Scientists Discover Monkeys Use Stones as Sex Toys

A group of long-tailed macaques in Indonesia have been observed by scientists to masturbate using stone tools.

Researchers from the University of Lethbridge in Canada, studying a group of monkeys at the Sacred Monkey Forest Sanctuary in Ubud, Bali, Indonesia, found that when the macaques would rub or tap their genitals with rocks or stones, they would subsequently show signs of sexual arousal.

In a study published in the journal Ethology: International Journal of Behavioral Biology, the scientists describe how the stone-tapping behavior would be followed by penile erection, or a previously present erection would last significantly longer if the monkey was stone-tapping compared to if it wasn't. These genital-stimulating behaviors were seen significantly more often in adolescent males than in males of any other age group.

Despite the difficulty in measuring female sexual arousal due to the lack of obvious exterior change, the researchers also found that the female monkeys were selective in the shape and size of rock that they used to rub their genitals, preferring rough edges or angular shapes.

"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," wrote the authors in the paper.

Masturbation in animals has been observed before in several species, including chimpanzees, dogs and even porcupines. Primates, our closest animal cousins, have been most observed to masturbate, albeit more commonly in captivity than in the wild, and macaques have even been found to sexually mount sika deer in the first observed case of inter-species sexual behavior.

The presence of masturbation in a species' behavioral repertoire is thought to be coincident with the mating system of their social groups.

"In a questionnaire-based study, [scientists] found a strong association between a primate species' mating system and the occurrence of male masturbation, with masturbation being more often displayed in species living in a multimale/multifemale organizational system, like macaques," wrote the authors of the Ethology: International Journal of Behavioral Biology paper.

The observation of the macaques using the stones to self-stimulate is a rare case of an animal using a tool to aid in its masturbation.

Tool use in general is a relatively uncommon trait across the animal kingdom, and is thought of as a hallmark of intelligence in a species. Originally assumed to only be a human capability, tool use varies in complexity, from instinctive and inflexible use of tools in accessing food, to problem-solving and innovation to achieve a goal, as seen in many primates.

The discovery of tool use for sexual pleasure is an unexplored and scarcely observed behavior.

"Since orgasm does not represent a necessary end goal for masturbation in nonhuman primates, biomechanical definitions of tool use can contribute to detecting instrumental components of object-assisted actions, by investigating the mechanical effect (here, a physiological change measured via penile erection) that the tools have on the target (i.e., the genital and inguinal area)," wrote the authors.

This has only been seen a few times before, including once in a subadult male chimpanzee in Uganda who was using a discarded plastic bottle as a masturbatory tool.

"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," wrote the authors in the paper.

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