Masturbation has been documented in many primate species beyond humans. Even the use of masturbatory tools – what we would call sex toys – has been documented in the wild, such as in male long-tailed macaques. Researchers now report witnessing for the first time a wild male chimpanzee using a human-made object as a masturbatory tool.

The report comes from Bulindi, Uganda in August 2018, where a juvenile male chimp named Araali was seen inserting his penis into a discarded plastic bottle. The scientists spotted the ape playing with an empty plastic herbicide bottle, making pelvic thrust movements, repositioning the bottle after his penis came out of it. The team report that the male chimp had a "play face", a relaxed open mouth expression they make when they are having fun.

The team couldn’t determine if the chimp ejaculated into the bottle. This is not just the scientists being overly nosy about what poor Araali gets up to – this event could give insights into the wider question of where the masturbatory behavior comes from in wild chimpanzees. The team doesn’t believe it is related to a way to improve sperm quality or as a sexual outlet, given that Araali was a subordinate male in the group.  

“Araali’s masturbatory behaviour most likely resulted from his motivation to inspect and play with a novel human object. Male chimpanzees exhibit penile erections in various contexts besides sexual arousal, such as food excitement and during some social interactions including play,” the researchers wrote in the paper.

"The physical properties of the open bottle presumably elicited Araali’s autoerotic response, suggesting he recognized its suitability for that purpose. Considering he exhibited a play face while ‘copulating’ with the bottle indicates his masturbatory behaviour was ‘pleasurable’ or ‘fun’."

Wild great apes including chimpanzees tend to be either indifferent or fearful of human-made objects, but the group that Araali is part of regularly comes into contact with our species. They feed on agricultural crops and they often found discarded human objects, such as that plastic bottle.

In captivity, things are different. Chimps in zoos and rescue centers have been seen performing masturbation by hand, foot, and mouth, against a cage wall or screen, and using manipulative objects, in one case including a very unfortunate frog. Witnessing a similar behavior in the wild expands our understanding of our closest relatives in the animal kingdom.