Intended receivers and functional significance of grunt and girney vocalizations in free-ranging female rhesus macaques

We investigated the intended receivers and contexts of occurrence of grunt and girney vocalizations in rhesus macaques (Macaca mulatta) to assess whether these calls are best interpreted as signals of benign intent or as calls that may function to attract the attention of other individuals or induce arousal. We focally observed 19 free-ranging adult female rhesus macaques. Female calls increased dramatically after infants were born, and most were directed toward mother-infant dyads. When infants were physically separated from their mothers, callers visually oriented toward infants in over 90% of the cases, suggesting that infants were the intended receivers of grunts and girneys. Approaches followed by vocalizations were more likely to lead to the caller grooming the mother, less likely to elicit a submissive response, and more likely to result in infant handling than approaches without calls. Infant handling, however, was not necessarily benign. Vocalizations were often emitted from a distance > 1 m and were rarely followed by approaches or social interactions. Our results suggest that grunts and girneys are unlikely to have evolved as signals that encode information about the caller's intention or subsequent behavior. Whereas girneys may be acoustically designed to attract infants' attention and elicit arousal, grunts may have no adaptive communicative function. Mothers, however, may have learned that other females' grunts and girneys are unlikely to be associated with significant risk and, therefore, are generally tolerant of the caller's proximity and behavior.