Acoustic correlates of caller identity and affect intensity in the vowel-like grunt vocalizations of baboons

Comparative, production-based research on animal vocalizations can allow assessments of continuity in vocal communication processes across species, including humans, and may aid in the development of general frameworks relating specific constitutional attributes of callers to acoustic-structural details of their vocal output. Analyses were undertaken on vowel-like baboon grunts to examine variation attributable to caller identity and the intensity of the affective state underlying call production. Six hundred six grunts from eight adult females were analyzed. Grunts derived from 128 bouts of calling in two behavioral contexts: concerted group movements and social interactions involving mothers and their young infants. Each context was subdivided into a high- and low-arousal condition. Thirteen acoustic features variously predicted to reflect variation in either caller identity or arousal intensity were measured for each grunt bout, including tempo-, source- and filter-related features. Grunt bouts were highly individually distinctive, differing in a variety of acoustic dimensions but with some indication that filter-related features contributed disproportionately to individual distinctiveness. In contrast, variation according to arousal condition was associated primarily with tempo- and source-related features, many matching those identified as vehicles of affect expression in other nonhuman primate species and in human speech and other nonverbal vocal signals.