The reliability of individual vocal signature varies across the bonobo's graded repertoire

Vocalizations often contain both ‘dynamic’ information, related to short-term fluctuations in the individual's emotional states, and ‘static’ information, related to long-term attributes such as age, sex, weight and body size which define an ‘individual vocal signature’. While both types of information may be of functional value to receivers, dynamic information requires acoustic versatility, while static information depends on acoustic stability. Here we investigated whether an individual vocal signature is present across the vocal repertoire of the bonobo, Pan paniscus. First, the analysis of the acoustic structure of its five most common tonal vocalizations emphasized the highly graded structure of its repertoire. We then evaluated the reliability of identity information across these call types. The results show that, while all call types supported information related to identity, the reliability of these vocal signatures was not consistent along the graded vocal continuum. Caller identity was strongly encoded at one end of the acoustic gradation (high-hoot) and decreased from bark, soft bark, peep-yelp to peep calls. Strikingly, the reliability of the individual signature thus decreased from calls used in high-arousal contexts to those used in low-arousal contexts. To the best of our knowledge, this is the first demonstration that an acoustic gradation that codes for ‘dynamic’ information can be accompanied by variation in the ‘static’ information that supports vocal individuality.