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.
