INDIVIDUAL VARIATION IN THE HEART-RATE OF PIGLETS - EVIDENCE AGAINST STABLE DIFFERENCES

Two hypotheses were tested, (a) that the heart rate of piglets would s how consistent, ordinal differences between animals within data collec tion sessions and (b) that the heart rate of piglets would show consis tent, ordinal differences between animals across data collection sessi ons, from birth to twenty-eight days of age. Heart rate was measured i n 15 animals and was analysed with respect to the following variables; individual animal, behaviour posture, locomotion, aggression and feed ing), age, weight, ranked weight, experimental treatment and data coll ection session (from data collection session 1 at one day old to data collection session 5 at twenty eight days old). Within a general linea r model all variables except ranked weight were found to be significan tly related to heart rate (p < 0.0001 in all cases). Residual heart ra te values were found, on the first, second and fifth data collection s essions, to show consistent ordinal differences between animals within data collection sessions. That is, the residual heart rate of an anim al in response to initial handling was correlated with that animal's b aseline residual heart rate (r = 0.80, 0.89, 0.89 on the first. second and fifth data collection sessions, respectively, P < 0.001 in all ca ses) and, on the first and fifth data collection sessions, with the re sidual heart rate in responses to subsequent handling (r = 0.84, p < 0 .001 and r = 0.71, p<0.01, respectively). However, correlations of res idual heart rate values across sequential data collection sessions wer e found not to be significant. That is, heart rate on data collection session n was not correlated with heart rate on data collection sessio n n + 1. it is suggested that either the dynamic environment within wh ich these animals existed precluded the observation of consistent, ord inal differences or that these types of differences do not exist in an imals of this age. (C) 1998 Elsevier Science B.V.