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.