Use of a novel human object as a masturbatory tool by a wild male chimpanzee at Bulindi, Uganda

8 INTRODUCTION Besides humans, self-stimulation of the genitalia (masturbation) has been documented in many primate species (Dixson 2012; Thomsen & Sommer 2017). Although masturbation by captive nonhuman primates is sometimes considered abnormal or, at least, undesirable behaviour (e.g., Mallapur & Choudhury 2003), masturbation also occurs under natural conditions and may represent a phylogenetically ancient and widespread trait in primates (Thomsen & Sommer 2017). Several studies have reported masturbation in wild male primates living in multimale–multifemale groups where sperm competition occurs (e.g., Temminck’s red colobus, Piliocolobus badius temminckii; Starin 2004; Japanese macaque, Macaca fuscata; Thomsen & Soltis 2004; rhesus macaque, Macaca mulatta; Dubuc et al. 2013). While the hypothesis that male masturbation functions to increase sperm quality (Baker & Bellis 1993) received support in one study of Japanese macaques (Thomsen & Soltis 2004), masturbation leading to ejaculation was observed rather rarely in most reports, suggesting alternative explanations require consideration (Starin 2004; Dubuc et al. 2013). In chimpanzees, masturbation occurs commonly in captive settings where it has been linked to restricted rearing, which can impede development of speciestypical social and sexual behaviour (Kollar et al. 1968; Rogers & Davenport 1969; Lopresti-Goodman et al. 2013). Masturbation by captive chimpanzees is performed by hand, foot or mouth, against a cage wall or screen, or sometimes using a manipulable object (Shefferly & Fritz 1992). In the wild, object-assisted masturbation occurs in male long-tailed macaques (Macaca fascicularis), which stimulate their genitals using stones (Cenni et al. 2020). Such self-directed employment of a manipulable object meets the criteria of an animal ‘tool’ (Shumaker et al. 2011). In contrast to captivity, masturbation appears to be rare among male chimpanzees in the wild. Male chimpanzees of all ages at Gombe and Mahale, Tanzania, manipulated (or ‘fumbled’ or ‘fiddled’ with) their erect penises occasionally, but this was never observed to lead to ejaculation (van Lawick-Goodall 1968; Nishida 1997). Recently, Nakamura (2018) described an infant chimpanzee at Mahale ‘copulating’ with a discarded fruit wadge, which was likened to a ‘sex toy’. Besides this, there seem to be no other reports of wild male chimpanzees using manipulable objects as tools in sexual behaviour. We observed a wild subadult male chimpanzee at Bulindi, Uganda, using a novel human object—a discarded plastic bottle—as a masturbatory tool. While anecdotal (Ramsay & Teichroeb 2019), this unusual observation raises questions about the function of masturbation in male chimpanzees, and contributes to an understanding of the range in behavioural responses of wild apes to novel objects.