In species that form one-male social groups, males may engage in contest competition via avoidance, direct contests during intergroup encounters and long-distance vocal signaling. These interactions may be influenced by the males' reproductive strategies, by differences in resource holding potential, and by the value of the resource. Because agonistic behavior can be costly, animals would benefit by assessing opponents and avoiding escalating a contest they were likely to lose. The male loud calls of many nonhuman primate species likely play an important role in this regard. Because energetic and physiological constraints may influence the quality of the signal, variation in these calls may enable males to assess competitors. This study will investigate male-male competition in pig-tailed langurs or simakobu monkeys (Simias concolor) with five main objectives. Firstly, by documenting intergroup encounters and loud calls, it will be determined if males defend mates, food, and/or infants from other males. Secondly, to understand the influences of resource holding potential and resource value on male interactions, detailed observations of contests between individuals with different fighting abilities and over different resources will be made. Thirdly, the long-term outcome of male-male competition will be explored by comparing home range sizes, group compositions and group productivity. Fourthly, to assess the honesty of vocal signals, intra- and inter-individual influences on male loud calls will be determined via acoustic analyses. Finally, by measuring male responses to loud calls in both natural and experimental conditions, the ability of conspecifics to perceive these differences will be investigate. Research will be conducted on four habituated one-male groups in a protected forest in northeastern Siberut, the northernmost island of the Mentawai island archipelago in Indonesia.
The study aims to refine our understanding of the strategies and signals used in male-male competition. The resource defense strategy in particular is hypothesized to be more widespread than once believed, especially in colobines. Furthermore, the role males might play as infant protectors has rarely been examined. This research will also provide insight into the information males can extract from vocal signals (in terms of male's quality and condition). In addition, this research serves as part of the Siberut Conservation Project's initiative to study the rare and little-known fauna of Siberut. Research will be carried out in close cooperation with an Indonesian doctoral student (shared data collection). Local guides and several Indonesian students will be trained in behavioral and ecological data collection and employed. Finally, this study will be the first to provide baseline data on the social, ecological and reproductive behavior of this highly endangered primate species, a much-needed basis for efforts to manage and conserve these animals.
