Many monkeys are skilled stone handlers, using rocks to dig up roots, cut plants and crack open an array of delicacies, including fruits and nuts.
But some monkeys in Bali, Indonesia also appear to be using stone tools for, erm, something else. 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 behaviours are associated with signs of physiological arousal that other stone-handling actions do not prompt.
