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Monkeys Practice Skilled Tool Use To Crack Open Their Nuts

February 28, 2013
Image Caption: A bearded capuchin monkey cracks open a stably-placed palm nut with a stone for a hammer. Credit: Barth Wright

WATCH VIDEO: [Bearded Capuchin Monkey]

Brett Smith for redOrbit.com – Your Universe Online

As biologists continue to study primate behaviors, they are beginning to understand the extent of their intelligence and how they apply that intelligence in their daily life.

According to a new report in the open access journal PLoS ONE, an international team of researchers, led by Dorothy Fragaszy and colleagues from the University of Georgia, has shown for the first time that bearded capuchin monkeys practice skilled tool use, or the ability to use a tool based on understanding certain tactile information, in an effort to crack open nuts for the tasty treat inside.

“Humans can use hand tools smoothly and effectively in varying circumstances; in other words, skillfully,” the report said. “A few other species of primates crack encased foods using hammer tools and anvils. Are they skilled?”

By observing the monkeys in study sites across northeast Brazil, the team saw the primates deliberately place palm nuts in a stable position on a cracking surface, or anvil, before trying to bust them open using a stone tool. The researchers also said that monkeys were looking to maximize their efforts by positioning the nuts flat side down more frequently than expected by random chance.

According to the report, previous research by the same team of biologists has found that capuchin monkeys living in the same area select hammer stones by weight, nuts by resistance to cracking, and cracking surfaces according to their recent use by others.

The study authors wrote that this systematic selection implies that the monkeys are paying attention to the various properties of these objects and conditions in an attempt to optimize their process.

During their latest round of observations, the team watched as the monkeys knocked the nuts on the surface a few times before nestling them into an ideal rut that held them in place. This strategic placement of the nuts on the anvil suggests that the monkeys were paying attention to the fit between the nut and the surface and adjusting their actions accordingly.

The team tested to see if the monkeys systematically placed the nuts in the anvil by presenting them with marked piassava nuts (Orbignya spp.) so that they could recognize on which side the nut stopped when rolled on a flat surface. In another experiment, the biologists blindfolded human participants and asked them to place similarly marked nuts as if to crack them, to compare the behaviors used by the two species.

They found that both the humans and monkeys in their experiments used identical methods to place the nuts on a cracking surface.

In their report, the researchers said their experiment confirms that the bearded capuchin monkeys are indeed skilled tool users.

“Capuchins apparently detect the relevant properties of the nut they will place in the pit on the anvil by repetitively knocking the nut against a hard surface,” they wrote. “An important conclusion from the current studies, and others from our work at (the study sites), is that bearded capuchin monkeys’ effective nut-cracking involves concurrent attention to several perceptual features of the problem and effective modulation of activity in accord with variable circumstances.”


Source: Brett Smith for redOrbit.com - Your Universe Online