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

► We studied acoustic cues to fitness-related caller attributes in male bison calls. ► Anatomically constrained formant frequencies were reliable cues to caller traits. ► Mass predicted formants, and formants predicted copulations. ► Heavier bulls produced lower formant frequencies. ► Bulls with lower formants obtained more copulations, even when controlling for mass.