A modulating effect of beta-endorphin on pigs' coping strategies during transport

Pigs were selected at random from three genetic lines (homozygote halothane positive and negative, heterozygotes), and were transported twice. Before and after each transport, blood samples were taken from the jugular vein to measure plasma cortisol and beta-endorphin content. During each transport behaviour and heart rate of the pigs were measured. Descriptive statistics of behavioural, endocrinological and heart rate data provided no evidence for a predetermined idiosyncratic response pattern (i.e., active vs. passive) with respect to transport stress, nor an interaction from the halothane gene, nor an habituation effect. However, from the endocrinological point of view a potentially of passive and active coping strategy (i.e., high vs. low basal plasma cortisol content) was observed before transport. But classification of pigs according to an active and passive coping strategy was unrepeatable during successive transports, when based on heart rate and plasma cortisol content measurements during successive transports. This was explained by a potential modulating effect of beta-endorphin on the relationship between cortisol and heart rate.