Amplitude of bison bellows reflects male quality, physical condition and motivation

Sound amplitude (measured as sound pressure level) is an acoustical parameter that has received little attention within communication research, especially in mammals. Although difficult to measure in the field, amplitude is a potentially important parameter of sexually selected signals. In North American plains bison, Bison bison, ‘bellows’ are low, guttural vocalizations made by bulls during the breeding season in the context of male–male contests. It has been hypothesized that bison use bellow amplitude to assess males during male–male competition or female mate choice. In this study, we tested the hypothesis that amplitude is significantly related to measures of bull competitive ability (quality, condition and motivation), and thus, could function as a sexually selected signal. During peak rut over 2 years, courtship and threat behaviours were recorded daily. During observation sessions, bellow amplitude was measured as peak sound pressure levels using a sound level meter. Subsequent genetic parentage analysis determined offspring sired by males. Based on aspects of signalling and game theory, we predicted positive associations between amplitude and mating and reproductive success, dominance, physical condition, motivation to retain females, age and weight. Our results supported a positive association between amplitude and both physical condition and motivation. Conversely, the results showed a negative association between amplitude and quality, as measured by mating and reproductive success. Supporting evidence and alternative hypotheses for these results are explored. Our findings provide support for the notion that bellow amplitude could be used as a sexually selected signal to assess rival males during male–male competition.