Date of Award:
Doctor of Philosophy (PhD)
Frederick D. Provenza
Social organization varies widely among herbivores, and the level of social organization in bison is widely debated. I hypothesized that when mother-daughter relationships are allowed to develop, bison form long-term associations. In my study, 25 treatment mothers were selected from a free-ranging herd and kept together with their calves, while 25 control females had their calves forcefully removed. Treatment mothers and offspring had by far the greatest number of associations with a greater percentage of individuals with a half weight index (HWI) > 0.50. The strongest associations (HWI > 0.31) were among treatment mothers and their offspring. Moreover, these associations persisted over multiple generations. Group coordination requires group decisions and these can vary between extremes. I hypothesized bison utilized both democratic and despotic decisions. I examined movement initiation and direction decisions following rest periods. For direction decisions older cows repeatedly made decisions despotically for the group; in 93% of the choices, group directions were within 95% confidence intervals. For movement initiation, bison used a more democratic decision-making process; group movements did not begin until an average of 47% of adult cows exited the group. Interestingly, the oldest females led this final post-rest movement behavior in 81% of the decisions. Presumably, living in properly functioning social groups has many benefits, including reduced stress. I hypothesized levels of stress was related to animal density. Consequently, yearling bison males were weaned and placed in either 1) tight confinement (TC), 2) loose (LC) confinement, or 3) free-ranging (FR, returned to herd). I measured fecal cortisol metabolites (FCM) as an index of stress. Fecal samples were collected in each group every 2 weeks from January to April 2009. Fecal cortisol levels were lowest for FR (23 ng/g DM), intermediate for LC (39 ng/g DM), and highest for TC (63 ng/g DM; P<0.0001). Fecal cortisol levels also varied by date (P<0.0001), and treatment and date interacted (P<0.0001). These results indicate bison live in extended families. Also, older females strongly influenced the direction of group movements, but bison also used democratic decisions. Finally, confinement greatly increased stress in young male bison compared with allowing them to free-range.
Shaw, Ryan A., "Social Organization and Decision Making In North American Bison: Implications For Management" (2012). All Graduate Theses and Dissertations. Paper 1204.
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