Recruitment Screams of Pigtail Monkeys (Macaca Nemestrina): Ontogenetic Perspectives

Abstract

AbstractIn several species of nonhuman primates screams given by victims of attack elicit interventions from allies in the social group. In the present study, the screams of 16 pigtail macaques (M. nemestrina) under three years of age were assessed with respect to production and contextual usage. Data obtained from these animals three years later were compared with those from the previous period for evidence of changes in usage and production. It was hypothesized that these monkeys (8 males and 8 females), when three years older, would show patterns of contextual usage and production for agonistic screams that were more similar to those of adult animals. For control and comparison, 10 monkeys (9 females and 1 male) who had been older than three years of age during the earlier study were also sampled during the subsequent period. Calls of all subjects were assigned to one of four agonistic contexts (defined by the severity of the aggression and the dominance rank of the opponent) using discriminant functions that had been generated from the earlier data. While the successful classification rate improved for the 16 subjects that were originally three years of age or younger, no significant change was found for screams of the 10 older control monkeys. The probabilities with which screams of the 16 younger subjects were assigned correctly to context (a measure of call production) also were significantly higher during the second period. No such change was evident in the calls of the 10 control subjects. Data from the earlier study period had suggested that, among juveniles, females appeared to be more proficient than males in both the production and usage of these calls. During the second period of study, the proportion of calls that was classified correctly increased significantly for both males and females. The results of tests comparing the proportion of correctly classified calls for individuals revealed no differences between males and females during the second period, confirming similar results for pooled data analyses. Furthermore, there were no significant differences between males and females in the assignment probabilities with which calls were correctly classified. These results suggest that, with age, the younger monkeys became more proficient in the contextual usage and production of screams.