Heart rate modulation in bystanding geese watching social and non-social events

Abstract

Simply observing other individuals interacting has been shown to affect subsequent behaviour and also hormones in 'bystander' individuals. However, immediate physiological responses of an observer have been hardly investigated. Here we present results on individuals' heart rate (HR) responses during various situations, which occur regularly in a flock of greylag geese (Anser anser, e.g. agonistic encounters, vehicles passing by). We recorded simultaneously HR and behaviour of 21 semi-tame free-roaming geese, equipped with fully implanted transmitters. We considered 304 social and 81 non-social events during which the focal individuals did not respond behaviourally. Independent of the spatial distance to the event, these HR responses were significantly greater in social contexts (e.g. departing or landing geese, agonistic interactions) than in non-social situations (e.g. vehicles passing by, thunder). Focal individuals showed a significantly higher maximum HR as well as a greater HR increase in response to agonistic interactions, in which the pair partner or a family member was involved, as compared with a non-affiliated goose. Also, HR was significantly higher when the bystander watched non-affiliated geese interacting, which were higher ranking than the focal. We conclude that these differences are due to different relevance of the recorded events for the focal individual, depending on the individuals involved in the observed interaction.

Figure 1

Examples of a social and a non-social bystander event of one focal individual. (a) The focal individual watched an agonistic interaction at a distance of approximately 5 m. The two interacting individuals were non-affiliated with the focal individual that was resting all the time. (b) A truck passed by, at a distance of approximately 10 m. In this case, the focal individual was vigilant and did not change its behaviour in response to the event.

Figure 2

(a) Maximum HR and (b) HR increase when watching social events as compared with non-social events in 21 greylag geese. Box plots show the median and the interquartile range from the 25th to the 75th percentiles. Whiskers above and below the box indicate the 10th and 90th percentiles. ^*p<0.05.

Figure 3

(a) Maximum HR and (b) HR increase in b.p.m. of eight bystanders showing no behavioural response when watching agonistic interactions with affiliated or non-affiliated geese involved. Other parameters are the same as given in figure legend 2.

Figure 4

(a) Maximum HR and (b) HR increase of focal individuals depending on the ‘rank’ relationship of the two interacting individuals with the bystander. In all events, the opponents were both either lower (n=7) or higher (n=4) ‘ranking’ than the observing focal individual. All opponents were non-affiliated to the bystander. Other parameters are the same as given in figure legend 2.