Social transmission of physiological and behavioural responses to castration in suckling Merino lambs

In social species like sheep, social context can modify both physiological and behavioural responses to stressors and normal behavioural patterns. Presence of conspecifics can ameliorate responses to noxious stimuli, an effect termed social buffering, whereas the presence of a distressed conspecific can invoke a distress response in an observer animal not receiving a noxious organic insult, an effect termed social contagion. Furthermore, synchrony of normal behaviours can occur within a group. Here a range of social contexts were created by grouping suckling lambs undergoing knife, ring or sham castration treatments either homogeneously or heterogeneously by castration treatment in pens within an animal house. The impact of social grouping on cortisol, rectal temperature, pain avoidance behaviours, postural behaviours and synchrony of behaviour in the first 12 h following castration treatment was examined. Irrespective of castration treatment, lambs grouped homogeneously by castration treatment had higher cortisol concentrations across the four time points measured than lambs grouped heterogeneously. They also spent 1.9% more of the time in restless behaviour in the first 1 h following castration, and in the 12 h following castration spent 6.7% more of their time lying ventrally, 4.9% less time standing normally, 1.6% more time walking normally, and 4.9% less time in total standing postures. Interactions between social grouping and castration treatment were not significant for physiological variables, pain related behaviours or postures with the exception of lateral lying which was decreased in knife castrated lambs grouped heterogeneously with other castration treatments from 1.9% (homogeneous grouping) to 0.4% (heterogeneous group). Synchronisation of behaviour was seen for walking in lambs grouped heterogeneously by castration treatment and for feeding at the trough in lambs grouped homogenously by ring and sham castration treatments. Lying and standing respectively, were negatively synchronised (occurred less frequently than predicted by chance alone) in ring and knife castrated lambs grouped homogeneously. The results indicate that mixing lambs that received the three castration treatments within a pen modified the activity profile of the lambs but did not substantially modify the behavioural or physiological changes that are stigmatic of responses to the specific castration treatments. The social contexts contrasted in the study did not result in marked contagion or social buffering between lambs of the measured responses and did not confound comparison of castration treatments. None-the-less, the comparability of responses observed in castration treatment groups penned homogenously or heterogeneously might not necessarily occur in other classes of sheep or in other social contexts. The absence of social transmission of behavioural and physiological responses indicates that grouping non-castrated lambs with castrated lambs did not compromise the welfare of the non-castrated lambs when their mothers were present.