Kinematic adjustments to arboreal locomotion in Japanese macaques (Macaca fuscata)

Although biomechanical adaptations to arboreal locomotion have been well investigated in primates and other mammals in laboratory settings, the results are not consistent, and more species need to be studied to build a comprehensive picture of this. Here, we used three-dimensional videography to quantify kinematic parameters thought to be associated with locomotor stability while two Japanese macaques walked on terrestrial and simulated arboreal substrates (a horizontal pole, which was narrow relative to the animal's body width). The parameters investigated included temporal-spatial gait variables, those associated with compliant walking, the height of the shoulder and hip, and hand and foot clearance during the swing phase. We found that there were many individual differences in kinematic adjustments made by the monkeys during arboreal locomotion. More importantly, the results were consistent between the monkeys for three parameters: maximum hand clearance, maximum hip height, and maximum foot clearance. The monkeys showed lower maximum hand and foot clearances during arboreal locomotion than during terrestrial locomotion, indicating that the hands and feet were kept close to the substrate surface during the swing phase. They also showed lower maximum hip heights during arboreal locomotion, suggesting that their whole-body centers of mass were lowered. These consistent kinematic adjustments can be interpreted as strategies for enhancing stability and reducing the risk of falling from a height. Overall, these results show that Japanese macaques make significant biomechanical adaptations to arboreal locomotion that are not fully consistent with existing data for other animals.