The effect of age structure on movement rate, group cohesion, and leadership in female bison (Bos bison)

Abstract

<p>As a result of the recent interest in bison (Bos bison) ranching, over 95% of bison are now found in private herds. Because management strategies used by both public and private herd managers are not significantly different from those associated with cattle, there is concern that bison may be undergoing domestication. Effects of management on bison behavior may include changes in group/herd dynamics, which may, in turn, may affect bison distribution and impacts on the landscape. Severely truncated age structures are typically maintained in both public and private herds, but effects on the behavior and social organization of bison are unknown. In this study I determined the effect of age structure on movement rate, fission/fusion rate, behavioral synchrony, and leadership in female bison. I observed 7 bison herds (18 sampling units) during the summers of 2004-2005 in the Nebraska Sandhills; 11 herds with an old age structure (oldest female [greater than or equal to] 10 y) and 7 herds with a young age structure (oldest female < 10 y). A herd size of about 200 animals had the lowest movement rate; larger and smaller herd sizes had higher movement rates. Movement rate decreased with increasing group size, and age structure of the herd had no independent effect on movement rate. Fission/fusion rate increased with pasture size, and fission/fusion rates were significantly lower in herds containing old females when differences in density were statistically controlled. Behavioral synchrony increased as age of the oldest female in the group increased in old herds, but not in young herds; however young herds were more synchronous than expected. Older females led the group more often than expected, given their proportion in the group. These results suggest that old herds may be more stable and provide leadership, when compared to young herds. Patterns of range use may be different in herds with different age structures, and as a result, revisitation rates may change based on age. Further studies are needed to determine differences in range use patterns, as well as impacts on the grassland environment, with respect to age structure.</p>