No abstract

The use of the hand in food grasping is a shared characteristic of primates. However, the factors involved in the elaboration of this function remain unclear. Grasping hands may have evolved in an arboreal habitat with narrow branches. Interestingly, grasping may also have an association with different types of feeding such as insect predation, fruit and flower exploitation, or both. No study has tested the importance of substrate diameter and food properties on the use of the hand in food grasping. Yet, both of these parameters likely impose important selective pressures on the origin and evolution of manual grasping strategies in the context of food acquisition. Here, we quantified whether (1) substrate diameter (narrow, wide) and (2) food properties (static, slow moving, fast moving) influence food grasping in a small primate, Microcebus murinus. Our results show that narrow substrates increase the use of hands in prey grasping. The mouth is preferentially used to grasp static food (banana), whereas the hands are preferred to grasp moving prey (mealworm and cricket) regardless of the substrate. Thus, the narrow branch niche may be an important selective pressure on the emergence of manual food grasping in primates, but predation likely also played a key role.

Among primates, apes and monkeys are known to use their hands and to exhibit independent control of their fingers. In comparison, Prosimii are thought to have less digital individualization and to use their mouth more commonly for prehension. Unfortunately, prehension and manipulation studies in Prosimii have been conducted in conditions constraining the subject to grasp with the hand. Moreover, the effect of food size remains unexplored, even though it could affect the use of the hands versus the mouth. Thus, whether prosimians use the hand or the mouth to grasp and manipulate food items of different sizes in unconstrained conditions remains unclear. To address this question, we characterized the eating and manipulation patterns of Microcebus murinus in unconstrained conditions, using three food sizes. The results showed that M. murinus showed (i) an eating pattern similar to that of rodents, with smaller food items being grasped with the mouth, (ii) a greater tendency to use the hands for prehension of larger foods, and (iii) plasticity during food manipulation similar to that which has been observed in rodents. These results are discussed in the framework of grasping in mammals and are used to discuss the origins of prehension in primates.

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