Hand posture in the grey mouse lemur during arboreal locomotion on narrow branches

Abstract

International audiencePrimates are typically subdivided into two fundamentally different groups: Strepsirrhini and Haplorrhini. These two suborders are differentiated by several anatomical characteristics, among which are features of the wrist and hand. Whereas strepsirrhines are characterized by an ectaxonic hand with a longer fourth digit, haplorhines display a mesaxonic hand with a longer third digit. Two complementary studies suggest that (1) an ulnarly deviated hand with respect to the forearm during locomotion is typical for ectaxonic hands and thin branches whereas mesaxonic hands display a less-deviated posture in relation to a more terrestrial type of locomotion; (2) ulnar deviations are not always produced by ectaxonic hands and may rather be associated with locomotion in an arboreal environment. The aim of this study was to explore how arboreal substrates influence the posture of the hand and the wrist in contact with the substrate. In this context, we assessed the grasping ability of the strepsirrhine Microcebus murinus, a highly arboreal species. Here we tested the effect of branch diameter (1 and 3 cm) and orientation (horizontal and vertical) on grasp choice during arboreal locomotion. Our results show that two hand postures were observed on horizontal substrates versus three-hand postures on vertical substrates. When ulnar deviation was observed, it was typically observed on vertical substrates, particularly on thick ones. In conclusion, our data show that vertical substrates increase the variability in grasping hand postures for M. murinus and include the use of uncommon grasps compared with horizontal substrates. We suggest that more than the arboreal substrate, the frequent use of vertical supports may influence the hand biomechanics towards ulnar deviation as observed for lorisids and indriids

The effect of substrate diameter and incline on locomotion in an arboreal frog

“…On wider substrates this grip type is utilized much less frequently, independent of the substrate orientation. These results are similar to data obtained for the gray mouse lemur (Reghem et al, 2012) as well as other arboreal mammals for which the use of this type of grip is reduced on wider substrates (Hershkovitz, 1977;Sargis, 2001). The grip type D34 is most common on wide, horizontal substrates and is nearly absent on inclined substrates.…”

“…Interestingly, on the incline, wider substrates invoke an increase in the use of this grip, potentially because of its better position to generate traction while maintaining adhesion with the substrate through the frictional forces generated by the toe pad of the third digit positioned flat and in line with the substrate (Endlein et al, 2013). Our results generally confirm those for primates in suggesting that hand positions and grip types are more dependent on substrate than on intrinsic anatomy, with animals changing their grip to optimize the interaction with the substrate (Lemelin and Schmitt, 1998;Reghem et al, 2012).…”

“…The mesaxonic grip with an axial position of the third digit (D234) is the most common grip type in P. azurea. Its use is diminished as the diameter of horizontal substrates increases, as has also been observed for the gray mouse lemur (Reghem et al, 2012). The use of this grip probably permits the animal to increase its stability when walking on very narrow substrates, needed for an efficient movement.…”

Hands of early primates

“…Typical hand postures may be strongly contingent on substrate size and orientation, as has recently been demonstrated for Microcebus (Reghem et al, 2012). When evaluating the degree and nature of wrist mobility (at both the proximal and distal carpal rows), facet shape is an important consideration.…”

“…Our definitions for hand and grasp types follow Cartmill (1974a), Napier (1960Napier ( , 1961, Preuschoft et al (1993), and Reghem et al (2012). Hand types generally refer to the pattern of differential length and robustness of digits.…”