In mammals, support by a social partner may reduce stress levels and ease access to resources. We investigated the effects of the passive presence of a nearby social ally on excreted corticosterone immunoreactive metabolites and behaviour in juvenile graylag geese (Anser anser). Two groups of hand-raised juveniles (N1 = 9, N2 = 3) were tested over 1 year by positioning humans of different familiarity (i.e., the human foster parent, a familiar human, a nonfamiliar human, no human) at a standard distance to the focal geese. Their success in agonistic interactions significantly decreased with age and with decreasing familiarity of the accompanying human. The humans present modulated the excretion of corticosterone immunoreactive metabolites, with the strongest effects recorded after fledging when corticosterone metabolites were also positively correlated with agonistic behaviour. This suggests that a human foster parent may provide similar supportive benefits as goose parents do in natural families. We discuss the benefits of social alliances with regard to the integration into the flock, access to resources, and life history.
