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

Abstract

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). 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.

Integrative experimental and morphological study of the metacarpal and metatarsal bones of the Japanese macaque (<i>Macaca fuscata</i>)

“…Both monkeys were trained to move in a straight path on a flat 6-m-long concrete floor (Figure 3; Higurashi et al, 2010Higurashi et al, , 2018. Without prompting by leash, the monkeys were either allowed to select their own preferred speeds or incentivized to move faster using food rewards.…”

“…Without prompting by leash, the monkeys were either allowed to select their own preferred speeds or incentivized to move faster using food rewards. Because the hand-pressure records were originally obtained for another study (Higurashi et al, 2018), only foot pressure was recorded, at 120 Hz using a 220 mm × 240 mm pressure mat (BIG-MAT quarter; Nitta, Osaka, Japan) at the midpoint of the path. The pressure mat quantified normal forces with an ar- The palmar surface of the hand and plantar surface of the foot of a Japanese macaque.…”

“…It follows that these bones also differ in diaphyseal robusticity because the strength of a structure depends on its length and the geometric properties of its cross-section, all other properties being equal (Case et al, 1999). Furthermore, the distal articular surfaces (heads) of MCs and MTs have been shown to make direct contact with the substrate during locomotion in some primates, including Japanese macaques (Higurashi et al, 2010(Higurashi et al, , 2018, white-handed gibbons (Vereecke et al, 2005), chimpanzees (Wunderlich and Ischinger, 2017), bonobos (Vereecke et al, 2003), and humans (Pataky and Goulermas, 2008;Stolwijk et al, 2013). Accordingly, the pressure, or force per unit area, exerted on these bones can be measured non-invasively through the use of plantar pressure measurement systems.…”

“…To our knowledge, no data have been published on the J values of MCs and MTs in this species. Some studies have reported hand and foot pressure during locomotion in Japanese macaques (Higurashi et al, 2010(Higurashi et al, , 2018, but detailed data on individual hand regions have not been reported. Although the design of this study did not allow us to reveal cause-and-effect relationships between morphological features and behaviors, understanding their correlations will facilitate more plausible interpretations of macaque hand and foot morphology.…”

Developments in development: What have we learned from primate locomotor ontogeny?

“…However, manual digitigrady in primates does not appear to be a speed‐related adaptation; primates that use manual digitigrady use it only when moving relatively slowly, becoming more palmigrade as speed increases (Patel, ). Palmigrady at higher speeds may be beneficial for distributing increased substrate reaction forces across a broader surface area, reducing stress on the hand (Higurashi, Goto, & Kumakura, ; Patel & Wunderlich, ). At lower speeds, though, digitigrady in primates appears to be beneficial for similar reasons to its benefits at higher speeds, serving to lower locomotor energetic costs as long as speed is not too high (Patel, ; Patel, Larson, & Stern, ).…”