Specialized for the reach: Fruit picking and positional behavior favor a reach over a grasp phenotype for Geoffroy’s spider monkey ( Ateles geoffroyi )

The Geoffroy’s spider monkey ( Ateles geoffroyi ) 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 well habituated to human observers in Sector Santa Rosa (SSR), Área de Conservación Guanacaste 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 (62%: 1,338 of 2,164 fruit items), 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 (466 observations) or only by mouth (360 observations). 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 (index finger), 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.

“ Spider monkeys cast a distinct morphological silhouette – long scrawny arms and a snaky prehensile tail arching from a narrow pot-belly torso, topped by a small round head and blunt face. The commitment of this relatively large-bodied platyrrhine to a large-tree, upper canopy milieu and to ripe fruit foraging is seen throughout its skeletal and craniodental morphology. ” Rosenbert et al (2008).