Motor planning in Colombian spider monkeys (Ateles fusciceps rufiventris)

The morphological constraint hypothesis suggests that primates with less dexterity show more consistent motor planning because there is less flexibility to compensate for inefficient actions. Spiders monkeys offer a unique opportunity to test this hypothesis because they do not have a functional thumb. Colombian spider monkeys (Ateles fusciceps rufiventris, n=8) housed at a wildlife park were tested on a standard motor planning task. A trial began when a dowel baited on one end was placed on an elevated apparatus and ended when a monkey grasped the dowel with the hands. Grip use was coded from video as radial (pronated overhand grip with the radius bone towards food) or non-radial. The percentage of radial grips was used in analyses for comparison to other platyrrhines. As a group, spider monkeys used a radial grip significantly more than chance (M ± SD=73.38% ± 18.91), indicating spider monkeys engage in second-order motor planning. Spider monkeys differed in radial grip use from captive cotton-top tamarins (Saguinus oedipus) but not captive (Sapajus spp) or wild capuchins (Sapajus libidinosus) tested on the elevated dowel task in prior work by other investigators. Together, these findings have mixed implications for the morphological constraint hypothesis that merit further study.