Endocrine and behavioural responses of male greylag geese (Anser anser) topairbond challenges during the reproductive season

Although greylag geese Anser anser establish long-term monogamous pair-bond s, some of the existing pairs do split up (divorce) and new pairs are forme d during the annual spring mating period. In this study, male greylag geese which were involved in the challenge of an existing pairbond (challenged m ales and challengers) were regarded as 'natural experimental' groups and co mpared with males in stable pairbonds (unchallenged males and male-paired m ales,a common male strategy when the availability of females is low). In to tal, 37 males were investigated. The analysis included a description of the seasonal patterns of hormone levels, aggression and courtship. We tested w hether hormone levels correlated with aggressive and courtship behaviours. Finally, we compared hormonal and behavioural patterns amongst the four gro ups. Immunoreactive testosterone (T) and corticosterone (B) equivalents wer e measured in faecal samples. Individual hormone levels were correlated wit h frequencies of agonistic male-male interactions and with frequencies of m ale-female courtship. During early mating and pre-laying phases, T was at i ts seasonal maximum, which may have masked hormone-behaviour correlations. During egg-laying, at the onset of seasonally decreased T, agonistic male-m ale interactions and the frequencies of courtship behaviour were significan tly correlated with T. Unchallenged males had higher rates of agonistic int eractions than ally other males. However, unchallenged and challenged males tended to excrete T at higher levels than challengers. The high rates of b eing attacked and elevated levels of faecal B were indicative of the social conflict experienced by challengers. No hormonal differences were observed between heterosexually paired males and male-paired males. Tn summary, pai rbond status and situations of social conflict had a modulating effect on T and B; however, in this study, the two hormones seem to be affected indepe ndently of one another.