Selective Forces Affecting the Predator Alarm Calls of Vervet Monkeys

Abstract

AbstractVervet monkeys in Amboseli National Park, Kenya are preyed upon by four types of predator: mammalian carnivores, eagles, baboons, and snakes. Over a 14 month period, adult males and females gave first alarm calls at comparable rates. Both observation on the frequency of alarm-calling and experiments on the duration of alarm-calling indicated that high-ranking adult males and females gave alarm calls more often than low-ranking adult males and females. Individuals who alarm-called most often did not vocalize most often during social interactions, nor did they spend more time than others surveying the habitat around them. There was some tendency, however, for females who alarm-called most often to precede other females in group progressions. Limited evidence suggests that adult males who gave most alarm calls were more likely than other males to have fathered the group's juveniles and infants. Among adult females, however, there was no correlation between number of offspring and frequency of first alarm calls. Females who gave alarm calls most often were not more likely than other females to spend large proportions of observation time more than 2 m from their offspring. Data on a small sample of confirmed predatory attacks suggest that the offspring of high-ranking females may have been more vulnerable than other immatures to predation. Such differential vulnerability may have resulted in part from the tendency of the offspring of high-ranking females to precede other juveniles in group progressions. Vervets of all age/sex classes alarm-called most at those species of predators to which they themselves seemed to be most vulnerable. Adult vervets gave relatively few alarm calls to predators to which their offspring, but not themselves, were vulnerable, even though such alarm calls would have been of low cost to themselves and of great potential benefit to their offspring. While some aspects of the alarm-calling behavior of vervet monkeys are consistent with the hypothesis that their alarms have evolved to benefit kin, in other respects their alarms appear to have the consequence of benefitting only the alarmists themselves. It is likely that both kin and individual selection, acting on an individual's inclusive fitness, have played a role in the evolution of vervet monkeys' alarm calls.