The use of vocal signals in the social play of barbary macaques.

Field studies in various species of Macaca (Cercopithecidae) provided evidence for specific visual displays that typically accompany playful interactions. The aim of our study was to examine whether and when playing individuals would use auditory displays, i.e. vocalizations that often occur during social play as well. The study was conducted on a population of semi-free Barbary macaques (Macaca sylvanus) with a special focus on the composition and dynamics of playful wrestling (synonymous term: 'rough-and-tumble play'). Analyses of dyadic encounters between subadult males allowed us to distinguish five types of playful behaviours and three types of vocalizations. The latter were clearly linked to encounters where effects of visual signals were impaired, e.g. during close body contact. During wrestling, vocalizations tended to increase in the beginning of an encounter, whereas the last seconds of wrestling often showed a decline in vocalization rate. Our results allowed us to conclude that these vocalizations may supplement or in many cases even substitute interactional effects of visual signals, e.g. the 'play face.'