Individual differences in tonic immobility may reflect behavioural strategies

Many species of animals have been reported to show tonic immobility (TI) in response to physical restraint. In this paper, we investigate the interrelationship between the behaviour of pigs in three different situations, which are assumed to present challenges to the pigs. Individual piglets were tested for TI at 2.5 weeks of age (susceptibility to and duration of immobility), for their response to routine administration of an anti-parasite injection at 4 weeks of age (relaxed (R), tense (T), and struggle (S); vocalising yes/no), and for their speed and ease of movement through an unfamiliar environment at 10 weeks of age. We found TI to be predictive of behaviour across the two month test period. In the handling/injection situation pigs who vocalised were either tense or struggled. Relaxed pigs did not vocalise. We interpret this link between response (tense or struggle) and vocalisations as suggesting that both T and S were responses indicating that the pigs found the situation challenging, while the absence of vocalisations in conjunction with the relaxed muscles of the R pigs may indicate that the situation was perceived as presenting little or no challenge to the pigs. T/S pigs did not differ from R pigs in their behaviour in the TI test at 2.5 weeks. However, T pigs did show longer TI durations than S pigs. In the movement test, pigs who had previously shown a low susceptibility to TI moved faster through the unfamiliar environment than those who had become immobile. We propose that TI is one possible way of assessing whether individual pigs are more likely to adopt a more active (low susceptibility/short duration of TI, struggle, move fast) or a more passive behavioural strategy (high susceptibility to/long duration of TI, tense, move more slowly) in a challenging situation.