Intra-individual variation in hand postures during terrestrial locomotion in Japanese macaques (Macaca fuscata)

The primate hand adopts a variety of postures during locomotion. Habitually terrestrial cercopithecine primates are known to use a palmigrade posture at faster speeds to possibly mitigate stresses on the hand skeleton; however, it is unclear whether arboreal or semi-terrestrial species use a similar strategy for adjusting hand posture. Here, we explored intra-individual variation in hand contact patterns during terrestrial locomotion in the Japanese macaque (Macaca fuscata), a semi-terrestrial cercopithecine primate. Two monkeys were required to walk on the ground at their own preferred speeds or were encouraged to move faster for food rewards. The contact area under the hand and ground reaction forces (GRFs) were measured simultaneously using a tactile pressure sensor, and then hand pressures were calculated offline. We found that hand contact patterns could vary within individuals. The monkeys used predominantly a palmigrade posture within the range of speeds covered in this study (0.72-2.56 m s^-1). There were two subtypes of palmigrade posture. In one subtype, the hypothenar pad did not contact the substrate, whereas the entire hand contacted the substrate in the other. The palm of the palmigrade hand with total-hand contact experienced similar or lower peak pressure and pressure-time integral than those of the palmigrade hand without hypothenar pad contact even though it experienced higher peak GRFs. The moderate peak pressure experienced by the palmigrade hand with total-hand contact was due to increased contact area under the palm. The total contact area of the fingers and peak GRF to the fingers were relatively unchanged with different patterns of hand contact. These findings provide evidence that when walking on the ground, semi-terrestrial Japanese macaques use a palmigrade posture with total-hand contact to attenuate stresses on hand bones, as do habitually terrestrial species.