Between-group contest competition for access to resources and mates is widespread among social animals. These interactions can impact individual fitness influencing chances of survival and inter-birth interval lengths. Fitness benefits derived from intergroup contest competition are reaped by groups able to displace their opponents. Long and aggressive contests are costlier in terms of fitness than short and peaceful interactions. Therefore, net benefits from intergroup contests can depend on contest escalation. Factors affecting intergroup encounter outcomes and intensity may have an effect on individual fitness. However, we know relatively little about these factors. The aim of this thesis was to investigate what factors affect outcomes and intensity of intergroup encounters in crested macaques (Macaca nigra). Crested macaques have frequent between-group interactions, which makes of them a suitable model species to investigate this topic. My team and I collected data on 163 intergroup encounters between three neighbouring groups of habituated crested macaques in Tangkoko Nature Reserve (North Sulawesi, Indonesia) between November 2015 and July 2016. We also collected data on demography and use of space of these three groups. In my first study, I investigated whether between-group differences in group size and location-based payoffs could be used to infer the probability of intergroup encounter outcomes and intensity. Likelihood of draw and contact aggression were higher when group sizes were similar. Probability of winning an encounter was higher for groups bigger than their opponent and with higher location-based payoffs. Groups with higher location-based payoffs were able to displace groups bigger than themselves. In my second study, I explored whether models accounting for female and male reproductive strategies could be used to infer the probability of intergroup encounter outcome and intensity. I also assessed whether these models fitted the data better than those of the first study. Female and male reproductive strategies played important roles in determining outcome and intensity of intergroup encounters. I found evidence of female resource defence and male mate access defence via intergroup aggression and intragroup sexual coercion. Overall, accounting for reproductive strategies improved model fit. In my third study, I reported several cases of coalitionary attacks against members of other groups. These attacks resulted in injuries, loss of infants and death for the victims. The aggressions resembled, in several key aspects, intergroup coalitionary aggressions described in chimpanzee and human raids. Intergroup coalitionary attacks in crested macaques may improve the chances of the attacking groups to displace the victims’ groups in future encounters. My thesis provides the first in depth investigation on factors affecting outcome and intensity of intergroup encounters in crested macaques. These data might be useful in comparative studies on how intergroup relationships vary depending on socioecological factors such as resource abundance and social structure. This is also the first report pf repeated intergroup coalitionary aggressions with severe consequences for the victims in an Old World Monkey (Cercopithecidae) species. My findings highlight the role of reproductive strategies in intergroup encounter outcome and intensity and the need to consider female and male strategies together when investigating between-group contests
