Dominance, age and weight in American bison males (Bison bison) during non-rut in semi-natural conditions

In the wild, male American bison (Bison bison) aggregate only during the breeding season or rut with female herds to reproduce. During the non-rut, they form bachelor groups. Very little is known about dominance structures in these bachelor herds. In this report, we evaluate social dominance in bachelor groups of bison bulls kept on a commercial bison farm in Belgium, where natural seasonal grouping dynamics are imitated. Observations were carried out during five different non-rut periods over several years. Agonistic behaviour was scored and used to determine dominance rank. The relationships between dominance and the individual factors age and weight were evaluated. We found a strong and significantly linear dominance hierarchy in the male bachelor groups. There was a reversal in rank among same-aged individuals between two subsequent study periods during one non-rut period. Dominance and age were strongly correlated in every study period, as were dominance and weight. We conclude that during non-rut bison bulls can form temporary groups with a linear dominance hierarchy in which older and heavier individuals occupy the highest social positions.