Social Interactions Between Rhesus Macaques (Macaca mulatta) and Assamese Macaques (M. assamensis) in Nepal: Why Do Male Rhesus Macaques Follow Social Groups of Assamese Macaques?

We report observations of male rhesus macaques ( Macaca mulatta ) following multi-male multi-female groups of Assamese macaques ( M. assamensis ), forming mixed species groups (MSGs), as well as intergroup encounters between rhesus and Assamese macaques at Shivapuri Nagarjun National Park (SNNP), Kathmandu District, Nepal. We observed both macaque species from July 2014 to April 2015 (Period 1) and from April 2017 to March 2018 (Period 2). In addition to wild groups, we observed the provisioned Rhesus Army (RA) and Assamese Army (AA) groups at the army camp in SNNP and the Rhesus Balaju (RB) group at Balaju Garden, adjacent to SNNP. During Period 1, macaques in the RA and AA groups entered the same provisioning site, sometimes showed aggressive behaviors between groups. During Period 2, macaques in the RB and AA groups had encounters at the army camp and Balaju Garden, showing severe aggressive behaviors between groups. Thus, intergroup relationships among these macaque species were agonistic. Nevertheless, two adult male rhesus macaques entered the provisioning site with the AA group of Assamese macaques, remained there, and then returned to the forest with the AA group during Period 2. Two other adult male rhesus macaques followed Assamese macaques in the wild Assamese Simpani (AS) group from morning until evening during Period 2. These males exhibited both aggressive and affiliative interactions with male Assamese macaques. Although these appeared to have been attracted to female Assamese macaques, we observed neither copulation nor hybrids between rhesus and Assamese macaques during our study. Since common leopards ( Panthera pardus ) inhabit SNNP, male rhesus macaques that could not immigrate into multi-male multi-female social groups of their own species might reduce predation risk and increase social advantages by joining groups of Assamese macaques, which would be a more effective strategy than remaining solitary or forming a small male group.