'Subordination style' in pigs? The response of pregnant sows to mixing stress affects their offspring's behaviour and stress reactivity

In various mammals stress during gestation can result in long term effects on the behaviour and physiology of the offspring. The aims of this study were (i) to characterise individual variation among primiparous sows (gilts) in their behavioural and physiological response to a commercially relevant social challenge (mixing with older sows) during gestation and (ii) to determine whether this variation was associated with a corresponding variation in the subsequent effects on the offspring. After assessing dominance in a food competition test, 24 gilts were studied in groups of six during two one-week periods (unmixed – U). These gilts were split into eight subgroups of three gilts, each of which weremixed with two older sows for one week (mixed – M1) before being returned to their original group of six for two weeks. Mixing was then repeated using different older sows during a second week (mixed – M2). Aggressive behaviour, skin lesions and salivary cortisol increase over baseline were higher, and weight gain was lower during mixed periods. During mixing gilts spent more time in feeder stalls, avoiding sows lying in a straw-bedded area, although there was considerable variation among individuals in the extent to which they interacted with or avoided sows. Gilts which interacted more with sows had lower weight gain and higher lesion scores. Previously dominant gilts had higher salivary cortisol increases during mixing. Measures of the impact of mixing, most notably M1 lesion scores, were associated with the behaviour of their offspring: gilts with more lesions had piglets that were less active and less vocal on the day of weaning and over 31 days post-weaning, and were less aggressive over 31 days post-weaning. Thirty-two piglets (from 16 of the gilts) were either restrained for 30 min or not restrained prior to euthanasia at 60 days of age. The restrained piglets from gilts with higher M1 lesion scores had higher levels of corticotropin-releasing hormonemRNA in the paraventricular nucleus and amygdala. This study demonstrates that differences between individual gilts in their experience of a stressful social challenge co-vary with the extent to which this pre-natal stress impacts on their offspring. 2010 Elsevier B.V. All rights reserved.