PECK ORDERS AND GROUP-SIZE IN LAYING HENS - FUTURES CONTRACTS FOR NONAGGRESSION

We analyse a simple model of the establishment and maintenance of domi nance hierarchies in hens. To be beneficial, dominance relationships r equire that the probability of meeting the same individual repeatedly is high, otherwise costs of establishing the dominance relation are ne ver recouped. Winners and losers benefit from dominance relationships, not necessarily from changing the rate at which they acquire resource s but by avoiding costly contests over them in future encounters. We s how that so-called 'loser effects', in which animals base their strate gies for contesting resources solely upon their past experiences of wi nning or losing dominance fights and not upon who their opponent is, c annot work - these strategies ('pragmatists') must additionally involv e either individual or status category recognition. As alternatives to dominance relationships, we show that signals of status or fighting a bility that determine access to contested resources are expected to ev olve in species with typically large groups because in such conditions the costs of establishing dominance relations are not recouped. Such signals do not depend upon recognizing others individually, but rather upon general category recognition. Status signals are not expected in small groups because dominance relationships are likely to be cheaper and just as effective. The results of the model have implications for the welfare of hens kept in large groups. (C) 1997 Elsevier Science B .V.