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

Primate hands and feet are versatile and offer an opportunity to examine how morphology reflects compromises among different functions in daily activities. In this study, we investigated the diaphyseal robusticity of metacarpals (MCs) and metatarsals (MTs) and discussed their correlation with locomotor and other behaviors in the semiterrestrial Japanese macaque (Macaca fuscata). The objectives of this study were to determine (1) whether more robust MCs and MTs experience higher forces during terrestrial locomotion than less robust bones; and (2) whether MT3, which is suggested to be the functional axis of the foot in Japanese macaques, is more robust than MT2, MT4, and MT5. Computed tomography of MCs1-5 and MTs1-5 was performed in 10 monkeys. As a measure of bone robusticity, the buckling strength of each MT and MC was calculated as J/L-2 where J is the polar second moment of area and L is the bone length. Hand and foot pressure were recorded using plantar pressure measurement systems while two monkeys moved on a flat floor over a range of speeds (0.72-2.56 m/s). The relationship between the bone robusticity and the load applied to the bones during terrestrial locomotion was analyzed. Our results did not support the two predictions. There was no positive correlation between diaphyseal robusticity and the peak force in both male and female Japanese macaques. There was no clear difference in bone robusticity among MTs2-5 in both males and females. These results suggest that the relation between MC and MT robusticity and mechanical loading during locomotion is not as straightforward as might be expected, possibly due to the complex multifunctionality of primate hands and feet. Additional integrative studies that similarly incorporate morphological and experimental approaches are expected to provide useful insights into macaque hand and foot morphology.