Male and female long-tailed macaques in Bali, Indonesia, frequently use stone tools as sex toys to pleasure themselves, according to a multi-year observational study.
at the University of Lethbridge in Alberta, Canada, and her colleagues noticed the behaviour while studying free-ranging long-tailed macaques (Macaca fascicularis) that live in and around the Sacred Monkey Forest Sanctuary in central Bali.
Between 2016 and 2019, they gathered video footage capturing hundreds of examples of males and females rubbing or tapping stones on their genitals while in seated positions.
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Males rubbed or tapped one or more stones on their penises while they were stretched out with one hand or erect. Females often rubbed stones on the ground first before pushing them under their lower belly to rub or tap their genitals.
This appeared to be for sexual pleasure, at least in males, since it usually preceded penile erections and was often performed in sexual contexts, like when males were inspecting females’ genitals.
It was harder to gauge sexual pleasure in females, but they tended to choose rougher, more angular stones to apply to their genitals, possibly because they provided greater stimulation, says Cenni.
Although masturbation has also been documented in many other primate species, it is normally performed using the hands, feet or mouth, without external tool use, and has primarily been observed in males.
Only a handful of examples of “tool-assisted masturbation” have been witnessed in non-human primates. For example, a wild male chimpanzee in Uganda was seen in 2018 and a male chimpanzee in Tanzania was observed in 2016.
The Balinese male and female macaques may have expanded their masturbation repertoires because they have more free time, says at the University of Western Australia. “These monkeys get food handouts from people and thus have to spend less time foraging for natural foods,” he says. “This frees up time in their activity schedule that can be devoted to non-essential activities, such as masturbation with sex toys.”Ěý
Other studies of primate masturbation have hypothesised that it may help to clear out low-quality sperm, but this doesn’t apply to the male Balinese macaques since their sex toy use doesn’t appear to lead to ejaculation.
Instead, the behaviour may simply be maintained because it feels good, says Grueter. “It doesn’t need to provide a function other than self-reward,” he says. “As long as a behaviour doesn’t negatively affect an individual’s fitness, it may not be weeded out.” In other words, there is no evolutionary reason for the macaques to stop masturbating as long as they still continue to mate and produce offspring.
A wide range of non-primate species have also been observed masturbating, including .
Ethology
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