The relationship between social stress and dominance is seasonal in greylag geese

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Studies of captive animals have suggested that social stress affects subdominants, whereas recent data from the wild have revealed that stress mainly affects dominants. We used a non-invasive approach to investigate, for the first time in a social bird, the circannual stress–dominance relationships between low-ranking single males, intermediately positioned paired ganders without offspring and high-ranking paired males with offspring from a flock of semi-tame, free-ranging greylag geese,Anser anser. We collected 933 faecal samples from 43 individuals, 12 singletons, 18 paired males without offspring and 13 paired males with offspring over an entire year and analysed them for corticosterone metabolites by enzyme immunoassay. During the mating season (February–April), singletons had marginally higher corticosterone than paired males (P<0.1), whereas during the parental season (May–January), the paired males with offspring had significantly higher corticosterone than both paired males without offspring and singletons. All three male categories had significantly higher corticosterone during the mating season than during the rest of the year. These results suggest that social stress in ganders is caused mainly by competition between males and by constrained access to females during the mating season, but by parental commitment during the rest of the year. We suggest that dominance per se may not be a direct cause of stress. Rather, the amount of social stress may co-vary with the behavioural investment individuals need to make to optimize their fitness and with the relationship between such demands and the individuals' rank positions. This relationship seems to be seasonal in geese.