Management of Synanthropic Macaque (Macaca fascicularis) Populations in Bali: Assessing the Implications of Sterilization on Female Social Dynamics

Abstract Growing contacts between humans and nonhuman primates at interface zones bring forth the need to better understand the efficiency and implications of synanthropic primates population management strategies. In this context, the expanding use of fertility control contrasts with the limited documentation of its potential consequences for primate behavior and social dynamics. Unlike other methods, tubectomy preserves the ovarian functions involved in sexual motivation of female macaques. However, sexual behaviors and aggression could intensify due to a higher proportion of cycling females within the group. In this study, we assessed whether tubectomy modifies the sociosexual interactions of female long-tailed macaques ( Macaca fascicularis ) in a primate-tourism site in Bali, Indonesia. Using focal sampling over a three-year period ( N = 56 females), we investigated changes in (a) female sociosexual activities (i.e., sexual and grooming interactions with males), and (b) female intrasexual aggression (i.e., female-female agonistic interactions). Using causal inference statistics, we found that (a) compared with intact females, sterilized females were more sexually receptive and attractive, and they received longer grooming bouts from male partners. Surprisingly, (b) tubectomy was associated with decreased intrasexual aggression among females, as sterilized females received aggression from fewer female opponents compared with intact females. This study showed that, at least in the short term, tubectomy modifies the sociosexual interactions, while not heightening female aggression. These findings may inform management decisions that maximize social stability and welfare of synanthropic populations. However, the long-term implications of female sterility for social dynamics warrant further investigation.