Habitual locomotor types and the shape of lower leg bones in primates, especially in hominoids

The mechanical dominance of the hindlimbs compared with the forelimbs in positional behavior in primates has been settled. Among primate hindlimb bones, the femur has received the main focus. The primate lower leg bones have not been studied precisely in relation to locomotor behavior. In this study, the lower leg bones of primates, 316 individuals in 80 species and 47 genera, were examined. The lengths and relative breadths of the tibia and fibula were compared across habitual locomotor types, with a focus on hominoids. Different locomotor types were characterized by hominoid, such as brachiation by lesser apes, knuckle or fist quadrupedalism and arm-swinging, including torso-orthograde forelimb locomotor behaviors, by great apes, and bipedalism by humans. The lower leg bones of great apes showed relatively large diameters, in particular, large distal breadths, in comparison with the length. Humans also showed relatively large diameters in many measuring points after those of great apes, but had differently shaped distal ends. The lower leg bones were relatively robust in these species, where a larger proportion of body weight was more habitually supported by lower/hindlimbs. Among quadrupedal monkeys, including prosimians, more terrestrially adopted animals showed larger diameters in comparison with length than did more arboreal ones in many measurements. Apes and monkeys who adopted brachiation or arm-swinging in their locomotor repertoire in arboreal environments exhibited relatively large distal end and tibial malleolus breadths compared with quadrupedal monkeys. A large breadth of the ankle compared with depth and a large malleolar breadth would help the inversion and eversion movements. The human ankle was relatively small in breadth compared with ape ankles and was adapted mainly to parasagittal rolling. The shape of the lower leg bones is therefore related to the differences in locomotor type and substrate use in primate species.