Specialized for the Reach: Visual Control of 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, elongated fingers and forelimbs, and a prehensile tail. The purpose of the present study was to understand how this derived morphology influences visually mediated reach and grasp movements during fruit picking. Wild spider monkeys, habituated to human observers, were filmed in Sector Santa Rosa (SSR), Área de Conservación Guanacaste in northwestern Costa Rica. We analyzed frame-by-frame video recordings of the monkeys picking 14 fruit species. The most frequent reach strategy was a branch-withdraw (62%; 1,338 of 2,164 fruit items), in which the monkeys hooked their fingers around a branch and pulled it toward themselves, to take the attached fruit by mouth. They sometimes used just their mouth to reach (17% of observations). Reaching with arm and hand extension to pick fruit by hand (21% of observations) was achieved using power grasps that relied on tactile cues. It is likely that picking fruit by hand or mouth involved foveal vision whereas grasping and manipulation branches used peripheral vision. Most fruit-picking sequences featured tail prehension on a branch, which extended the monkeys’ reach horizontally and ventrally into the small distal branches of the canopy. The observed reach strategies and the configuration of visually guided reaching skills of the hands and mouth—contrasted with the absence of visually mediated grasps—indicate that spider monkeys exhibit a reach phenotype. This contrasts with the grasp phenotype of the sympatric capuchin monkey, which uses the thumb and fingers to pick individual fruit items. These findings support the idea that the reach and the grasp have separate evolutionary histories in spider and capuchin monkeys, potentially facilitating fruit harvesting in different microcanopy locations and thereby contributing to niche partitioning.