Manual function and performance in humans, gorillas, and orangutans during the same tool use task.

Abstract

Humans are known to possess more complex manual abilities than other primates. However, the manual abilities of primates have not been fully explored, and we still do not know if the manipulative abilities we attribute to humans are unique. The aim of this study was to compare the manual function and performance developed by humans, gorillas and orangutans while performing the same experimental tool use task. The study was conducted on 20 humans, 6 gorillas, and 7 orangutans. Each individual had to use a tool to collect food from a maze during six experimental sessions while maintaining the same unconstrained body posture condition. We quantified the different manual techniques used and the manual performance. Each species used different techniques. Humans used bimanual grip techniques, pad-to-pad precision grasping postures, and in-hand movements involving fingertips. Gorillas used unimanual grip techniques and simple in-hand movements while orangutans used a variety of strategies (e.g., hand or mouth). With these techniques, humans performed the task better than both gorillas and orangutans (e.g., by being quicker to collect the food). This study highlights other ways in which humans' manual dexterity differs from that of other species and emphasizes the distinct manipulative function of orangutans. The differences between the species could be due to the differing muscular anatomy and morphology of the hands, with hand proportion possibly placing particular biomechanical constraints on each species. The differences between gorillas and orangutans could result from their different locomotor behaviors, and we hypothesize terrestriality facilitates the development of complex manipulation.