Some Monkeys Use Stone Tools for Pleasure, Study Suggests

" In a paper published this month in the journal Ethology, researchers report that some macaques frequently rub or tap stones around their genitals, and that these behaviors are associated with signs of physiological arousal that other stone handling actions do not prompt.

In other words, the monkeys appear to engage in “a form of self-directed, tool-assisted masturbation,” said Camilla Cenni, a doctoral student at the University of Lethbridge in Canada, who conducted the research as part of her dissertation."
https://psoriasinews.net/news/some-monkeys-use-stone-tools-for-pleasure-study-suggests/3984/