New primate behavior study: perceiving kinetic energy

photo of bearded capuchin monkey with stone tool

capuchin teaser.jpgUGA behavioral scientists in the department of psychology Primate Behavior Laboratory led by Professor Dorothy Fragaszy have a long-established expertise in problem-solving, perception, manipulation, and skill learning in primates, particularly bearded capuchin monkeys of South America.

Now a new study published January 10 In the Royal Society journal, Biology Letters, presents new insights on tool making by the capuchins and how their methods might arise out of perceptual differences between monkeys and humans

“We examined the patterns of coordination of striking movement and perceptuomotor control of stone hammers in wild bearded capuchin monkeys. The monkeys predominantly relied on the movement of their hindlimbs (hip and knee) and their torso (lumbar) to lift and lower a hammer, and to a limited extent, on the movement of their forelimbs (shoulder) to lift a hammer. They altered their patterns of coordination of movement to accommodate changes in hammer mass.

The findings that the strike's amplitude and the hammer's velocity at impact did not vary across hammers of different mass suggest that the monkeys actively altered their patterns of coordination of movement to control these parameters.”

Humans began to make their own tools by producing stone flakes millions of years ago.  No other primate flakes stones intentionally, even though some use stones as hammers to crack open encased food items such as nuts, as Fragaszy has extensively documented. 

“Why don’t other primates flake stones? We thought we might find an answer by looking at how monkeys cracked nuts using stone hammers,” said Madhur Mangalam, Ph.D. candidate in the Primate Behavior lab and first author on the study.

“We showed that monkeys, even when they are skilled at using stone hammers, do not control the hammer’s kinetic energy at impact, as humans do to flake stones. We suggest that monkeys do not perceive the hammer’s kinetic energy, as do humans – they cannot integrate information about the hammer’s velocity and mass to arrive at perception of the hammer’s kinetic energy.”
Mangalam said the team expected to find that the monkeys would handle the hammers dexterously – that is, they would alter their movements when using stones of different masses, which they did observe. However, they were surprised to find that monkeys controlled different parameters than do humans while using hammers. 
“Our study raises the possibility that perceptual processes differ between humans and other primates in ways consequential for flaking stones,” Mangalam said.

The full study is available at

Image: A wild bearded capuchin monkey is striking an intact piaçava nut with a quartzite stone hammer (Credit: Dorothy M. Fragaszy).