Differences in the vertical components of substrate reaction forces between two modes of infant carrying in Japanese macaques (Macaca fuscata fuscata)

Abstract

Objectives: Japanese macaque mothers carry their infants ventrally (VC) or dorsally (DC). As the infant weight increases, the dominant mode of infant carrying transitions from VC to DC. We biomechanically tested the hypothesis that the transition is associated with relieving vertical components of substrate reaction forces (SRF vrt s) acting on the forelimbs, as macaques prefer to support their body weight mainly using the hindlimbs. The test is based on two expectations: (1) during VC, higher SRF vrt s act on the forelimbs, and (2) SRF vrt s during DC do not significantly differ from those of a control condition in which the mother walked alone.Materials and methods: Two female Japanese macaques rearing 1-year-old infants were used for terrestrial locomotor experiments, and five females were used for arboreal experiments. We collected the SRF vrt s acting on the fore-and hindlimbs at midstance when the macaques walked on the ground and pole and statistically compared the values among the control condition, VC, and DC.Results: Higher relative SRF vrt s normalized by the total weights of the mother and infant acted on the forelimbs during VC compared with the control condition and DC, regardless of the substrate type. The relative forelimb SRF vrt s during DC exhibited similar values to those during the control condition.Discussion: The transition to DC could relieve the excessive loads acting on the forelimb. Despite the biomechanical costs of VC, the carriage mode has evolved in the order primates likely because VC might enable the mother to protect her infant from predators.

Diagonal‐couplet gaits on discontinuous supports in Japanese macaques and implications for the adaptive significance of the diagonal‐sequence, diagonal‐couplet gait of primates

“…The LSLC region should theoretically contain positive AISOTs (Figure 6a) where the duty factor was greater than 75%, a higher value than those observed in this study. The excessively high‐duty factor may render locomotion impractical because the amplitude of the substrate reaction force required to propel the animal forward and upward decreases (Goto et al, 2022; Granatosky et al, 2018; Schmitt, 1994).…”