Tests for cooperative behaviour between stallions

Breeding groups with multiple stallions occur sympatrically with single-sta llion breeding groups in feral horse, Equus caballus, populations. Mutualis m and reciprocal altruism between stallions have been proposed to explain t he origin and functioning of multistallion bands. However, empirical suppor t for these hypotheses is contradictory and incomplete. Furthermore, there are no explicit tests of the predictions that each hypothesis makes about s tallion behaviour and social structure. We compared nine multistallion and 18 single-stallion bands in the Kaimanawa Ranges, New Zealand. Compared wit h agonistic behaviours, affiliative behaviours were relatively unimportant in the relationships between stallions within bands. The number of stallion s in the band did not have a positive influence on mare group size, stabili ty, home range quality or reproductive success in bands. Furthermore, there was a positive relationship between aggression ('intolerance') by the domi nant towards subordinate stallions and the subordinates' effort in mare gro up defence ('helping') but a negative relationship between helping effort b y subordinates and their proximity to, and mating with, the bands' mares. T herefore, the predictions of the mutualism and reciprocal altruism hypothes es were not supported. Indeed, for some of the predictions we found the opp osite outcomes to be true. Multistallion bands had significantly poorer rep roductive success, and dominant stallions were less tolerant of subordinate s that helped most and reduced their access to mares. Nevertheless, in all other respects Kaimanawa stallions in multistallion bands behaved like thos e described elsewhere. Thus, we reject cooperative hypotheses for multimale breeding groups in horses and discuss the mate parasitism and consort hypo theses as better alternatives. (C) 2000 The Association for the Study of An imal Behaviour.