Agonistic behaviour associated with orphan bison (Bison bison L.) calves released into a mixed resident population

Aggressive interactions of 43 bison (Bison bison L.) calves (10 months old) released motherless into a 288-member resident bison population of mixed composition were observed. Based on expected observations of aggressive interactions from chi(2) analyses, these 'orphan' calves were recipients of significantly more aggression than any resident age class, including similarly aged resident calves. Most aggression directed towards orphans was initiated by resident yearlings. We did not observe orphan calves initiate any aggression towards other animals during the study, even towards other orphans. These findings support previously suggested hypotheses that: (1) dominance rank in bison is linear and established by older cohorts over each subsequent year's cohort; and (2) maternal presence is socially beneficial to young bison by reducing aggression directed towards the offspring from other herd members and facilitating their social integration. Surprisingly, no differences were found in growth rates between orphan calves and resident calves with mothers, despite initial weight differences that were significant. This finding suggests that the higher levels of hostility experienced by the orphan calves had no significant physical effects. (C) 1997 Elsevier Science B.V.