University of São Paulo Researcher Documents Yellow Capuchin Monkeys Using Sticks for Food
In order to study how the lifestyle of yellow capuchin monkeys ( Sapajus libidinosus ) affected their use of tools, a researcher from USP ended up observing a new range of behaviors of these animals in the Ubajara National Park, in Ceará. There, the group used stones and sticks to extract food from the ground. This is the first population of capuchin monkeys to use sticks to probe spiders in underground burrows and the second to use stones to dig. The discovery may help to understand the culture of these primates and the techniques used by human ancestors to obtain underground food.
“It was by chance that I realized that they had other behaviors in their tool use repertoire. One day I was observing and began to see them using stones to dig: they pick up a stone, hit it on the soil, which becomes a little looser, and they can access roots, tubers and trapdoor spiders,” says Tatiane Valença, a doctoral student who authored the article and is associated with the Capuchin Monkey Culture project at the School of Arts, Sciences and Humanities (EACH) at USP and the Neotropical Primates Research Group (Neoprego).
The surprise is that not all species of capuchin monkeys use tools and, in the populations that do, the functionality is related to breaking hard fruits, mainly coconuts. Until now, the use of stones for digging had only been observed in the Serra da Capivara National Park, in Piauí, by a group of the species Sapajus libidinosus.
In addition to stones, sticks and branches are also used by primates, a behavior that is unprecedented for this species. Using branches detached from trees or loose on the ground, primates disturb the holes of trapdoor spiders (which live underground) and manage to capture them when they escape from their burrows. “This was a behavior that was not common in other populations. In Serra da Capivara, sticks are used for other things, but not for this purpose with spiders. This is very interesting because, above all, it shows a cultural variation among these populations,” says Tatiane Valença.
The researcher began her doctorate in 2021 and, for two years, followed a group of around 30 yellow capuchin monkeys. To observe the group, she accustomed the animals to the presence of humans and gave each one a name, which helped her when recording behaviors and collecting information related to frequency.
During this period, the research recorded 214 episodes of excavations, with and without tools. The occurrences of obtaining tuberous roots accounted for 49% of them; for reaching spiders, 19%. In 32% of the cases, it was not possible to identify the food.
It is not yet known exactly how these monkeys choose the most appropriate technique for digging. One of the researchers’ hypotheses is that the composition of the soil matters. For example, with stones embedded in the soil it is more difficult to dig with the hands and the use of stones may be useless, so choosing sticks is the best option. “We believe that they use their hands when the soil is softer, that they use stones when the soil is a little harder but not so stony, and that they use sticks when the soil is very stony.”
Another characteristic of these behaviors that surprised the researcher was the difference between males and females in digging. Although the use of tools to break open fruit is similar between both sexes and different groups of monkeys, female yellow capuchin monkeys more often use only their hands to dig, while males prefer stones and sticks. “It’s not a question of skill, because female capuchin monkeys can use these stick tools in captivity. The main hypothesis for this sexual difference is that females are more sensitive to food returns. In other words, since the success of these tool uses is low, perhaps they don’t care as much about this type of use,” explains Valença.
The use of tools was very important for the evolutionary process of human beings; sticks and branches were essential for digging up roots and tubers, but these materials are rarely preserved in the archaeological record. “We don’t know how long humans have been using tools to dig. They have probably been using these tools for much longer than we imagine, but we still don’t know very well how to identify these tools in the archaeological record,” says Tatiane Valença.
Even though they are millions of years apart, the behavior of these capuchin monkeys can also help us understand more about human evolution. The researcher explains that populations that dig offer a model of what marks appear when animals dig. With this, it is possible to look at human archaeological records and identify when we first used tools to dig.
In ecological terms, the discovery made by the research is important to understand more about the behavioral variation between populations of capuchin monkeys and their cultures. This new perspective is relevant to create conservation actions for different groups, since these cultures can be important for the survival of the species.
“This reveals that capuchin monkeys have a more complex culture than we imagined and that they are extremely intelligent animals,” he says.