Changes in Activity Patterns and Intergroup Relationships After a Significant Mortality Event in Commensal Long-Tailed Macaques (Macaca Fascicularis) in Bali, Indonesia

Abstract

Little is known regarding behavioral and social responses of free-ranging primates to demographic changes emerging from significant mortality events. Here, we report on the activity patterns and intergroup sociospatial relationships in a commensal population of long-tailed macaques (Macaca fascicularis) in Bali, Indonesia, that underwent a significant mortality event in summer 2012. During the period of interest, we noted heightened mortality in three of the five social groups present in this population, with adult females and juveniles experiencing higher mortality rates than adult and subadult males. Limited diagnostic data regarding pathogen identification and a lack of any conclusive etiology of the deaths prevent our ascertainment of the agent(s) responsible for the observed mortality, but given the characteristics of the event we assume it was caused by a transmissible disease outbreak. Comparing the pre- and post-mortality event periods, we found significant differences in activity patterns, including a decreased proportion of affiliation in adult females. This result is likely indicative of enhanced social instability induced by the high mortality of adult females that constitute the stable core of macaque social structure. A higher social tension between groups after the mortality event was indicated by more frequent and intense agonistic intergroup encounters. Intergroup conflict success was inversely proportional to the rate of mortality a group suffered. Our results illustrate how changes in demographic structure caused by significant mortality events may have substantial consequences on behavior and social dynamics in primate groups and at the level of a population.