The evolution and functional significance of bovid horns.

Researchers generally agree that the horns of male bovids are a product of sexual selection and are adapted for use as weapons of intraspecific combat. However, little attention has been given to the problem of accounting for the occurrence of horns in females or to explaining interspecific variation in male horn morphology-- why are structures that serve the same function so morphologically diverse? This study evaluates several hypotheses that address these questions. Morphological data were collected from the horns and skulls of museum specimens (22 African species) and interpreted in the light of behavioral data from the literature. Chapter I reviews the phylogenetic history of the family Bovidae, with emphasis on the evolution of bovid horns; Chapter II examines interspecific variation in male horn size, and evaluates the hypothesis that relative horn size reflects variation in the intensity of male-male competition; Chapter III examines the evolution and functional significance of horns in female bovids; and Chapter IV examines the relationship between male horn morphology and intraspecific fighting behavior. Across species, male horn size increases disproportionately with body size. The hypothesis that interspecific variation in horn size reflects variation in the intensity of male-male competition is supported by the finding that polygynous species have larger horns, relative to body size, than monogamous species. However, the pattern of interspecific variation in horn size might alternatively be explained by differences in fighting behavior. Compared to male horns, female horns show a reduction in attributes important to male intraspecific fighting (e.g. robustness, upright stem, well developed catching arch) and enhancement of characters that might be advantageous in anti-predator defense (e.g. straightness, thinness). However, more behavioral data are needed before intraspecific aggression can be ruled out as an important function of female horns. Morphological data support the hypothesis that interspecific differences in male horn morphology reflect differences in fighting behavior. In particular, differences in horn reach and relative development of the catching arch are associated with differences in the frequency of stabbing vs. wrestling or fencing behavior, and differences in horn robustness are associated with differences in the frequency of ramming behavior.PhDBiologyUniversity of Michiganhttp://deepblue.lib.umich.edu/bitstream/2027.42/162085/1/8907088.pd