Fighting for what it's worth: participation and outcome of inter-group encounters in a pair-living primate, the Javan gibbon (Hylobates moloch)

Abstract

When resources are limited and defensible, inter-group encounters in animals are often of aggressive nature. Individuals can participate in inter-group encounters to defend mates, infants, and food resources, but also to attract out-group individuals for additional mating opportunities. Since inter-group conflicts have mainly been studied in group-living species, we examined the mate, infant, and food resource defense and mate attraction hypotheses in pair-living Javan gibbons (Hylobates moloch) in Gunung Halimun-Salak National Park, Indonesia. To this end, we investigated factors influencing male and female participation and outcome of encounters (i.e., win vs. lose). We observed 234 complete encounters between three habituated and five unhabituated gibbon groups over 43 months, of which 72% were aggressive. Males were the main participants and they were more likely to participate when cycling females or dependent infants were present, supporting the mate and infant defense hypotheses. Males were also more likely to participate when more fruits were available, contradicting the food resource defense hypothesis. Females participated by singing more often when they were cycling and when there were singing opponents, suggesting an advertisement function of their reproductive status through songs. The probability of winning an inter-group encounter was only higher when cycling females were present, supporting the mate defense hypothesis. The intensity of space use or aggression level had no effect on the outcome of inter-group encounters. Our results highlight that mate and infant defense are crucial for male Javan gibbons, especially in view of their pair-living system, long interbirth intervals, and slow infant development. Significance statement While animal groups interact aggressively with each other to defend valuable resources, they can also interact to increase additional mating opportunities. Here, we examined male and female participation and the outcomes of inter-group encounters in a wild pair-living primate, the Javan gibbon. Crucially, we found that the presence of cycling females had a significant impact on male participation, female singing, and the outcome of encounters. Our findings suggest that Javan gibbon females may advertise their reproductive status through singing during inter-group encounters while Javan gibbon males rather participate to defend their mates and infants.