Age affects action planning in wild bearded capuchin monkeys (Sapajus libidinosus)

The study of how animals anticipate future actions is crucial to understand cognitive processes that guide behavior. Action planning involves the abilities to plan how to perform an action and to coordinate effective movements within this plan. Planning is fundamental to the execution of most daily activities, including object manipulation. The End-State Comfort (ESC) effect is the capability of grasping an object in a way that enhances hand comfort and object control while performing the next action; ESC demonstrates second-order motor planning in grip selection. In humans, anticipatory planning skills develop slowly over the course of sensory-motor maturation. Age-related differences in ESC effect have not been investigated in other primates so far. We assessed the anticipatory motor planning of infant, juvenile and adult capuchin monkeys (N=15) belonging to a wild group. The study area was located in Fazenda Boa Vista in the North-Eastern Brazilian state of Piauí. We used a modified version of the "elevated spoon task" in which a horizontal stick was supported by two Y-shaped forks; the stick was baited either on its left or right side. Subjects grasped the stick and brought the bait to the mouth. We found that "comfortable" grips significantly increased with age. All age classes performed "comfortable" grips significantly more than expected by chance. However, their planning ability varied as a function of age: adult individuals used them significantly more frequently than infants, whereas juveniles were in-between adults and infants and did not significantly differ from both groups. Thus, we found a positive relation between age and sensitivity to the ESC effect. Our findings in capuchins parallel those reported in humans and indicate that simple grasping tasks allow drawing and comparing developmental trajectories of planning skills in primates, also in natural settings.