Polyadic Grooming Patterns and Network in a Free-Ranging Group of Japanese Macaques at Awajishima

Grooming is the most common affiliative behavior in many primate species. While traditionally viewed as an exclusively dyadic interaction, polyadic grooming involving more than two individuals also occurs. Few studies have explored the characteristics or functions of polyadic grooming in comparison with dyadic grooming. However, some studies suggest that polyadic grooming may have distinct characteristics and could be associated with social tolerance. We analyzed polyadic grooming patterns and its network structure in a free-ranging group of Japanese macaques at Awajishima, Japan. This group exhibited higher social tolerance than typical Japanese macaque groups. We found that polyadic grooming was common in this group, with higher frequency than other Japanese macaque groups as well as other primate species except chimpanzees. We also found that polyadic grooming typically occurred with multiple groomers sharing a single groomee, a pattern commonly observed across most primate species. Social network analyses showed that the polyadic grooming network was centralized around high-ranking males, reflecting the frequent grooming from multiple females to a single adult male. In addition, the network of polyadic grooming was less dense and more cliquish than that of dyadic grooming. This potentially suggested that polyadic grooming was more strongly kin-biased than dyadic grooming. Our results support the prediction that polyadic grooming may be associated with higher levels of social tolerance. Moreover, polyadic grooming may function to access valuable social partners more efficiently than dyadic grooming.