Brief communication: Dynamic plantar pressure distribution during locomotion in Japanese macaques (Macaca fuscata)

Differential leg and trunk operation during skipping without and with hurdles in bipedal Japanese macaque

When locomoting bipedally at higher speeds, macaques preferred unilateral skipping (galloping). The same skipping pattern was maintained while hurdling across two low obstacles at the distance of a stride within our experimental track. The present study investigated leg and trunk joint rotations and leg joint moments, with the aim of clarifying the differential leg and trunk operation during skipping in bipedal macaques. Especially at the hip, the range of joint rotation and extension at lift off was larger in the leading than in the trailing leg. The flexing knee absorbed energy and the extending ankle generated work during each step. The trunk showed only minor deviations from symmetry. Hurdling amplified the differences and notably resulted in a quasi-elastic use of the leading knee and in an asymmetric operation of the trunk.

Forward dynamic simulation of Japanese macaque bipedal locomotion demonstrates better energetic economy in a virtualised plantigrade posture

A plantigrade foot with a large robust calcaneus is regarded as a distinctive morphological feature of the human foot; it is presumably the result of adaptation for habitual bipedal locomotion. The foot of the Japanese macaque, on the other hand, does not have such a feature, which hampers it from making foot-ground contact at the heel during bipedal locomotion. Understanding how this morphological difference functionally affects the generation of bipedal locomotion is crucial for elucidating the evolution of human bipedalism. In this study, we constructed a forward dynamic simulation of bipedal locomotion in the Japanese macaque based on a neuromusculoskeletal model to evaluate how virtual manipulation of the foot structure from digitigrade to plantigrade affects the kinematics, dynamics, and energetics of bipedal locomotion in a nonhuman primate whose musculoskeletal anatomy is not adapted to bipedalism. The normal bipedal locomotion generated was in good agreement with that of actual Japanese macaques. If, as in human walking, the foot morphology was altered to allow heel contact, the vertical ground reaction force profile became double-peaked and the cost of transport decreased. These results suggest that evolutionary changes in the foot structure were important for the acquisition of human-like efficient bipedal locomotion.

Ontogeny of hallucal metatarsal rigidity and shape in the rhesus monkey (Macaca mulatta) and chimpanzee (Pan troglodytes)

Life history variables including the timing of locomotor independence, along with changes in preferred locomotor behaviors and substrate use during development, influence how primates use their feet throughout ontogeny. Changes in foot function during development, in particular the nature of how the hallux is used in grasping, can lead to different structural changes in foot bones. To test this hypothesis, metatarsal midshaft rigidity [estimated from the polar second moment of area (J) scaled to bone length] and cross-sectional shape (calculated from the ratio of maximum and minimum second moments of area, Imax /Imin ) were examined in a cross-sectional ontogenetic sample of rhesus macaques (Macaca mulatta; n = 73) and common chimpanzees (Pan troglodytes; n = 79). Results show the hallucal metatarsal (Mt1) is relatively more rigid (with higher scaled J-values) in younger chimpanzees and macaques, with significant decreases in relative rigidity in both taxa until the age of achieving locomotor independence. Within each age group, Mt1 rigidity is always significantly higher in chimpanzees than macaques. When compared with the lateral metatarsals (Mt2-5), the Mt1 is relatively more rigid in both taxa and across all ages; however, this difference is significantly greater in chimpanzees. Length and J scale with negative allometry in all metatarsals and in both species (except the Mt2 of chimpanzees, which scales with positive allometry). Only in macaques does Mt1 midshaft shape significantly change across ontogeny, with older individuals having more elliptical cross-sections. Different patterns of development in metatarsal diaphyseal rigidity and shape likely reflect the different ways in which the foot, and in particular the hallux, functions across ontogeny in apes and monkeys.

Hand before foot? Cortical somatotopy suggests manual dexterity is primitive and evolved independently of bipedalism

People have long speculated whether the evolution of bipedalism in early hominins triggered tool use (by freeing their hands) or whether the necessity of making and using tools encouraged the shift to upright gait. Either way, it is commonly thought that one led to the other. In this study, we sought to shed new light on the origins of manual dexterity and bipedalism by mapping the neural representations in the brain of the fingers and toes of living people and monkeys. Contrary to the ‘hand-in-glove’ notion outlined above, our results suggest that adaptations underlying tool use evolved independently of those required for human bipedality. In both humans and monkeys, we found that each finger was represented separately in the primary sensorimotor cortex just as they are physically separated in the hand. This reflects the ability to use each digit independently, as required for the complex manipulation involved in tool use. The neural mapping of the subjects’ toes differed, however. In the monkeys, the somatotopic representation of the toes was fused, showing that the digits function predominantly as a unit in general grasping. Humans, by contrast, had an independent neurological representation of the big toe (hallux), suggesting association with bipedal locomotion. These observations suggest that the brain circuits for the hand had advanced beyond simple grasping, whereas our primate ancestors were still general arboreal quadrupeds. This early adaptation laid the foundation for the evolution of manual dexterity, which was preserved and enhanced in hominins. In hominins, a separate adaptation, involving the neural separation of the big toe, apparently occurred with bipedality. This accords with the known fossil evidence, including the recently reported hominin fossils which have been dated to 4.4 million years ago.

Planar covariation of limb elevation angles during bipedal walking in the Japanese macaque

We investigated the planar covariation of lower limb segment elevation angles during bipedal walking in macaques to elucidate the mechanisms underlying the origin and evolution of the planar law in human walking. Two Japanese macaques and four adult humans walking on a treadmill were recorded, and the time course of the elevation angles at the thigh, shank and foot segments relative to the vertical axis were calculated. Our analyses indicated that the planar law also applies to macaque bipedal walking. However, planarity was much lower in macaques, and orientations of the plane differed between the two species because of differences in the foot elevation angle. The human foot is rigidly structured to form a longitudinal arch, whereas the macaque's foot is more flexible and bends at the midtarsal region in the stance phase. This difference in midfoot flexibility between the two species studied was the main source of the difference in the planar law. Thus, the evolution of a stable midfoot in early hominins may have preceded the acquisition of the strong planar intersegmental coordination and possibly facilitated the subsequent emergence of habitual bipedal walking in the human lineage.