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

Abstract

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.

Medial metatarsal rigidity in extant non‐human catarrhine primates

“…Additionally, primarily arboreal cercopithecines and colobines along with orangutans are inclined to show mixed morphology with neither Mt1 or Mt2 consistently being absolutely more rigid. Moreover, the findings in the present study utilizing a more comprehensive sample of catarrhine primates echoes previous research focused on H. sapiens (Griffin & Richmond, 2005) and M. fuscata (Higurashi et al, 2019) in demonstrating that differences between metatarsals in bone rigidity do not correlate with regional differences in peak plantar pressure in the foot during weight support.…”

“…One consistent finding in many of these studies is that metapodials tend to differ in their absolute and relative rigidity within individuals and species (Griffin et al, 2008; Griffin & Richmond, 2005; Higurashi et al, 2019; Marchi, 2005; Patel, Jashashvili, et al, 2018; Patel, Organ, et al, 2018; Patel et al, 2020). Moreover, some taxa show greater asymmetry in inter‐digit metapodial rigidity compared to others.…”

“…Biological anthropologists have a long history of studying musculoskeletal morphology of extant primate hands and feet in order to reconstruct grasping performance, substrate preference, positional behaviors, and locomotion in fossils. In studies focused on locomotor behavior, one aspect of skeletal morphology that has recently received more attention is the distribution of cortical bone near the midshaft of metapodial diaphyses (i.e., its cross‐sectional geometry, CSG) (Dowdeswell et al, 2017; Griffin et al, 2008; Griffin & Richmond, 2005; Higurashi et al, 2019; Jashashvili et al, 2015; Marchi, 2005; Patel, Jashashvili, et al, 2018; Patel, Organ, et al, 2018; Patel et al, 2020). Specifically, measures of CSG such as polar second moment of area have been quantified to estimate metapodial bending and torsion rigidity.…”

“…Patel, Organ, et al (2018) found similar patterns in adult specimens of Pan which had Mt1s that were on average 1.96 times more rigid than the Mt2s. Asymmetry between Mt1 and Mt2 second polar moment of areas in Macaca fuscata is more moderate than humans and African apes, but Mt1s are still on average 1.08 times more rigid than the Mt2s (Higurashi et al, 2019). In contrast, Pongo has Mt2s that are on average 1.07 times more rigid than the Mt1s, but the asymmetry in strength between both bones is generally lower and more variable compared to other hominids (Marchi, 2005).…”

“…and Higurashi et al (2019) for M. fuscata. Such poor correlations likely reflect the large amount of variation generally seen in primate plantar pressure across all studies.…”