Monkey-deer rodeo: exploring the mounting behaviours of Japanese macaques (Macaca fuscata) on Sika deer (Cervus nippon)

Interspecies interactions present diverse forms and functions, contributing significantly to ecological and social dynamics. This study focuses on the mounting behaviours of Japanese macaques (Macaca fuscata) towards Sika deer (Cervus nippon), a rare and still unexplained phenomenon, with minimal emphasis on the behaviour of the deer. Using video data from two distinct sites, Yakushima and Minoh, Japan, we documented 45 sequences of macaque–deer interactions and analysed macaques’ behaviours and deer behaviours using focal and behavioural sampling techniques. We identified four primary activities performed by macaques during mountings—sexual, play, grooming, and resting—with resting being predominant. The study evaluates six hypotheses regarding the functions of these mounting behaviours: (1) interactions are sex-specific, (2) macaques use deer for transportation, (3) mutual warming through physical contact occurs, (4) deer provide support for resting, (5) macaques engage in grooming to consume parasites or play to strengthen interspecies bonds, and (6) aggressive signals relate to resource disputes or rejection of mounting behaviour. Statistical analyses using the Kruskal–Wallis test and Dunn’s post-hoc test revealed significant differences between resting and other active behaviours, but no significant differences in behaviour duration between lying and sitting positions. Adults engaged in mounting for longer periods than juveniles, suggesting potential age-related differences in social and reproductive roles. These findings enhance our understanding of interspecies interactions by focusing on macaque behaviours and emphasise the need for longitudinal studies to clarify the ecological and social implications of these interactions.