How bearded capuchin monkeys (Sapajus libidinosus) prepare to use a stone to crack nuts

Moving

Examples of realized scientific careers can provide ideas and inspiration for others aiming to pursue such careers. Here I recount in brief the story of my long career in primatology (1973 to the present), focusing on one enduring theme in my research: the nature and genesis of goal-directed action (evident in movement). The story begins in graduate school, passes through developing my own laboratory, on to pursuing a spectrum of studies with mentees and collaborators, developing a theoretical explanatory framework for goal-directed action that I think holds promise for the field as a whole, and ends with an exciting field project that seems a suitable finale to my career. I mention the value to me, the field, and society of participation in scientific societies, including the American Society of Primatologists, throughout my career.

How tufted capuchin monkeys (Sapajus spp.) and humans (Homo sapiens) handle a jointed tool

The embodied theory of tooling predicts that when using a grasped object as a tool, individuals accommodate their actions to manage the altered degrees of freedom in the body-plus-object system. We tested predictions from this theory by studying how 3 tufted capuchin monkeys (Sapajus spp.) and 6 humans (Homo sapiens) used a hoe to retrieve a token. The hoe's handle was rigid, had 2 segments with 1 planar joint, or had 3 segments with 2 (orthogonal) planar joints. When jointed, rotating the handle could render it rigid. The monkeys used more actions to retrieve the token when the handle had 1 joint than when it had no joints or 2 joints. They did not use exploratory actions frequently nor in a directed manner in any condition. Although they sometimes rotated the handle of the hoe, they did not make it rigid. In a follow-up study, we explored whether humans would rotate the handle to use a 2-jointed hoe in a conventional manner, as predicted both by the embodied theory and theories of functional fixedness in humans. Two people rotated the handle to use the hoe conventionally, but 4 people did not; instead, they used the hoe as it was presented, as did the monkeys. These results confirm some predictions but also highlight shortcomings of the embodied theory with respect to specifying the consequences of adding multiple degrees of freedom. The study of species' perceptual sensitivity to jointed object's inertial properties could help to refine the embodied theory of tooling. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

Adult and juvenile bearded capuchin monkeys handle stone hammers differently during nut-cracking

Wild bearded capuchin monkeys (Sapajus libidinosus) habitually use stone hammers to crack open palm nuts and seeds on anvils. This activity requires strength, balance, and precise movement of a large stone with respect to the item placed on an anvil. We explored how well young monkeys cope with these challenges by examining their behavior and the behavior of adults while they cracked palm nuts using a stone. Using video records, we compared actions of six juvenile (2-5 years) and six adult (7+ years) wild monkeys during their first 20 strikes with one unfamiliar ellipsoid, quartzite stone (540 g), and the outcomes of these strikes. Compared with adults, juveniles cracked fewer nuts, performed a more diverse set of exploratory actions, and less frequently placed one or both hands on top of the stone on the downward motion. Adults and juveniles displayed similar low frequencies of striking with a slanted trajectory, missing the nut, and losing control over the nut or stone after striking. These findings indicate that young monkeys control the trajectory of a stone adequately but that is not sufficient to crack nuts as effectively as adults do. Compared with juveniles, adults more quickly perceive how to grip the stone efficiently, and they are able to adjust their grip dynamically during the strike. Young monkeys develop expertise in the latter aspects of cracking nuts over the course of several years of regular practice, indicating that perceptual learning about these aspects of percussion occurs slowly. Juvenile and adult humans learning to use stones to crack nuts also master these features of cracking nuts very slowly.