Studies have reported social association between animals of different species. Japanese macaques (Macaca fuscata yakui) in Yakushima are often spatially associated with sika deer (Cervus nippon yakushimae). In rare instances, their association may involve physical contact. The most prominent interaction is so-called “monkey-deer mounting”, where a macaque climbs onto the back of deer. Pelé et al. (2017) and Gunst et al. (2018) reported monkey-deer mounting by an adult male and immature female macaques (aged 3-4 years), both described as heterospecific sexual behavior in nature. Here, we provide some evidence for more affiliative aspects of monkey-deer interactions. First, in our study site in Yakushima, juvenile macaques (aged 1-2 years) also engaged in monkey-deer mounting, yet too young to be sexually motivated. Second, only 30% of monkey-deer mounting was accompanied by pelvic thrusting that indicates its sexual nature. In particular, juveniles rarely performed pelvic thrusting; rather, they tended to groom deer while sitting on their back in a various positions (i.e., backwards, sideways). Taken together, juveniles are more likely to interact with deer in the context of social play, leading us to suggest that sexually-oriented behavior is only a partial view of monkey-deer mounting.