In Bali’s Sacred Monkey Forest, long-tailed macaques are the main attraction. Lounging around in lush greenery, hundreds of the primates entertain visitors and feast on food provided by visitors.
Their leisurely lifestyle leaves them with a lot more free time than the average wild monkey, and in these spare hours they’ve taken to playing with stones—in some cases, as researchers recently found, even using them as “sex toys.”
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In a research paper published on Aug. 4 in the journal Ethology: International Journal of Behavioural Biology, researchers analyzed how this population of Balinese monkeys play with stones. Among more innocuous uses, including biting and gathering the stones, video footage collected between 2016 to 2019 also shows hundreds of instances of monkeys in the sanctuary, both male and female, tapping and rubbing the stones on their genitals—seemingly in a state of sexual arousal.
For the male monkeys, their genital stone-tapping and rubbing occurred more often and lasted longer when they also had an erection. But despite rubbing the stones on their erect penises, the male monkeys never managed to ejaculate.
Camilla Cenni, the paper’s co-author and a PhD candidate at the University of Lethbridge in Canada, told VICE World News that while masturbation in primates is not uncommon, the use of tools for this process is rare.
“It’s hard to give a very solid explanation, but it really seems that they do it because it feels good,” she said. “There is some sort of tactile stimulation from the contact of those stones with their genitals and it feels good. And there is no reason to stop.”
While the study found strong evidence that the male monkeys were using stones for masturbation, it was harder to ascertain what was going on with the female monkeys, who were also touching themselves with stones.
“For females, it’s a little bit harder because we don’t really have an easy signifier of arousal,” said Cenni. They did, however, observe some selectivity in the stones that the mature female monkeys chose to tap or rub their genitals with, often preferring ones with sharp edges or a grainy texture.
Playing with stones, which has also been observed in Japanese macaques, is considered a cultural behavior passed among certain populations of monkeys within the same species. Cenni said that stone play is quite common among macaques in Bali, and that the stone-assisted masturbation likely emerged from this.
“If you stop and start watching those monkeys as they play with stones, you may very likely see it,” she said.
Native to Southeast Asia, long-tailed macaques often live in habitats that overlap with humans. In Bali, the macaques are known for their intelligence, having learned to rob tourists and hold their possessions hostage until food is offered as ransom.
When faced with a shortage of food handouts from tourists during the pandemic, the macaques living in southwestern Bali’s Sangeh Monkey Forest ventured out of their sanctuary to raid homes.
The monkeys in the Sacred Monkey Forest are surrounded by human settlements and given fruits and vegetables at least three times per day by temple staff, researchers say. Cenni pointed to this food security as one factor in why the monkeys may have started playing with stones, adding that the populations of macaques that were found to engage in these acts were all well-fed.
“I think we can probably confidently say that the free time they have as a result of provisioning, it’s a big explanation for [stone play],” she said.
“But [free time] is not a sufficient explanation,” she added. “I think it’s only part of the story. The other part, it’s hard to pinpoint because you would have to see the first [monkey] doing it.”
