By Andrea Hargraves 
The effort some of the Piaui’s capuchin monkeys make to break the coconuts they are feeding on is no joke—the animals even lift rocks twice their weight to split the fruits in half.
A new study shows that this hard work pays off: thanks to the use of stones as tools, primates get 50% more calories and a more balanced diet.
The data, obtained by Brazilian researchers, shows that capuchin monkeys’ craving for tools has a direct impact on their survival. But the implications also extend to what we know about the evolution of our species, indicating how the use of simple stone tools made life easier for humanity’s distant ancestors.
“The idea of using tools because they contributed to the diet, reducing the time needed to acquire food resources, is something everyone assumes makes sense in models of human evolution, but has not been tested in practice,” explains Patricia Isar, professor in the Department of Experimental Psychology at USP is the new study coordinator.
“No one has analyzed the caloric and dietary contribution of this tool-use ability in non-human primates, in a rigorous and quantitative manner, as we have now done,” Izar says. The researcher’s work with colleagues from Brazil, Europe and the United States has just appeared in the journal Current Biology.
Capuchin monkeys occupy a privileged position in this debate. Like chimpanzees, humanity’s closest surviving relatives are primates with great brains, great manual dexterity, complex social lives, and the ability to use all kinds of tools in ways that seem to vary from region to region — or that, as if there were different “cultures” of species.
On the other hand, their lineage and the lineage that would give rise to chimpanzees and sane man They were separated for more than 40 million years, when the ancestors of South American monkeys left the African continent and came to this side of the Atlantic. This means that the use of tools between them arose independently and makes it even more interesting from a scientific point of view.
The residents of the capuchin monkeys spotted in the new study live in Gilbois municipality in south Piauí and have become specialists in cracking hard fruit, especially those produced by different types of palm trees. Among them is the coconut of piasava (tree of the genus Orbignya), were evaluated in detail in the survey. Animals also apply this technique to break cashews.
Generally, solid food is placed over a large rock or tree root, which acts as an “anvil,” while the monkeys hold the rock, or “hammer,” with both hands to strike the fruit.
“They adjust the size of the stone and the effort involved in each type of food resource. They adjust the movement of the body, with a lower or greater amplitude, depending on the hardness of the shell or if the coconut has already begun to open up. Being aware of the properties of the materials and how they need to modify their behavior is incredible. ‘ says Izar.
To assess whether all this specialization is really beneficial to the animals, it was necessary to make a detailed record of the energy expenditure and food consumption of each of the monkeys that broke the coconut. Biologist Lucas Petrinelli dos Santos was one of those responsible for the mission, who is also responsible for the research.
“Every day, we would attract one individual from the group, who would be monitored all the time,” he says. The “big brother” of the monkeys began around 6:30 AM, when the group of monkeys left the caves in the nearby mountain where the animals had been spending the night, and continued until the moment it got dark and they returned. to their shelter.
“They are well accustomed to the presence of the observer and spend a lot of time on the ground and in low trees. Since the vegetation is not very dense, visibility is usually good,” says the researcher. Good, because the job requires a complete inventory of the animals’ movements throughout the day, including the tiring activity of cracking the coconut, to estimate their energy expenditure.
Peternelli-dos-Santos and his colleagues also had to count each bite the animals took of the piasava fruit, mango, or insects they fed. Finally, samples of the fruits were analyzed in the laboratory to estimate the amount of calories and key nutrients (carbohydrates, proteins and fats) present in the monkeys’ diet.
On the days devoted to cracking the coconut, the primates had a net calorie gain of 50% compared to the other days. This is mainly due to the high fat content in the food. At the same time, the days they ate coconut were less variable when it came to eating protein, another important type of nutrient. This likely means that the tool-broken coconut acts as a safe food base, helping to balance the animals’ diet.
An even more interesting and suggestive fact is the fact that when compared with other groups of species, capuchin monkeys can spend more time socializing and resting. Something like this is also thought to have accompanied tool use among distant human ancestors, spurring the development of larger brains and more complex behaviours.