Motor planning for grasping in different contexts by tufted Capuchin monkeys ( Sapajus spp.)

Studies on motor planning and action selection in object use reveal that what we choose to do in the present moment differs depending on what we intend to do next. In particular, many studies have shown that human adults adopt initially uncomfortable hand postures to accommodate later task demands (i.e., the end-state comfort effect). Recent studies on action planning in non-human primates belonging to different taxonomic groups provided contrasting results. Here we tested whether capuchin monkeys (Sapajus spp.), natural tool users, would show the end-state comfort effect in two different contexts. In Experiment 1, ten capuchin monkeys had to grasp a horizontal dowel with either the left- or right-end baited. If action selection is influenced by the intended subsequent movement (comfortably bringing the baited end of the handle to the mouth), then capuchins would prefer the grip that places the thumb-side of the hand towards the baited end; i.e. they would prefer a radial grip. In Experiment 2, seven capuchins needed to use a dowel that rested on a bracket that could be horizontally rotated (thus altering how the subject could grasp it) to dislodge an out-of-reach food reward. Also in this case, a radial grip allows the possibility to complete the task in a more comfortable way. In both experiments, capuchin monkeys adopted significantly more a radial grip than other types of grips to accommodate subsequent task requirements, thus demonstrating the end-state comfort effect. Cognitive and motor skills potentially affecting action planning in primate species are discussed. Our research protocols complied with the European Directive 2010/63/EU and were approved by the Italian Health Ministry (License n. 132/2014-C).