Measuring amplitude in North American bison bellows: Environmental, social, and individual impacts

Abstract

Communication is a key aspect of any mating system and is a dramatic feature in the mating behavior of North American bison. During their reproductive season, bison bulls defend females from other males by establishing dominance through contests involving multiple signals, the most prominent being the bellow vocalization. While acknowledged that amplitude may be involved in sexual selection, it is rarely measured directly in the field (especially in large mammals) due to difficulties in accurately assessing this parameter. Using strict methodologies accounting for many sources of potential error, the impact that a variety of factors had on bellow amplitude was examined. Amplitude was measured in dB with Lamax and Lcpeak weighting. Parameters increasing amplitude (order of decreasing impact): Number of surrounding bulls bellowing, wind levels, and number of surrounding bulls. Parameters decreasing amplitude (order of decreasing impact): Distance, head orientation, days spent in rut, time of day, number of copulations, dominance level of bull, and temperature. Since amplitude is correlated with measures of bull quality, condition, and surrounding social environments (beyond physical impacts of distance, etc.), it is likely that amplitude plays a significant role in competition over mates. [Work supported by NSF.]