Of all the non-human primate species studied by researchers, the rhesus macaque (Macaca mulatta) is likely the most widely used across biological disciplines. Rhesus macaques have thrived during the Anthropocene and now have the largest natural range of any non-human primate. They are highly social, exhibit marked genetic diversity, and display remarkable niche flexibility (which allows them to live in a range of habitats and survive on a variety of diets). These characteristics mean that rhesus macaques are well-suited for understanding the links between sociality, health and fitness, and also for investigating intra-specific variation, adaptation and other topics in evolutionary ecology.

Resource partitioning reduces the competition between different species within the same habitat, promoting their coexistence.To understand how such species co-adapt to reduce conflicts, we examined the behavior of two primates, Assamese macaque (Macaca assamensis) and rhesus macaque (Macaca mulatta), from April 2017 to March 2018 in Sivapuri Nagarjun National Park (SNNP), Kathmandu Valley, Nepal. We performed 1580 and 1261 scan sessions on wild multi-male/multi-female groups of Assamese and rhesus macaques, respectively, at 15-min sampling intervals. Assamese macaques consumed fewer plant species (38 species) than rhesus macaques (88 species). Overlapping food sources between the macaque species resulted in a Pianka index of 0.5. Assamese macaques consumed more items of tree, climber, and vine species, whereas rhesus macaques fed on more shrub, herb, and grass species. The proportions of plant parts consumed by the two species differed-more leaves, fruits and cones were used by Assamese macaques than rhesus macaques, whereas more flowers, seeds, and pods were consumed by rhesus macaques than Assamese macaques. Assamese macaques had a smaller home range (0.55 km 2 ) than rhesus macaques (4.23 km 2 ), and Assamese macaques had a shorter daily moving distance (1.6 km) than rhesus macaques (4.0 km). Although feeding time did not differ between the two macaque species, less time was devoted to social activities by Assamese macaques (16.0%) than by rhesus macaques (33.7%). Assamese macaques were generally arboreal, with 94.0% of their activities in trees, whereas rhesus macaques were largely terrestrial, with 58.5% of their activities on the ground. These differences in food selection, home-range size, ranging and activity patterns, and habitat use suggest that Assamese and rhesus macaques reduce resource competition through resource partitioning to coexist in a landscape matrix.

A wide range of taxa including fish, birds, and mammals form mixed species groups (MSGs) to increase feeding efficiency, reduce predation risk, and increase social or reproductive advantages (Stensland et al., 2003). MSGs typically involve multiple individuals of two or more species, but a single animal has sometimes been observed to join another species group (Baraff & Asmutis-Silvia, 1998). Various primate species form MSGs (Chapman & Chapman, 2000; Heymann & Buchanan-Smith, 2000; Waser, 1986), yet there is little information on MSGs in macaques. For example, Japanese macaques (Macaca fuscata) formed a MSG with sika deer (Cervus nippon) (Tsuji et al., 2007). Rhesus (M. mulatta) and long-tailed macaques (M. fascicularis) formed a MSG, in which hybridization occurred (Jadejaroen et al., 2015; Malaivijitnond & Hamada, 2008). Besides released macaques (e.g., Kawamoto et al., 2007; Malaivijitnond & Hamada, 2008), however, reports of macaques forming MSGs with another species-group in Macaca are rare, even when habitats are shared. This study is the first to report that male rhesus macaques in a fascicularis species-group joined a social group of Assamese macaques (M. assamensis), which belong to a sinica species-group. We report social interactions within the MSG and intergroup encounters between rhesus and Assamese macaques at Shivapuri Nagarjun National Park (SNNP) in central Nepal.