INDIVIDUAL-RESPONSES TO ACUTE AND CHRONIC STRESS IN PIGS

Pigs can be characterised as resistant (R) or non-resistant (NR) at an early age (1 to 2 weeks) by means of a backtest. In the test the anim al is put on its back and the number of bouts of resistance is used to characterise the animal. The test is performed twice with 1 week inte rval and only pigs that show a consistent response in both tests are c lassified as either Ii or NR pigs. On average eighty percent of a popu lation can be classified by this test. ii and NR pigs show consistent behavioural, physiological and immunological differences when tested i n various challenge test in later life. The NR pigs are more sympathet ically dominated and showing an active coping style (fight/flight) as described in rats and mice. The NR pigs are more para-sympathetically dominated, resembling the passive coping style (conservation/withdrawa l). In intensive husbandry, breeding sows are housed individually and often tethered. After long term tethering these sows show signs of chr onic stress; overreaction of the sympathetic nervous system, hypercort isolaemia and disturbed behaviour. The most common disturbed behaviour found in tethered sows is stereotyped behaviour. Most sows develop st ereotypies within 1 month after first tethering. Again great differenc es are found in the amount of stereotypies shown between sows. Some so ws spent up to 80% of their active time on this behaviour while others hardly develop stereotypies. Sows showing high levels of stereotypies manage to counteract the sympathetic overreaction caused by the chron ic stress of tethering as was shown by a decrease in heart rate during bouts of stereotyped behaviour. In this view stereotypies help the an imal to cope with the averse situation of tethering. However, after 8 months of tethering stereotypies are no longer effectively attenuating heart rate. The effect of stereotypies is limited to the initial phas e of chronic stress when the animal is striving to regain control. Whe n chronic stress persists stereotypies get dissociated from their effe ct on the sympatho-adreno-medullary system and the animal loses contro l.