Abstract detail

Bison maternal social groups in Custer State Park, South Dakota

Student: Alyce Johnson
Faculty Mentor: Dr. Josh Millspaugh, Fisheries and Wildlife

Presented at the 2007 Summer Undergraduate Research and Creative Achievements Forum

Abstract

We studied maternal bison groups in Custer State Park, South Dakota, June–July 2007. We used two methods to evaluate social groups. In one method, we recorded observations of the numbers of cow pairs of the same cohort. We used a chi-square test to determine if cohort pairs occurred more than expected among cows that were force-weaned versus cows that were naturally-weaned. For the second method, naturally-weaned tagged bison cows with known maternal lineage were observed to determine the proportion of time they are in the same group. We used a chi-square test to determine if cows spent more time in groups with their maternal relative than would randomly be expected. We grouped observations into "forced-weaned" and "naturally-weaned" cohorts and analyzed these with a chi-square test. We found that the "force-weaned" group was found with individuals of the same cohort greater than would be expected at random (χ27 =19.3; P = 0.007) and the "naturally-weaned" group was not found with cows of the same cohort greater than expected (χ22 = 0.250; P = 0.882). We observed that tagged yearling heifers spent more time in groups with their mothers than in other groups (χ28 = 167; P < 0.000), however, older cows spent more time in groups without their mother (χ22 = 44.5; P < 0.000). We also observed that three "sister" cows spent did not spend more time in groups selectively with or without each other (χ22 = 0.160; P = 0.689). We conclude that the maternal social structure of CSP bison herds is complicated, yearlings demonstrate close bonds with mothers but this relationship appears to become less important as cows age. Moreover, the bond among animals that were force-weaned appears to be stronger than maternal bonds of naturally-weaned cows. More research is needed to better understand this relationship.