Heart rate responses to agonistic encounters in greylag geese, Anser anser

As in most social groups, agonistic interactions of various intensities are common in a goose flock. This may cause social stress, modulating heart rate (HR), which may serve as a measure of energetic investment and also of individual emotional involvement. We investigated HR responses to social encounters in 24 free-living greylag geese in an intact social environment. We recorded 1602 social interactions of various intensities in which the focal individual either attacked another member of the flock or was attacked itself. We analysed five HR parameters (mean HR, maximum HR, HR increase, duration until maximum, time until the baseline value was reached again). Generally, HR scaled positively with increasing intensity of agonistic interactions as well as with increasing duration. Individuals showed higher HRs when attacking than when being attacked. In addition, focal individuals responded with a greater HR increase when confronted with an opponent winning a higher percentage of interactions than itself. Repeated agonistic interactions against a specific opponent were related to greater HR responses than single events and focal individuals responded more strongly to male opponents than to females. Our results indicate a differential HR response depending on the intensity and duration of an interaction as well as the identity of the opponent. This differential physiological investment may reflect differences in emotional involvement depending on the social context of a particular agonistic interaction.