The Reproductive Trajectories of Bachelor Geladas.

Abstract

Sexual selection strongly influences patterns of male behavior. Fertilizations cannot be shared, therefore male-male relationships tend to be agonistic rather than affiliative. Despite this fact, males in many species form all-male groups that vary in composition and stability. All-male groups have been observed in many mammalian taxa including cetaceans, ungulates, and primates. Because few studies have examined these groups directly, we know little of how they form, the social relationships between the males that comprise them, or if such relationships influence future reproductive success. Geladas (Theropithecus gelada) are an Old World monkey species where males disperse from their natal units and join all-male groups prior to acquiring reproductive access to females. In this study, I examine the behavioral, hormonal, and genetic factors that shape the reproductive trajectories of bachelor geladas living in all-male groups. This study has four major components. First, I address how gelada all-male groups form. I demonstrate that relatedness influences all-male group formation, although only few pairs of males were considered close kin. Second, I ask how bachelors interact within their groups. I show that bachelors form stratified social bonds within their all-male groups. These bonds were strongest between males that were closely related and similar in age. Third, I attempt to answer how bachelors become dominant “leader” males. Males that eventually became dominant had higher testosterone levels during the bachelor period. I draw on extended field-based observations of wild geladas (2006-2011) as well as hormonal and genetic data to answer my study questions. These findings represent the first study of gelada all-male groups and one of the few studies on such groups in related species.