Long-tailed macaques (Macaca fascicularis) are frisky devils. They often roam the ancient site of the  Uluwatu temple in Bali, where they rob unsuspecting tourists, clinging to their possessions until the food is offered as ransom. The monkeys don’t steal any worthless junk. The shrewd macaques are clever enough to judge which items their victims value most –things such as mobile phones, wallets, and glasses — and use this information to maximize their profit.

Clearly, long-tailed macaques are clever, playful, and inquisitive. They’re the only Old World monkeys who use stone tools in their daily foraging, especially around the coasts to prey on oysters, crabs, and mussels. The monkeys also employ stones to have fun, throwing them around in a non-threatening manner. But nothing could really prepare scientists for the next iteration of stone tool use.

Scientists collected video footage from 2016 to 2019 filmed at the Sacred Monkey Forest Sanctuary in Ubud, central Bali, finding hundreds of examples of both male and female monkeys using stones to pleasure themselves. A not very safe for work video exemplifying this behavior is embedded below.

Males carefully chose the right stones and tapped or rubbed their genitals against them. The behavior lasted longer when they had an erection, although researchers at the University of Lethbridge in Canada have yet to document an instance in which they ejaculated.

Females tended to push stones under themselves and thrust against them to stimulate their genitals. The females were more selective in the kinds of stones they chose, opting for toys that were rougher and more angular.

Auto-sexual behavior, or masturbation, is far from a novelty among non-human primates. Chimpanzees, orangutans, golden lion tamarins, and Goeldi’s monkeys have been observed stimulating their own penis or clitoris using their fingers or tails. Masturbation is more prevalent in species that exhibit a polygynandrous (multi-male, multi-female sexual partners) breeding system but it also occurs in harems and in monogamous populations. This suggests that masturbation is an ancestral and widespread trait within the order of primates.

In some rare cases, orangutans of both sexes have been observed fashioning leaves or twigs for use in genital stimulation. But the new findings represent the first evidence of the use of stones as sex toys among non-human primates.

“It’s hard to give a very solid explanation, but it really seems that they do it because it feels good,” Camilla Cenni, the paper’s co-author and a Ph.D. candidate at the University of Lethbridge in Canada, told VICE World News. “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.”

According to the researchers, stone play is quite common among macaques throughout Bali and is culturally transmitted. Stone-assisted masturbation likely emerged from this behavior, likely facilitated by the fact that the monkeys have nothing better to do.

Spoiled is an understatement when referring to the monkeys at Sacred Monkey Forest. They’re fed at least three times per day by temple staff with tasty fresh fruits and vegetables, and often receive treats from tourists looking to buy their good favor. The researchers note in their study that food security is at least one factor why the monkeys started playing with the stones to sexually satisfy themselves. That and the fact that it feels good, I guess.

The authors reported their findings in the journal Ethology: International Journal of Behavioural Biology.