Cashew nuts, West Indian locust seed pods, jatobá in Brazil, and other hard foods are cracked open using stones by Capuchin monkeys in the Cerrado and Caatinga as hammers and anvils. The instruments are bits of quartzite and sandstone discovered in locations known as processing sites. The only reason the animals visit these locations is to search for these stones to use as hammers and anvils. A nut or seed is pounded between two stones, one of which serves as an anvil.

In a new study, Brazilian researchers demonstrate that the correlation between food hardness and tool size is not always as strong as previously believed in an article that was published in Scientific Reports1.

The researchers measured the hardness of the food being processed, tool size and weight, and the accessibility of stones in three populations of beard capuchin monkeys for their study. The discoveries were the outcome of a lot of effort. Foods including babassu, West Indian locust, cashew, and wild cassava that were most frequently found in the processing sites were noted by the researchers. They also recorded the sizes and weights of the tools they discovered, the sizes and weights of the available stones, the hardness of each type of food using a particular tool, and the tool usage in each research region.

(a) Capuchin monkey from CVNP recorded on camera-trap using a 4650 g stone tool to crack open an Attalea palm nut. (b) The stone tool used. 10 cm scale. Credit: *Scientific Reports* (2022). DOI: 10.1038/s41598-022-18661-3

Even when monkeys have stones that are ideal for usage on a specific food resource, they may utilize excessively heavy tools, presumably indicating a cultural feature of that group, as was shown in one of the populations they studied. They came to the conclusion that culture, which they described as knowledge passed down through social learning from one generation to the next, can also affect behavior in this area.

The population mentioned is found in Brazil's Center-West state of Goiás, specifically in Chapada dos Veadeiros National Park. This population's characteristics were compared in the study to capuchins who reside in Serra das Confuses National Park in Piau, a state in the Northeast, and to a different group who resides in Serra da Capivara National Park, which is located about 100 km apart in the same state. The monkeys in Serra das Confuses use smaller tools to open smaller, softer fruit but big, heavy hammers to break open the very hard coconut shells.

Even for delicate cuisine, monkeys in Chapada dos Veadeiros use the heaviest stones because they have a variety of sizes to select from. Unsurprisingly, the researchers found the biggest stone lifted by capuchins in this latter park. They observed someone lifting a hammer stone that was later determined to weigh 4.65 kg, despite the fact that the average adult male weighs 3.5 kg. Although there are stones of all sizes available and people can choose a smaller size, the population in Chapada dos Veadeiros primarily used the larger ones, contrary to the authors' expectations that they would find a very close correlation between the type of food and the size and weight of the tool. They most likely picked up this behavior from their forebears.

When compared to other populations, there is a cultural difference.

Studies in other regions, including Serra de Itabaiana in Sergipe and Chapada Diamantina in Bahia (both states in the Northeast), involving capuchine as well as stones and the same kinds of fruit and seed, have not discovered processing sites or the use of stone tools for this purpose. This supports the cultural learning hypothesis.

Although cashew nuts are plentiful, the capuchins in Serra das Confuses need tools to crack open other types of food. Their actions are a result of their cultural legacy rather than the availability of resources. To determine whether cultural differences can be connected to genetic variations, researchers are currently examining the genomes of all three populations.

Some capuchins consume the powder created by pounding stones, or they use it to anoint themselves. They might also brush their teeth with the powder. Although the researchers are unsure of their motivations, they think one explanation could be to fight parasites.

In the experiments, homogeneous material-containing anvils that broke apart also created flakes. The flakes closely resembled the lithic tools discovered by archaeologists at excavations around the world, but the monkeys did not use them. According to the researchers, flakes were accidently acquired by the first hominins before they were purposefully produced for use as tools. If an inventive individual begins using flakes as tools in the future and others pick it up from watching, Capuchins may also utilize them as tools. Therefore, we can use these primates as a model to better explain the evolution of humans.

The lithic tools used by the capuchin population in Serra da Capivara had various wear patterns depending on the activities engaged, according to a previous study by the same group of researchers2. It may be possible to determine how our earliest ancestors used lithic tools by comparing the wear marks on tools used by monkeys and hominins. Because of this, learning more about the evolution of humans through research on Brazilian capuchin monkeys may be possible.

1

Tiago Falótico et al, Stone tools differences across three capuchin monkey populations: food's physical properties, ecology, and culture, Scientific Reports (2022). DOI: 10.1038/s41598-022-18661-3

2

Lydia V. Luncz et al, A primate model for the origin of flake technology, Journal of Human Evolution (2022). DOI: 10.1016/j.jhevol.2022.103250

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