Capuchin monkeys are choosy about the best nutcrackers

From the carpenter choosing the right strength of drill, or the artist selecting the right weight of pencil, humans have a natural talent for picking the right tool for the job. Now, it seems that monkeys are similarly selective about their tools. In the first study of its kind, Elisabetta Visalberghi from the National Research Council, Italy, found that capuchin monkeys are able to pick stones with the right properties for nutcracking.

Capuchins often use stones to crack otherwise impenetrable nuts upon hard, flat surfaces, turning innocuous forest objects into their own hammers and anvils. By examining their cracking sites, Visalberghi deduced that the animals were picky about their hammers, for the sites were littered with hard and heavy rocks that weighed as much as 40% of an adult. However, it was entirely possible that the monkeys used any old rocks and those that remained were simply the ones that hadn’t eroded yet.

Visalberghi decided to put eight wild capuchins to the test by allowing them to choose between sets of potential hammers. These monkeys regularly use stones to break into palm nuts and they need heavy, hard materials for the job. Visalberghi removed all the stones from the experimental arena and provided the capuchins with a choice of two – one of which would excel as a nutcracker and another which would not. The monkeys selected the right stone at least 90% of the time, correctly picking solid siltstone over crumbly sandstone, and big quartzile stones over small ones.

In both cases, the stones were natural ones of the type that the monkeys regularly encounter in their habitat. No surprise then that the animals immediately and confidently singled out the right tool. But Visalberghi found that they did just as well when choosing between artificial stones – they just spent more time examining the different options on offer.

Normally, capuchins know that a bigger stone is probably going to be a heavier one. When they were given two stones of the same size but different densities, they spent a bit of time on investigation. They picked both up, weighed them in their hands and tapped them, before making off with the heavier one. Tapping was important – capuchins use it to tell full nuts from empty ones, and while they never tapped natural stones, they often used the trick to discern between artificial stones of unexpected densities.

If Visalberghi gave them a light, large stone and a heavy, small one, those that picked the small one immediately walked away with it; if they picked up the lighter stone first, they checked the other option out before choosing that instead. These monkeys have a very good idea about what they want from their hammers and again, they made the right choice on over 85% of the trials.

As final proof of the monkeys’ deliberate decision-making, Visalberghi showed that when they examined both options, they switched from the wrong stone to the right one far more often than they did the other way round.

In a way, the excellent performance of these capuchins shouldn’t come as a surprise – they are, after all, very intelligent animals. But it’s worth noting that captive capuchins behave differently; they’re still choosy but much less successful in their choices than their wild counterparts. It just goes to show that we need to be careful about making generalisations about wild behaviour by looking at captive individuals that lack the full breadth of experience that their free-range peers have.

Reference: Current Biology 10.1016/j.cub.2008.11.064

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