Dominance and Its Fitness Consequences in American Bison Cows

1. Dominance relations among adult female American bison (Bison bison) were studied between June 1980 and August 1982 at the National Bison Range, Moiese, Montana, and Yellowstone National Park, Wyoming, U.S.A. 2. Contested interactions, or "battles", between cows were rare at both sites. Only one of 362 observed interactions among known Bison Range cows was contested, and only four such interactions were observed during the entire study. 3. Among Bison Range cows, reversals of dominance status within dyads occurred at an extrapolated rate of once every 7.5 dyad-years. 4. Success in aggressive encounters between Bison Range cows was strongly, positively correlated with age, but uncorrelated with body weight or horn damage. Hence, dominance appears to be poorly correlated with fighting ability. Instead, dominance may be determined by cost-benefit asymmetries; older cows may have less to lose from aggression than younger cows. 5. The cow dominance "hierarchy" at the Bison Range does not show a higher degree of linearity than expected given the age-dominance correlation. 6. At neither Yellowstone nor the Bison Range was there evidence of aggressive inhibition of female reproduction, or of harassment of calves by dominant cows. 7. In a variety of environments, a cow that received aggression during foraging suffered a reduction in the proportion of foraging time she spent cropping. Since aggression frequencies varied considerably between habitats, however, the influence of dominance on cropping times also varied between habitats. 8. Aggression frequencies were highest in habitats where forage was sparse and heterogeneous in quality. Accordingly, aggression was most frequent, and probably had the strongest effects on feeding efficiency, in habitats where there was significant travel time between suitable grazing patches.