Abstract
In general, support by social allies may reduce stress, increase success in agonistic encounters and ease access to resources. Social support was mainly known from mammals, particularly primates, and has been studied in birds only recently. Basically two types are known: (i) 'active social support', which describes the participation of a social ally in agonistic encounters, and (ii) 'passive social support' in which the mere presence of a social ally reduces behavioural and physiological stress responses. In greylag geese (Anser anser) offspring stay with their parents for an entire year or even longer and therefore are a candidate avian model to study support by social allies. We investigated the effects of active and passive social support in ten families (ten males, ten females, 33 juveniles) in a free-roaming, semi-tame flock of greylag geese. Focal individuals were observed during three time periods: (i) re-establishment of the flock in the fall, (ii) stable winter flock, and (iii) disintegration of the flock and break-up of family bonds. We recorded all agonistic interactions of the members of one focal family during morning feedings for two consecutive days: a control day, in which food was distributed widely, and a social density stress situation, in which the same amount of food was spread over a much smaller area. In addition, we collected faeces of all individuals within this family for three hours from the beginning of the feeding situation for determining excreted corticosterone immuno-reactive metabolites by enzyme immuno assay. We found that the small families, i.e. pairs with one or two accompanying young, were involved in more agonistic interactions, mainly through the lack of active social support, as compared to large families in the same situation. Members of greylag goose families lost agonistic encounters significantly less often when actively supported. In addition, the excretion of corticosterone metabolites was significantly decreased in large families during a social density stress situation, probably as an effect of passive social support. Via such a socially induced decrease in hormonal stress response during challenging situations, an individual's long term energy management may benefit.