Long-tailed macaque monkeys at the Sacred Monkey Forest Sanctuary in Ubud, central Bali, are using stones for sexual pleasure, according to a report by a science magazine.
Both male and female long-tailed macaques were observed tapping or rubbing stones on their genitals during a study led by Camilla Cenni of the University of Lethbridge in Alberta, Canada, as reported by New Scientist magazine.
Cenni and her colleagues’ findings are made in a research article “Do monkeys use sex toys? Evidence of stone tool-assisted masturbation in free-ranging long-tailed macaques”, published in the latest issue of Ethology: International Journal of Behavioural Biology.
The researchers collected video footage from 2016 and 2019, showing hundreds of examples of males and females using stones to pleasure themselves while sedentary in and around the 12.5-hectare sanctuary.
Males tended to rub or tap at least one stone on their penises, but this did not seem to result in ejaculation. Females were recorded pushing stones under themselves to stimulate their genitals. The study found that adult females are more selective in choosing the texture of the stones, opting for those that are rougher and more angular.
While masturbation is not unknown in primates, it does not usually involve the use of external tools and is considered more common among males. The stone tapping and rubbing by the long-tailed macaques in Bali is believed to be associated with mating behaviour, as well as physiological responses, such as estrus in females and penile erection in males.
Cenni had earlier reported some of her findings in a research article dryly titled “Inferring functional patterns of tool use behavior from the temporal structure of object play sequences in a non-human primate species”, published in the May 2020 issue of the Physiology & Behavior journal.
According to Cyril Grueter, a biological anthropologist at the University of Western Australia, the Balinese macaques may have started using stones because they have more free time, as they are fed by people and therefore spend less time looking for food. The Monkey Forest’s website reminds visitors not feed the monkeys, as they are fed by staff three times a day, so extra food is not required. Visitors are warned not to hide any food because “because the monkeys will know and try to find it” and to never try to pull back any food seized by the monkeys.
Online travel agency Traveloka notes that Ubud’s Monkey Forest is home to more than 1,000 long-tailed monkeys and states, “it won’t be hard to find them eating, playing, or grooming each other throughout the forest”. Entry to the forest costs Rp40,000 (US$2.70).