Matrilineal Signatures in the Recruitment Screams of Pigtail Macaques, Macaca Nemestrina

Abstract

In macaques and baboons, scream vocalizations play a major role in the recruitment of allies against agonistic opponents. Pigtail macaques make use of 4 acoustically distinct scream types, with each associated with a particular agonistic context (defined in terms of the opponent's relative dominance rank and the intensity of the aggression). Information about caller identity must also be encoded in recruitment screams if spatially distant allies are to make decisions about intervention. In this study, the agonistic screams of pigtail macaques were analyzed for evidence of vocal signatures that may serve to identify matrilineal kin groups. Maternal genealogical relationships were known for all individuals in the 56 member study group. Direct discriminant analysis was used to classify calls of individuals on the basis of their acoustic structure to one of 3 groups defined by matrilineal relatedness (two homogeneous groups, or matrilines, and one heterogeneous control group). Two scream types associated with higher-ranking opponents were analyzed separately: contact aggression screams and noncontact screams. A highly significant proportion of calls was classified to the correct matrilineal group for both scream classes. The acoustic basis for matrilineal vocal signatures in these calls apparently exists. Efficient vocal communication may require monkeys to classify group members at different levels, depending upon the degree of specificity needed.