AGONISTIC BEHAVIOR 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 we re recipients of significantly more aggression than any resident age c lass, including similarly aged resident calves. Most aggression direct ed towards orphans was initiated by resident yearlings. We did not obs erve orphan calves initiate any aggression towards other animals durin g the study, even towards other orphans. These findings support previo usly suggested hypotheses that: (1) dominance rank in bison is linear and established by older cohorts over each subsequent year's cohort; a nd (2) maternal presence is socially beneficial to young bison by redu cing aggression directed towards the offspring from other herd members and facilitating their social integration. Surprisingly, no differenc es were found in growth rates between orphan calves and resident calve s with mothers, despite initial weight differences that were significa nt. This finding suggests that the higher levels of hostility experien ced by the orphan calves had no significant physical effects. (C) 1997 Elsevier Science B.V.