Digit contact strategies among primates according to the volume of the object E. POUYDEBAT*†, P. GORCE† and V. BELS‡ †Handibio, University of Toulon-Var, La Garde, France ‡USM 302, National Museum of Natural History, Paris, France 1. Introduction The study of grasping behaviour in non-human primates has become a general paradigm to use some monkeys as models to understand grasping mechanisms in humans (Bortoff and Strick 1998). However, monkey’s use of their upper limbs differs from that of humans. To grasp food involve various hand and digits postures divided into two main groups: power (involving the palm) and precision grips (involving the tips of the digits). Although there are classifications of grasping patterns of hand and digits used by humans (Cutkosky 1989), there is very few ethological study of the variations in grasping types in other primates (Christel 1993) in spite of the fact that the evolution of Primates, including human, is strongly related to the evolution of grasping behaviour, from power grasping toward precision. The goal of this study is to investigate individual differences of six species in use of different grips, taking into account the volume of the food. We aim to discuss the simplest model of grasping evolution in the light of functional and kinematical grasping abilities of primates. 2. Materials and methods The data presented in this study are based on five species of Primates observed in captivity: 9 capuchins (Cebus apella), 14 chimpanzees (Pan troglodytes), 7 orangutans (Pongo pygmaeus), 9 macaques (Macaca fuscata), 9 baboons (Papio papio), 9 children from 2 to 5 years old (Homo sapiens) and 9 adults (Homo sapiens). Species studied here belong to highly separated phylogenetic groups and have different hand structures. Areas of contact between the fingers and the object were determined to assess the relationship between grip types and the volume of the food. The volume of the spherical foods was calibrated according to the length of the hand. Percentage of categories of grasping were statistically compared by ANOVAs followed by PCAs, for both volumes of the objects. 3. Results and discussion Five categories of grasping behaviour based on the contacts between the digits and the food were defined as following: Precision: distal phalanxes of the thumb and the index finger; Thumb-distals: distal phalanxes of the thumb and fingers; Thumb-lateral: thumb and the lateral side of the index finger; Without thumb: grasp without the thumb; Power: grasp with the palm. All species were able to use precision grip. Moreover, species had a widespread effect on the percentages of Computer Methods in Biomechanics and Biomedical Engineering ISSN 1025-5842 print/ISSN 1476-8259 online q 2007 Taylor & Francis http://www.tandf.co.uk/journals DOI: 10.1080/10255840701479453 *Corresponding author. Email: pouydebat@mnhn.fr Figure 1. Principal component analysis on the five categories used to grasp large objects. Computer Methods in Biomechanics and Biomedical Engineering, Supplement 1, 2007, 147–148