Monkeys are using stone tools... for pleasure

Human sexual behavior is the source of nearly endless conflict both interpersonally and societally, not to mention serving as the central motivator of countless movies, including Sex Ed (now streaming on Peacock!). Despite being one of the most natural activities any animal — including humans — can engage in, we hang a lot of baggage on sex. It’s something which only happens in private, isn’t spoken of in polite company, and is otherwise shielded behind blackout curtains and locked doors. The long-tailed macaque monkeys of Bali, however, don’t have the same hang-ups.

According to a recent study published in the journal Ethology, macaques have been observed using specially selected stone tools in order to enhance masturbatory sexual pleasure. The study took place in the Sacred Monkey Forest Sanctuary in central Bali, where researchers observed over 170 monkeys of all ages and sexes and categorized their use of stone tools.

It’s commonly believed that new technologies quickly filter themselves into one of two uses, those of warfare and sex. Certainly, a review of human technological advancement, both contemporarily and in the past, supports this notion. Whenever new technological ground is broken, we pretty quickly ask ourselves how we use it to fight or f… well… you know.

Most non-human animals don’t use tools so this hasn’t been a particular area of interest, but over the last several years it has been reported that several species of monkeys are entering their own stone age and may, in fact, have entered it up to 3,000 years ago. Those initial studies focused on the more readily apparent uses of stone tools, that of resource acquisition.

The use of tools is often explored through the lens of a survival advantage, the ways in which they can enhance the gathering of resources or defense. Indeed, monkeys have been observed using stone tools to dig for food or smash open seed pods, but now we know their tool usage extends to more personal activities without any direct survival advantage.

The new study builds upon previous work which demonstrated that male long-tailed macaques used stone tools to stimulate their genitals through repetitive tapping or rubbing. We have to be careful when observing animals not to project our own biases onto their behavior, but the so-called “Sex Toy” hypothesis appears to be borne out by these additional observations, which also looked at the behaviors of female macaques as well as juveniles.

The use of stone tools for “self-directed tool-assisted masturbation,” as it is described in the study, reveals some interesting demographic differences. Notably, researchers found that female monkeys are more selective about which stones they choose to stimulate themselves with, while male monkeys are less discriminating but engage in the activity more often. That disparity is largely a consequence of juvenile males. They were most likely to engage in tool-assisted sexual play while adult males were the least likely of any demographic. It’s possible that’s a result of access to mature females, minimizing the need for adult males to go solo.

In an interview with Live Science, Camilla Cenni, lead author of the study, said that researchers didn’t have to wait long to see the behavior. If the monkeys were holding a stone, it was likely it was heading toward their genitals sooner than later. Perhaps most interesting is that the use of stone tools — either for masturbation or more broadly — appears to be cultural and is only seen in some populations. The emergence of this behavior among monkeys living in Sacred Monkey Forest Sanctuary could be a result of their comparatively lavish lifestyle. The monkeys there are often fed by people. The fact that they don’t have to expend as much energy on foraging could be freeing up time and energy for tool manipulation and, ultimately, exploration of their bodies. Idle hands and all that.