New York: Many monkeys are skilled stone handlers, using rocks to dig up roots, cut plants and crack open an array of delicacies, including fruits and nuts.

But some monkeys also appear to be using stone tools for, erm, something else. In a paper published this month in the journal Ethology, researchers report that some macaques frequently rub or tap stones around their genitals, and that these behaviours are associated with signs of physiological arousal that other stone-handling actions do not prompt.

In other words, the monkeys appear to engage in “a form of self-directed, tool-assisted masturbation,” said Camilla Cenni, a doctoral student at the University of Lethbridge in Canada, who conducted the research as part of her dissertation.

Some of the best-known examples of tool use by wild animals revolve around the never-ending, life-or-death quest to find enough to eat: chimpanzees use sticks to collect termites, crows use twigs to extract larvae from logs, sea otters use rocks to smash snails.

The macaque study is not the first report of object-assisted masturbation in wild animals, but it provides new evidence that, in some cases at least, animals appear to use tools simply to give themselves pleasure. “It’s arguably not really adaptive or useful,” Cenni said.

The study is based on observations of a population of free-ranging, long-tailed macaques that live in or near the Sacred Monkey Forest Sanctuary in Ubud, a Balinese town in Indonesia, where the animals regularly receive food from humans.

Scientists had previously observed that these macaques frequently handle stones with what appears to be no obvious purpose. The monkeys might clack the stones together in their hands, for instance, or pick them up and drop them over and over again.

“It’s some sort of playful manipulation, in which there doesn’t seem to be an apparent function,” Cenni said.