Monkeys in Indonesia use stones as sex toys, study suggests, in new example of creative tool use in animals

• A new study suggests long-tailed macaques in Indonesia engage in tool-assisted masturbation.
• Researchers observed the monkeys tapping and rubbing stones against their genitals.

Tool use in animals is well documented, but researchers recently discovered some monkeys in Indonesia that appeared to be using stones in a very specific way: as sex toys.

While studying a population of 173 long-tailed macaques, researchers observed the macaques tapping and rubbing stones on their genitals and groin area, according to a paper published this month in the journal Ethology. The behavior led the researchers to the "Sex Toy" hypothesis.

The researchers evaluated the theory in several ways. They observed young males were more likely than older males to partake in the behavior, and that it was often followed by sexual physiological responses, such as an erection. They also found actions with the stone were associated with other sexual behaviors, such as mounting.

Altogether, the researchers said the data partly supported the theory that the behavior was "sexually motivated." Meaning that the monkeys seemed to be performing "a form of self-directed, tool-assisted masturbation," Camilla Cenni, a PhD student at the University of Lethbridge in Canada and author of the study, told The New York Times.

The authors said the paper suggested monkeys over time may continue using tools for behaviors, such as masturbation, that are not directly tied to survival but have "pleasurable" or "self-rewarding" aspects, like "underlying playful and sexual activities."

Many animals are known to use tools. Otters use stones to crack open shells for food, like clams and mussels. Dolphins use sea sponges to kick up sand and uncover prey on the seafloor. And monkeys have been documented on video using rocks to crack open nuts and shellfish.

Animals have also been documented masturbating with objects, though it's less common. Most tool use serves a clear survival purpose, like eating.

"It's arguably not really adaptive or useful," Cenni told The Times of the object-assisted masturbation.

The authors noted the macaques that were observed lived in an area where they regularly have access to human food, so they may spend less time foraging than other populations.