The effects of social stress on heart rate, heart rate variability and the occurrence of cardiac arrhythmias were studied in 12 growing pigs. Social stress was induced during a good competition test with a pen mate, and subsequently during a resident–intruder test with an unacquainted pig in which the experimental pig was the intruder. The outcome of a test was determined using observations of agonistic behaviour. Five pigs won the food competition test. All pigs were defeated in the resident–intruder test with an unacquainted pig. For all pigs, heart rate was significantly higher and thus the R-R interval significantly lower during the food competition test and resident–intruder test than during baseline recordings. However, pigs that were first defeated in the food competition test had a higher heart rate during the first 7 min of the resident–intruder test than winners of the food competition test. Parameters of heart rate variabiality did not significantly change during the food competition test and the resident–intruder test relative to baseline recordings. Thus, the parasympathetic and the sympathetic nervous system remained in balance during the social stress situations. This may explain why the occurrence of cardiac arrhythmias did not increase during the food competition test and the resident–intruder test relative to baseline. We showed that social status, based on agonistic encounters during the food competition test, may influence the heart rate responses of pigs during the resident–intruder test. When heart rate is used as an index of stress, results indicate that subordinate pigs may experience more stress during an agonistic encounter with an unacquainted pig than dominant pigs.
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