Many monkeys are adept at stonehandling, using stones to dig up roots, cut plants, and crack open an array of delicacies including fruits and nuts.
But some monkeys also seem to use stone tools for, er, something else. In an article published this month in the journal Ethologyresearchers report that some macaques frequently rub or tap stones around their genitalia, and that these behaviors are associated with signs of physiological arousal that other stone handling actions do not elicit.
In other words, the monkeys appear to engage in “a form of self-directed, tool-assisted masturbation,” said Camilla Cenni, a doctoral student at the University of Lethbridge in Canada, who led the research in the framework of his thesis.
Some of the best-known examples of tool use by wild animals revolve around the never-ending, life-and-death quest to find enough to eat: chimpanzees use sticks to pick up termites, crows use twigs to extract larvae from logs, sea otters use stones to crush snails.
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The macaque study isn’t the first report of object-assisted masturbation in wild animals, but it does provide new evidence that, in at least some cases, animals appear to use tools simply to give themselves pleasure. . “It’s probably not really adaptive or useful,” Cenni said.
The study is based on observations of a population of free-ranging long-tailed macaques that live in or near the Sacred Monkey Forest Sanctuary in Ubud, a Balinese town in Indonesia, where the animals regularly receive food. humans.
Scientists had previously observed that these macaques frequently handled stones with no obvious purpose. The monkeys can for example clap the stones in their hands, or pick them up and drop them again and again.
“It’s kind of playful manipulation, in which there doesn’t seem to be any apparent function,” Cenni said.
Monkeys also sometimes use rocks to rub or tap around their genitals and groin, prompting what researchers have called the “sex toy” hypothesis.
To systematically investigate the hypothesis, Cenni and his colleagues analyzed the rock-handling actions of 173 monkeys. They found that young males practiced genital tapping and rubbing more than adult males, and were particularly likely to do so in sexually charged situations, such as when they, or another nearby macaque, were soliciting a mate. or showed signs of sexuality. awakening. Additionally, the behavior tended to precede physical signs of male sexual arousal, and it lasted longer when arousal occurred—patterns that were not true for other types of stone manipulation.
“The data is very compelling,” said Elisabetta Visalberghi, a primate cognition expert at Italy’s National Research Council’s Institute of Cognitive Science and Technology, who was not involved in the research. “What they discovered was that indeed there was something sexual going on.” (Like any good scientist, she made sure to add the appropriate caveats. “The pleasure of masturbation is very difficult to assess,” she noted.)
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Although using a stone as a sex toy may seem unpleasant, Cenni said she has never seen the monkeys make sounds suggesting they are in pain, as they sometimes do when bitten when bitten. a fight, for example. Still, she noted, the idea that this particular pursuit might cause pain as well as pleasure couldn’t be ruled out without further testing.
The researchers speculate that the monkeys may have stumbled upon the genital tapping and rubbing while engaging in other seemingly aimless rock handling actions. If true, it would fit with a scientific theory that tool use can evolve from the playful manipulation of objects.
“There could be a transformational effect from playing to using tools,” Cenni said.
Yet it is unclear to what extent one can generalize from a single population of macaques, a population that is regularly fed by humans and therefore needs to spend less time feeding than other macaque populations. monkeys. Maybe it’s a leisure advantage, Visalberghi said: “When you have free time and you find a way to use the stones for a given purpose, why not?”
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