Behavioral correlates of non-random mortality among free-ranging female vervet monkeys

Abstract

Over a 3.5 year period, illness and predation operated in a non-random manner on free-ranging vervet monkeys in Amboseli National Park. As a result, there was no correlation among adult females between dominance rank and reproductive success. Deaths due to illness were concentrated among low-ranking individuals, and appeared to occur as a result of restricted access to food and water during the dry season. In contrast, deaths due to predation were concentrated among high-ranking individuals. The precise cause of such increased vulnerability could not be determined. High-ranking females alarm-called at higher frequencies than low-ranking females, and were also more aggressive than low-ranking females during intergroup encounters. In contrast, low-ranking females were more likely to initiate friendly interactions with the members of other groups. The non-random distribution of causes of mortality suggests that individuals living in the same social group may confront different selective pressures. Perhaps as a result, individuals appear to respond differently to similar social and environmental variables.