution of arboreality in early mammals.