Twig-assisted masturbation in Balinese long-tailed macaques

We documented evidence of idiosyncratic but repeated twig-assisted genital stimulation by Balinese long-tailed macaques (Macaca fascicularis), living in Ubud, Indonesia. This population is known to customarily engage in genital stone-tapping/-rubbing, a behaviour expressed by males and females of different age classes as part of a broader cultural repertoire of stone-directed and overall playful actions known as stone handling. However, genital stimulation using other objects has never been reported in this population and species. We observed repeated and extensive twig-assisted genital stimulation in one adult female and one adult male. We also recorded three instances of twig-assisted genital stimulation in three additional individuals. Our results indicate that this behaviour is not accidental and may be sexually motivated; that is, it may be another form of tool-assisted masturbation in this primate population. This report provides a valuable addition to the increasing literature on tool-assisted behavioural innovations pertaining to self-care and possibly pleasure.