Monkeys in Indonesia get their rocks off with real rocks, supporting the so-called sex toy hypothesis, a new study finds.
Researchers studying long-tailed macaques (
Macaca fascicularis) found that the monkeys repeatedly tapped and rubbed rocks on their genitals to please themselves, according to the study first reported by new scientist (opens in new tab). This finding provides further evidence for the sex toy hypothesis, proposed by the same researchers in a previous study, which presents the activity as a form of tool-assisted masturbation.
The team found that men and women of different age groups all used rocks to play with themselves, but there was some variation between the groups:
apes were more picky about the stones they used, while young men were most involved in the activity. Researchers who watched the monkeys usually didn’t have to wait long to observe the behavior.
“You see this genital stone tapping and rubbing quite regularly,” lead author Camilla Cenni, a doctoral student at the University of Lethbridge in Alberta, Canada, told Live Science. “They don’t do it constantly, of course, but if you stop and see them and they start playing with bricks, they probably will.”
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Some macaque populations regularly manipulate rocks as part of their behavioral repertoire, seemingly as a form of play. They carry rocks around, rubbing them on surfaces and hitting them together. This stone manipulation is likely cultural, as it’s only seen in certain populations, Cenni said.
The “self-directed, tool-assisted masturbation” described in the new study likely stems from this wider use of stones. However, it has been documented in only one population of macaques in Bali, Indonesia.
“When we talk about resource use in animals, we normally think of survival-dependent cases,” Cenni said. For example,
chimpanzees ( pan cavemen) use rocks to crack nuts so they can eat them. “There is an increasing body of research suggesting that using objects as tools does not 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 and Behavior in 2020. The study first proposed the sex toy hypothesis in male macaques, as the new research looked at males and females and their potential motivations.
Young men spent significantly more time on the activity than adult men. In fact, mature males were the least partial to stone-masturbation, possibly because they had access to mature females. However, there was a lot of variation between individual macaques of both sexes. “Within those groups, you have monkeys that do it a lot more than others,” Cenni said.
The monkeys were macaques that lived in the city and lived in and around the Sacred Monkey Forest Sanctuary in the city of Ubud. They roam free and are fed by humans. The researchers suggested that feeding could reduce the pressure on the monkeys to forage, making them more involved in the stone behavior. 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
Originally published on Live Science.