TOOL USE IN CAPTIVITY AND IN THE WILD

Capuchins are natural born tool users

“Some of those cats (monkeys) are so astute that many things they see man do, they imitate and also do. In particular, there are many that when they see how to smash a nut or a pine nut with a stone, they do it in the same way and, when leaving a stone where the cat (the monkeys) can take it, smash all that are given to them” (Sumario de la Natural Historia de las Indias, de Oviedo 1526/1996). This is the first report of tool use in Cebus, dating about 500 years ago. After hundreds of years of research, we know that capuchins are very skilled at manipulating objects in captivity.

We investigate the behavioural and cognitive ingredients of capuchins’ success in tool use and the extent to which naive individuals benefit from expert models. Overall, our results show that capuchins use tools in a very flexible way, generalizing how to use an object from one context to another. They can also solve the same task using different objects and they can also modify an object in relation to the task to be performed. Moreover, capuchins demonstrate a good appreciation of the relation between the tool properties and the task requirements. For example, they are able to efficiently select a tool on the basis of a non-visual property such as rigidity. When give them a chance to manipulate new stick-like tools differing in flexibility, they were able to choose the right one to insert into a tube to obtain food both manipulating the tools theirselves or observing a human demonstrator repeatedly bending them.

Video. The experimenter introduced the three tools inside the compartments so that Gal, an adult male, could freely manipulate them. Only one of the three tools, the flexible one, is useful to reach the yogurt in the baited apparatus, a tube presenting an angle of approximately 90°. Once Gal stopped manipulating the tools, they were retrieved and placed on their preassigned location on the sliding platform. After Gal chose one of the three tools, the experimenter gave the selected tool to him and allowed him to move to the baited tube.

Video. The experimenter manipulated the three tools in front of Pedro, an adult male. Only one of the three tools, the rigid one, is useful to rake in a food reward (a 1-cm-thick banana slice) located on a rectangular wooden platform beyond the cage mesh. Once the experimenter stopped manipulating the tools, Pedro could select the desired tool by inserting a finger into the corresponding hole. After the experimenter gave the selected tool to him, Pedro could go to retrieve the banana from the table.

In 2005, the members of the EthoCebus Project observed wild bearded capuchin monkeys (Cebus libidinosus, now Sapajus libidinosus) living in the dry woodland habitat of Piaui (Brazil) routinely using stones to crack palm and other hard fruits they have previously placed on stone or wood anvils. As in captivity, capuchins proved to be very expert users. For example, when faced with stones differing in functional features such as friability and weight, they choose the functional tool on the basis of material and weight, even when weight could not be judged by vision. Moreover, prior to pounding, the monkeys achieve a stable position of the nut on the anvil. Monkeys' strategic placement of the nut reveals their capacity to improve their tool-use skills based on an understanding of shape.

Stone tool use was the first to be observed systematically in a New World monkey species. Apart from capuchins, in fact, the only non human primates who use stones and anvils to process food in the wild were chimpanzees of Western Africa. Nowadays, the EthoCebus project is going on studying all the ecological, developmental, social, physical and historical implications of this behaviour (more on http://www.ip.usp.br/ethocebus/)

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Figure. Wild capuchin monkeys using hammer stone on a log anvil (left) and a sandstone anvil (right) to crack open resistante palm nuts (photo Elisabetta Visalberghi).

Why does it take such a long time to discover that capuchins are so good at using tools in the wild? Because they are essentially arboreal: capuchins spend most of the time on trees, where it is more difficult to use tools.In contrast, capuchins living in Fazenda Boa Vista (Piauí) have terrestrial habits, an ecological condition which has favoured the emergence of this demanding behaviour. In fact, when Wright and colleagues (2019) studied the positional behavior and substrate use behaviors of these monkeys and found that they spend 27% of time on terrestrial substrates.

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