The effort that some capuchin monkeys in Piauí make to break the coconuts on which they feed is no joke — the animals even lift rocks twice their own 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, show that the capuchin monkeys’ passion for using instruments has a direct impact on their survival. But the implications also extend to what we know about the evolution of our own species, indicating how the use of simple stone tools may have made life easier for humanity’s remote ancestors.
“The idea that tools came to be used because they contributed to the diet, reducing the time for the acquisition of food resources, is something that everyone assumes as logical in models of human evolution, but which has never been tested for real”, explains Patrícia Izar, professor at the Department of Experimental Psychology at USP and coordinator of the new study.
“No one has really analyzed the caloric and nutritional contribution of this ability to use tools in non-human primates, in a rigorous and quantitative way, as we have done now”, says Izar. The researcher’s work, along with colleagues from Brazil, Europe and the USA, has just appeared in the specialized journal Current Biology.
Capuchin monkeys occupy a privileged position in this debate. Like chimpanzees, humanity’s closest living relatives, they are primates with big brains, considerable manual dexterity, a complex social life, and the ability to use all sorts of tools in ways that seem to vary from region to region — or that is, as if there were different “cultures” of the species.
On the other hand, their lineage and the one that would give rise to chimpanzees and the Homo sapiens they have been separated for more than 40 million years, when the ancestors of the South American monkeys left the African continent and came to this side of the Atlantic. This means that the use of instruments between them arose independently and makes it even more interesting from a scientific point of view.
The population of capuchin monkeys followed in the new study lives in the municipality of Gilbués, in southern Piauí, and has become a specialist in breaking hard fruits, especially those produced by different types of palm trees. Among them, the coconuts of piassava (tree of the genus Orbignya), evaluated in detail in the survey. The animals also apply the technique to break cashew nuts.
In general, hard food is placed on top of a large rock or tree root, which serves as an “anvil”, while monkeys hold the rock, or “hammer”, with both hands to strike the fruit.
“They adjust the size of the stone and the effort for each type of food resource. They modulate the movement of the body, with less or greater amplitude, depending on the hardness of the shell or if the coconut has already started to open. The perception of the characteristics of the materials and how they need to adjust their behavior is incredible,” says Izar.
To assess whether all this specialization is really worthwhile for the animals, it was necessary to make a detailed record of the energy expenditure and food consumption of each of the coconut-breaking monkeys. One of those responsible for the task was biologist Lucas Peternelli-dos-Santos, who is also responsible for the research.
“Every day, we would draw one of the individuals from the group, who would be monitored all the time”, he says. The “Big Brother” of the apes started around 6:30 am, when the band of monkeys left the caves in the neighboring mountain where the animals used to spend the night, and continued until the moment when it started to get dark and they returned to their shelter.
“They are well used to the presence of the observer and spend a lot of time on the ground and in lower trees. As 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 breaking coconuts, to estimate their energy expenditure.
Peternelli-dos-Santos and his colleagues also had to count each bite the animals took on the piassava fruit, mangoes, or insects on which they fed. Finally, samples of the fruits were analyzed in the laboratory to estimate the amount of calories and the main nutrients (carbohydrates, proteins and fats) present in the monkeys’ diet.
On the days when they were dedicated to breaking the coconuts, the primates obtained a net gain of calories of 50% in relation to the other days. This was mainly because of the high fat content of the food. At the same time, the days they had the coconuts were less variable when it came to protein intake, another important type of nutrient. This likely means that the coconuts broken with the instruments act as a safe food base, helping to balance the animals’ diet.
An even more interesting and suggestive fact is the fact that, when compared to other populations of the species, the capuchin monkeys in the study can afford to spend more time socializing and resting. It is thought that something like this also accompanied the use of tools among the remote ancestors of humans, spurring the development of larger brains and more complex behaviors.
