Alpha Male Capuchins (Cebus capucinus imitator) as Keystone Individuals

Abstract

A k eystone individual is defined as an individual that has a disproporA keystone individual is defined as an individual that has a dispropor- tionate impact on group dynamics relative to its representation in the population. Here we use over 30 years of behavioural, physiological, paternity, and demographic data collected on the Santa Rosa, Costa Rica, capuchin population to address the question of whether or not alpha male white-faced capuchins (Cebus capucinus imitator) are keystone individuals. Within groups of white-faced capuchins, the alpha male is easily distinguished from other adult males. He is usually of prime age (10–15 years) and often the largest male due to his pronounced secondary sexual characteristics. He is the most central adult male and the recipient of the highest rates of grooming. He is also the most active participant during encounters with predators and extragroup individuals. Using naturally occurring dispersal events, we assess the impact of the removal of an alpha versus subordinate adult male on group dynamics, specifically infant mortality. We found that infant mortality fol- lowing the removal of an alpha male was more than double the rate observed fol- lowing the removal of a subordinate adult male. The removal of an alpha male has additional consequences for group success, individual life histories, and population conservation that extend far beyond the immediate aftermath of an alpha male replacement. Based on these findings, we conclude that alpha male white-faced capuchins are keystone individuals, and future research should focus on identifying the factors that enable some males to attain alpha status while others live out their lives as subordinates.