Unconstrained 3D-kinematics of prehension in five primates: lemur, capuchin, gorilla, chimpanzee, human.

Primates are known for their use of the hand in many activities including food grasping. Yet, most studies concentrate on the type of grip used. Moreover, kinematic studies remain limited to a few investigations of the distal elements in constrained conditions in humans and macaques. In order to improve our understanding of the prehension movement in primates, we analyse here the behavioural strategies (e.g., types of grip, body postures) as well as the 3D kinematics of the whole forelimb and the trunk during the prehension of small static food items in five primate species in unconstrained conditions. All species preferred the quadrupedal posture except lemurs, which used a typical crouched posture. Grasp type differed among species, with smaller animals (capuchins and lemurs) using a whole-hand grip and larger animals (humans, gorillas, chimpanzees) using predominantly a precision grip. Larger animals had lower relative wrist velocities and spent a larger proportion of the movement decelerating. Humans grasped food items with planar motions involving small joint rotations, more similar to the smaller animals than to gorillas and chimpanzees, which used greater rotations of both the shoulder and forearm. In conclusion, the features characterising human food prehension are present in other primates, yet differences exist in joint motions. These results provide a good basis to suggest hypotheses concerning the factors involved in driving the evolution of grasping abilities in primates.

Diversity of grip in adults and young humans and chimpanzees (Pan troglodytes).

Grasping is essential for primates in numerous behaviors. A variety of different grasping techniques are used for obtaining food. Among humans, several studies have shown that the properties of the objects such as the size or the form influence grasp patterns. In addition, other works have tested the individual variability through grasping strategies and age and several studies have revealed some similarities between great apes and humans in grip types. Finally, results on hand preference are still equivocal and, for non-human primates, object parameters and age effect are rarely tested together, even though it is a methodological aspect important to consider. The study sought to determine whether grip type varied according to the age of the subject, the species (human versus chimpanzee), the size of the object and the hand used. Frame-by-frame analysis of hand contact strategies and statistical results indicated that (1) adults of both species used fewer contact strategies than juveniles and that there was a greater variability of contacts for small than for large objects (2) young juvenile chimpanzees and human children follow a similar grip types development, i.e. more frequent use of precision grips with age (3) juvenile chimpanzees used all five categories of grip and the adults used the "thumb-fingerpad(s)" more than the "precision grips" in addition to the "power grip" and (4) a right hand preference was greater for the grasping of small objects with "precision grips" in adults for both species. These results are discussed in relationship with neurology, morphology and grasping evolution.