Fitness Consequences of Sexually Selected Acoustical Signaling in American Bison

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It is now clear that sexual selection (selection for traits that increase mating success) has led to the evolution of sexually dimorphic attributes in males. Whether in the context of male-male contests or female mate choice, many of these traits act as signals that contain information about mate quality. Studies of invertebrates and small vertebrates have revealed that such signals are often assessed by receivers so as to choose tactics that maximize reproductive success. Little is known regarding how signal assessment works in large mammals, in which multiple and changing signals must be evaluated concurrently over the course of the breeding season. The goal of this research is to investigate the production and assessment of the bellow vocalization of male bison, which is the most conspicuous hallmark of the bison breeding season. The aim is to clarify the role of the bellow signal in the reproductive success of breeding males. The central hypothesis is that useful information is contained in bellows that other bison can evaluate and use to their advantage. This hypothesis is based on strong preliminary findings that indicate that a number of bellow acoustical features reflect the quality of the signaling male. A necessary prerequisite to this study is an understanding of how reproductive success relates to mating performance and attributes that other bison may use to assess male quality. Therefore, an integral component of this project will be the use of molecular techniques to determine genetic parentage for comparison with male mating behavior. Bison will be studied at Fort Niobrara National Wildlife Refuge where all adults are individually marked and open grassland terrain enables constant surveillance of breeding behavior. A multidisciplinary team of five collaborators will perform the laboratory analyses for genetic parentage, acoustics, and behavioral endocrinology that will complement behavioral observations. Dawn-to-dusk observations will document breeding behavior, and genetic parentage analyses will assign maternity and paternity to all calves. Bellowing will be recorded from dominant and subordinate males, and recordings analyzed to compare acoustical features with measures of signaler quality (reproductive success, competitive ability, condition, physiological state) and motivation (estrous condition of tended female, number of challenging males). Playback experiments will be conducted at night to evaluate the response of males to the acoustical features of synthesized bellows. Fecal samples will be used to measure male sex and stress hormones (associated with dominance and aggression), and female sex hormones (to determine hormonal estrus). The results will provide insights into similar species in which vocalizations play a role in breeding. Field and lab participation by undergraduate and graduate students will offer valuable learning experiences and preparation for future careers in science. Finally, information obtained on bison reproductive biology can be applied to the conservation of this keystone species and symbol of the American West.