It is well documented that in the wild, some species of monkey use stones as tools to gain access to the tasty insides of nuts and fruits. It seems clever, but is there more to the movements that underpin this behaviour than meets the eye?

To investigate, a collaborative team of researchers, led by Madhur Mangalam from the University of Georgia, USA, studied the biomechanics of strikes made by capuchin monkeys when using stones to crack open nuts. They wanted to know exactly how the monkeys use these stone hammers. Do they alter their methods for different nut types? And what happens once the nut has cracked? Not only is this interesting in itself, but studying the way non-human primates use tools could help to inform scientists about how ancient humans may have used them.

The team studied seven wild bearded capuchin monkeys (Sapajus libidinosus) at a natural study site in north-east Brazil. They provided the monkeys with stones of known mass, and two types of locally available nut (tucum and piaçava), which differ significantly from each other. Tucum nuts are composed of two layers: a soft outer hull and a harder inner shell housing a single soft edible kernel. The tougher piaçava nuts have an outer hull and a very resistant inner shell, surrounding several kernels each within their own seed compartment. Knowing that grazing cattle often dislodge the outer hull of piaçava nuts before the monkeys get to them, the researchers first stripped off the hulls before giving them to the capuchins. By filming each of the monkeys as they used a stone to crack a nut, the team could measure how high the monkeys raised the stone before striking the nut and the speed of the stone at the moment of impact. They then compared these values between strikes, noting whether the nut had cracked.

When hitting the tucum nuts, the monkeys altered their technique based on the condition of the nut. They began by striking it with a moderate force and, if the nut failed to crack, they increased the height and velocity of the stone, hitting the nut harder. If the soft outer hull had partly cracked, they decreased the height and velocity of the stone for the next strike, probably because it requires less force to break a partially cracked nut. Once the hull had been breached completely, reaching the hard inner shell, the monkeys hit the nut harder by increasing the speed of the stone at impact. And once this shell had begun to crack, they again reduced the force of their strike, presumably to reduce damage to the nutritious kernel within. In contrast, when striking the piaçava nuts, the capuchins simply hit them repeatedly with the stone as hard as they could until the nut cracked.

Although hitting a nut more times with a smaller force is the safest way to extract the kernel without smashing it, the presence of seed compartment structures in the piaçava probably disrupts energy transferred during a strike. Maybe this is something each monkey has learnt by trial and error, discovering that the best way of cracking these nuts is to hit them with maximum force. With each crack in a nut, its material properties change and so the requirements of the next strike alter. These results show that capuchin monkeys can adjust the biomechanics of tool use in response to quickly changing tasks such as nut kernel extraction.

By comparing morphological features between today's primates and ancient humans, this type of research can help suggest how tool use by our ancestors may have evolved until we eventually began shaping stones to use as weapons and in construction. Perhaps the key to understanding how human civilisation began is locked within the nut-cracking abilities of our primate pals.

D. M
Task-specific temporal organization of percussive movements in wild bearded capuchin monkeys
Anim. Behav.