How Capuchin Monkeys Use Their Semi-Prehensile Tails

Among primates, prehensile/semi-prehensile tails have evolved independently in the families Atelidae and Cebidae of the infraorder Platyrrhini (Neotropical monkeys). They facilitate maintaining stability during locomotion on thin, flexible branches and while reaching for food on challenging substrates. How a prehensile/semi-prehensile tail is coordinated with the hind limbs to facilitate controlled, flexible adoption of postures remains unknown. In an experimental set-up in the wild, we induced capuchin monkeys to adopt a tail-assisted, head-down tripodal posture to reach for food, documenting from slow-motion video recordings (120 fps) both qualitative changes in the monkeys’ positional behaviour – the relative orientation of their limbs and semi-prehensile tail – and quantitative changes in the left knee angle. The monkeys coordinated their tail and hind limbs in an online manner by preparing to anchor the tail over a substrate when beginning to adopt a tail-assisted, head-down, tripodal posture, but anchoring it only at the moment when they extended their hand to grasp the food. Coordination of their semi-prehensile tail with their limbs enables these capuchin monkeys to adjust their posture more flexibly compared to anchoring their tail over a substrate in anticipation and subsequently changing posture.