Whishaw, I. Q.

Geoffroy spider monkey has distinctive features, including a vestigial external thumb, long fingers and forelimbs and a prehensile tail. To better understand how its derived morphology is used during natural foraging, we filmed wild spider monkeys in northwestern Costa Rica. We analyzed frame-by-frame video recordings to examine the influence of this morphology on picking 14 fruit species. The spider monkeys most frequent reach strategy was a branch withdraw in which they reached equally with either hand for a branch, hooked their fingers around it, and pulled it toward themselves to take the attached fruit by mouth. Less frequently they picked fruit by hand or only by mouth. Hand and arm extension, with rotatory movements at the wrist, elbow, and shoulder, assisted reaching and transferring food to the mouth, and tail prehension further assisted extending reaches horizontally and ventrally into the tree canopy. To pick fruit by hand, they used tactile guidance and finger pad to palm grasps, mainly with the second digit, and rotatory arm and head movements to deliver fruit from the hand to the mouth. Fruit was grasped by the incisors and chewed with molars to release the endocarp for swallowing, with the exocarp sometimes ejected by spitting. The combination of branch-withdrawal, mouth grasping and postural extension via a prehensile tail constitutes a reach phenotype that contrasts with the grasp phenotype of the sympatric capuchin monkey. The behavioral and morphological commitment to a reach phenotype in spider monkeys supports the idea that the reach and the grasp have separate evolutionary histories and, with respect to spider and capuchin monkeys, contribute to niche partitioning.