Capuchin monkeys choose the right tool for the nut

Wild capuchins carefully choose the most effective stones for cracking nuts, suggesting primates used stone tools earlier than thought

Wild capuchin monkeys don’t thoughtlessly grab any handy piece of stone to crack open hard-shelled nuts at snack time. These slender, agile primates select the best tool for the job, a new study finds.

Much like people, capuchins translate past experiences into action, say primatologist Elisabetta Visalberghi of the Institute of Cognitive Sciences and Technologies in Rome and her colleagues. These monkeys draw on a reservoir of knowledge about a variety of stones and nuts to select suitable nut-cracking implements, the scientists assert in a study published online January 15 in Current Biology.

Capuchins make mental plans for fracturing a particular nut before selecting an appropriate stone for the task, Visalberghi’s team proposes.

“The present findings make capuchins a compelling model to track the evolutionary roots of stone-tool use,” Visalberghi says. Because capuchins last shared a common ancestor with humans approximately 35 million years ago, the team writes, the capacity for stone-tool use evolved earlier than thought.

In Visalberghi’s investigation, wild monkeys living in a forested area of Brazil individually approached two or three stones that differed in hardness, size or weight. One stone was best for cracking nearby palm nuts. Nearly all the time, animals chose the superior stone.

In one case, an adult male capuchin briefly touched each of two stones of comparable hardness before picking up the larger, heavier one. He then took that stone to a nearby fallen log. After placing a palm nut on the log, he stood upright on the makeshift anvil and broke the nut’s shell with his rock.

Such findings underscore wild capuchins’ proficiency at trial-and-error learning, remarks primatologist Frans de Waal of Emory University in Atlanta. Thus, planning need not have contributed to the animals’ tool preferences, as argued in the new report, de Waal says.  After having spent their lives learning to associate certain types of stones with nut-cracking success and others with failure, monkeys in the new study automatically applied that knowledge to experimental choices, he suggests.

Field observations suggest that wild chimpanzees also find especially effective stones for cracking different types of nuts. It will be difficult to run comparable experiments with wild chimps, de Waal notes, since many are wary of people due to frequent run-ins with poachers. But other evidence indicates that chimps do in fact solve problems using planning and insight, he says.

Visalberghi’s team received permission to study wild capuchins on privately owned Brazilian land. Eight monkeys participated in experiments.

Animals completed 10 trials in each of five conditions. In the first two conditions, choices consisted of pairs of stones that differed either in hardness or in size and weight. So, a monkey might select between a big piece of quartzite and a small piece of quartzite, or between same-sized pieces of quartzite and sandstone, a type of rock more apt to break on impact.

In the next three conditions, capuchins confronted artificial but realistic-looking stones constructed of a hard material. Animals chose between stones of the same size but different weights; between a light and large stone and a heavy and small stone; and among a light and large stone, a light and small stone and a heavy and large stone. In each condition, heavier stones worked best as nut crackers.

Capuchins chose the most effective stone for cracking nuts more than 90 percent of the time in four conditions. That figure fell slightly to 85 percent when the monkeys selected from artificial stones of the same size and different weights.

Monkeys usually touched the more-effective natural stone first, indicating that they could tell by sight which one was harder or heavier. With artificial stones, which had weights that couldn’t be detected visually, monkeys usually moved, lifted or tapped each stone before making a selection.

Bruce Bower has written about the behavioral sciences for Science News since 1984. He writes about psychology, anthropology, archaeology and mental health issues.

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