Many monkeys are skilled at using stones, using them to dig out roots, cut plants and open various goodies, including fruits, nuts and chestnuts.
But it seems that some apes are also using stone tools for something a little different. In an article published this month in the journal Ethology, researchers reported that some monkeys often rub stones around their genitals or gently tap them in that area. And that these behaviors are accompanied by signs of physiological sexual arousal not produced by other actions involving stone manipulation.
In other words, said Camilla Cenni, a doctoral candidate at the University of Lethbridge in Canada who carried out the research for her dissertation, monkeys appear to practice “a form of self-directed, tool-assisted masturbation.”
Some of the best-known examples of tool use by wild animals are linked to the incessant life-or-death struggle to find enough food: chimpanzees use sticks to gather termites, crows use sticks to extract larvae from logs, and sea otters crush snails with help. of stones.
The monkey study is not the first report of wild animals using objects to masturbate, but it does provide new evidence that, at least in some cases, animals appear to use tools solely for pleasure. “One could argue that it’s not really helpful behavior or that it promotes adaptation,” Cenni said.
The study is based on observations of a population of long-tailed Macaca monkeys that roam a large area and live in or near Sacred Forest Monkey Sanctuary in Ubud, a Balinese city in Indonesia, where they are routinely fed. of humans.
Scientists had previously observed that these apes often handle rocks for no apparent purpose. Monkeys can, for example, knock stones together in their hands, or they can lift and drop them repeatedly.
“It’s some sort of playful manipulation that doesn’t seem to have an obvious function,” said Cenni.
Monkeys also sometimes use stones to rub or pat their genitals and groin. It was this behavior that led to what the researchers called the “sex toy” hypothesis.
To systematically investigate the hypothesis, Cenni and his colleagues analyzed the actions of 173 stone-handling monkeys. They found that young males practiced genital patting and rubbing more often than adult males and that they tended to do so especially in sexually tense situations, such as when they or another monkey nearby were trying to attract a mate for sex or giving signals. of sexual arousal. Furthermore, the behavior tended to precede the males’ physical signs of sexual arousal and lasted longer when arousal occurred. These standards did not hold for other types of stone handling.
“The data are very convincing,” said Elisabetta Visalberghi, an expert on primate cognition at the Institute of Cognitive Sciences and Technologies at the Italian National Research Council, who was not involved in the study. “What they found is that there was indeed something of a sexual nature going on.” (Like any good scientist, she did not fail to make the appropriate caveats. “It’s very difficult to assess the pleasure of masturbation,” she observed.)
Using a rock as a sex toy can seem unpleasant, but Cenni said he never witnessed the monkeys making any sounds that suggested they were in pain, as they sometimes do when they are bitten in a fight, for example. Even so, she noted, the idea that this particular activity can cause pain as well as pleasure cannot be ruled out without further testing.
The researchers theorize that the monkeys may have come across the patting and genital rubbing while performing other seemingly random actions involving rock handling. If true, it would fit the scientific theory according to which the use of tools may have evolved from the playful manipulation of objects.
“There can be a transformational effect, from play to tool use,” said Cenni.
But it’s not clear how much you can generalize from a single population of apes, a population that is regularly fed by humans, so it doesn’t have to spend as much time looking for food as other ape populations. Perhaps it is a benefit of leisure, Visalberghi suggested. “If you have free time and find a way to use the stones for a good purpose, why not?”
Translation by Clara Allain
