Macaques Are Pleasuring Themselves With Stone Sex Toys In Bali

Silly as it may seem, monkey sex tools could provide some surprisingly profound insights.


Tom Hale

Senior Journalist

clockAug 16 2022, 16:43 UTC
A male Macaque sits proudly on a temple in Bail.
A male macaque sits proudly on a temple in Bail. Image credit: Andrea Izzotti/

Monkeys are one of the many animals that can master tools and, staying true to their spirit, they may have used this ability to fashion sex toys out of stones. That’s according to a new study published in the journal Ethology: International Journal of Behavioural Biology

To reach these findings, scientists at the University of Lethbridge in Canada looked closely at a group of long-tailed macaques living around the urban environment of the Sacred Monkey Forest Sanctuary in central Bali, Indonesia. This entailed some 700 to 1,000 monkeys, comprised of five to seven different groups with overlapping home ranges. 


Previous research has highlighted how Bali’s long-tailed macaques would repeatedly tap and rub pebbles onto their genital area. Intrigued by this unusual – although oddly familiar – behavior, the researchers from this latest study wanted to assess whether the stones could be strictly considered a masturbatory tool akin to a “sex toy.” 

Observations and statistical analysis showed that the stone-tapping and stone-rubbing behavior was clearly sexual in nature. This was especially noticeable in the males, who would rub their groin with the stone and achieve an erection. It was also evident that the males were more likely to use the stones in a sexual context, such as when females flashed their genitals. 

Female macaques were seen touching themselves with pebbles around the groin region, although it appears they didn't stone-rub quite as frequently as the males. The researchers note that the females were especially keen on using stones that had rough edges or angular shapes, for reasons we can only imagine.

Since females don’t have a visible signifier of sexual arousal like an erection, the researchers say it's harder to discern their motive, but it would be hard to deny that this behavior is much different from the males’.


“The fact that no other stone handling patterns have a temporal association with penile erection suggests that genital stone-tapping/ rubbing is distinctly sexually motivated, compared with other seemingly playful actions,” the study authors conclude.

Primates are no stranger to sex toys and masturbatory aids. Researchers have witnessed wild male chimpanzees using a human-made object to masturbate and Japanese macaques have been observed rubbing up against deer’s back for sexual pleasure. 

As silly as the topic may seem, the evolutionary origins of masturbatory tools could provide some surprisingly profound insights. The mastery of tools undoubtedly provides an advantage for survival among a handful of species, and it was clearly a huge factor in the success of our own species, the Homo Sapien. 

In concluding their study, the researchers ponder whether our ability to harness tools first started with functional objects, such as hunting weapons, or if it evolved indirectly through play and pleasure as some kind of "junk behavior" that later proved very useful. 


“Our research suggests that instrumental behaviors of questionably adaptive value may be maintained over evolutionary time through pleasurable/self-rewarding mechanisms, such as those underlying playful and sexual activities,” they write.

“Our results support the view that tool use evolves in stages from initially non-functional behaviors, such as object play, through affordance learning,” the researchers added.

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