Individual differences in behaviour: A test of 'coping style' does not predict resident-intruder aggressiveness in pigs

Temperament traits in animals are those which are stable across time and predictive of behaviour in other situations. Individuals can be identified as showing either active (proactive) or passive (re-active) responses to a variety of challenges, and these are often referred to as 'coping styles'. Although most studied in rodents, coping styles are found in a variety of vertebrate species. In rodents, the resident- intruder aggressiveness test is a key measure of coping style, whereas in pigs, several recent studies have used a 'backtest' as the key measure of coping: A piglet is held on its back for one minute and the frequency of struggling bouts ('escape attempts') is recorded. In this study, we compared pig behaviour in backtests performed on 3 and 9 days of age, with aggressiveness was measured using 2 resident-intruder tests between 16 and 19 days post-weaning. A smaller unfamiliar intruder was introduced into part of the resident pig's home pen, and resident attack latency was recorded. Struggling bout frequency in the backtests showed a unimodal distribution. Data were analysed both as a continuous distribution and by comparing only the piglets from either extreme of the distribution (for comparison with other studies, top and bottom 25%). There was good repeatability of outcome (struggling frequency) over the two backtests, and over the two resident-intruder tests (attack latency and occurrence of resident attacks). However, no relationship was found between behaviour in the backtests and aggression in the resident-intruder tests, despite exhaustive analysis. Other authors have found that pigs show consistent responses to challenges, suggesting a degree of stable temperament. In line with the present findings though, most found no correlation between social and non-social responses. Other published experiments show that in pigs, the backtest can predict several physiological differences characteristic of coping styles. However, the present study shows that in pigs, aggressiveness and struggling in a backtest are not correlated, suggesting that they do not represent different expressions of the same underlying 'coping' trait.