Monkeys in Indonesia use rocks as sex toys, study suggests

• A new study suggests that long-tailed macaques in Indonesia engage in instrument-assisted masturbation.
• The researchers observed the monkeys hitting and rubbing rocks on their genitals.
• Tool use is well documented in animals, although typically as survival feeding.

The tool on animals is well documented, but researchers recently found some monkeys in Indonesia that seem to use stones in a different way.

When researchers studied a population of 173 long-tailed macaques, they observed the macaques kicking and scratching their genitals and groin with rocks, according to a study published this month in the journal Ethology. The behavior led the researchers to the “sex doll” hypothesis.

The researchers evaluated the theory in several ways. Young men are more likely to engage in the behavior than older men, and it often follows sexual physiological responses such as erection. In addition, actions with stones are associated with other sexual behaviors, such as planting.

In general, the researchers said that the data partially supports the theory that the behavior is “sexually motivated”. That is, the monkeys appear to be performing “self-directed, instrument-assisted masturbation”; Camilla Seeni, a PhD student at the University of Lethbridge in Canada and the author of the study, told the New York Times.

According to the authors, the paper suggests that over time, the apes’ use of the apes for behaviors such as masturbation are not directly related to survival, but “pleasant” or “self-rewarding” aspects such as “underlying playful and sexual activities” may continue.

Many animals are known to use tools. Otters use rocks to crack open shells for food, such as clams and mussels. Dolphins use sea sponges to trample sand and uncover prey on the sea floor. And monkeys have been recorded on video using stones to crack open nuts and shellfish.

Although less common, animals have been recorded masturbating to objects. Most tool use serves an obvious survival purpose, such as eating.

“It’s not really adaptive or useful,” Senni told The Times of Object-Assisted Masturbation.

The authors suggest that the observed macaques lived in areas where they regularly had access to human food, so they may have spent less time foraging than other populations.