Monkeys in Indonesia use rocks as 'sex toys'

A macaque sitting next to a stone sculpture of a monkey at Ubud monkey sanctuary.
A macaque sitting in the Ubud monkey sanctuary in Bali, Indonesia. (Image credit: Nora Carol Photography via Getty Images)

Monkeys in Indonesia get their rocks off using actual rocks, supporting what's known as the sex toy hypothesis, a new study finds.  

Researchers studying long-tailed macaques (Macaca fascicularis) found that the monkeys repetitively tapped and rubbed their genitals with stones to pleasure themselves, according to the study first reported by New Scientist. This finding provides further evidence for the sex toy hypothesis, proposed by the same researchers in an earlier study, that presents the activity as a form of tool-assisted masturbation.

The team found that males and females of different age groups all used stones to play with themselves, but there was some variation among the groups: Female monkeys were pickier about the stones they used, while young males engaged in the activity the most. Researchers watching the monkeys usually didn't have to wait long to observe the behavior.

"You do see this genital stone tapping and rubbing quite regularly," lead author Camilla Cenni, a doctoral candidate at the University of Lethbridge in Alberta, Canada, told Live Science. "They are not, of course, constantly doing it, but if you stop and see them and they start playing with stones, they are likely going to do it."

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Some macaque populations regularly manipulate stones as part of their behavioral repertoire, seemingly as a form of play. They carry stones around, rub them on surfaces and bash them together. This stone manipulation is likely cultural, because its only seen in certain populations, Cenni said.

The "self-directed tool-assisted masturbation" described in the new study likely stems from this wider stone use. However, it's been documented in only one population of macaques in Bali, Indonesia. 

"When we talk about tool use in animals, we normally think about survival-dependent instances," Cenni said. For example, chimpanzees (Pan troglodytes) use stones to crack nuts so they can eat them. "There is an increasing number of studies that are suggesting that using objects as tools doesn't have to be a matter of survival. This is clearly an example." 

The new research builds on a study led by Cenni and published in the journal Physiology & Behavior in 2020. The study first proposed the sex toy hypothesis in male macaques, while the new research looked at males and females and their potential motivation. 

Young males spent significantly more time engaging in the activity than mature males did. Mature males, in fact, were the least partial to stone masturbation, possibly because they had access to mature females. However, there was a lot of variation among individual macaques of both sexes. "Within those groups, you have monkeys that do it way more than others," Cenni said.

The monkeys were urban-dwelling macaques living in and around the Sacred Monkey Forest Sanctuary in the town of Ubud. They are free-roaming and fed by people. The researchers suggested that the feeding might relax pressures on the monkeys to forage, leading them to engage more in the stone behaviors. In other words, they have more time on their hands than other monkeys, and they choose to spend it with the stones.  

The study was published online Aug. 4 in the journal Ethology

Originally published on Live Science.

Patrick Pester
Live Science Contributor

Patrick Pester is a freelance writer and previously a staff writer at Live Science. His background is in wildlife conservation and he has worked with endangered species around the world. Patrick holds a master's degree in international journalism from Cardiff University in the U.K.