The effects of social stress on heart rate, heart rate variability and the
occurrence of cardiac arrhythmias were studied in 12 growing pigs. Social s
tress was induced during a good competition test with a pen mate, and subse
quently during a resident-intruder test with an unacquainted pig in which t
he experimental pig was the intruder. The outcome of a test was determined
using observations of agonistic behaviour. Five pigs won the food competiti
on test. All pigs were defeated in the resident-intruder test with an unacq
uainted pig. For all pigs, heart rate was significantly higher and thus the
R-R interval significantly lower during the food competition test and resi
dent-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 com
petition;test. Parameters of heart rate variabiality did not significantly
change during the food competition test and the resident-intruder test rela
tive to baseline recordings. Thus, the parasympathetic and the sympathetic
nervous system remained in balance during the social stress situations. Thi
s may explain why the occurrence of cardiac arrhythmias did not increase du
ring the food competition test and the resident-intruder test relative to b
aseline. 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 s
tress, results indicate that subordinate pigs may experience more stress du
ring an agonistic encounter with an unacquainted pig than dominant pigs.