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

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 (SRFvrts) 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 SRFvrts act on the forelimbs, and (2) SRFvrts 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 SRFvrts 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 SRFvrts 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 SRFvrts 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. Two modes for mothers to transport her infant.