Acoustic cues to size and quality in the vocalizations of male North American bison, Bison bison

Source-filter theory provides a framework to interpret the acoustic structure of vertebrate vocalizations in relation to biophysical production, and it predicts that specific acoustic parameters can encode information about callers. Because formant frequencies are determined by vocal tract dimensions, with longer vocal tracts producing lower formants, they can be reliable indicators of body size, as well as other important traits. In polygynous species, reliable acoustic cues to fitness-related traits are expected to be under strong sexual selection pressure through male competition and/or female choice. This study investigates whether formant frequencies of male North American bison bellow vocalizations encode information about fitness-related caller attributes. Bison exhibit male-dominance female-defence polygyny, with dominance displays involving bellows. We hypothesized that physical attributes (mass, age) would predict formants and that formants would in turn predict quality indices (dominance, copulations, offspring sired). Our results showed that heavier bulls produced lower formants and that lower formants predicted higher mating success (copulations), even when controlling for mass. Given positive associations between mating success, dominance and reproductive success (offspring sired) in bison, we conclude that bellows with lower formants reflect greater fitness in bulls. We discuss the importance of reliable acoustic cues to size and quality indices in sexual selection contexts.