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 animal 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 interval and only pigs that show a consistent response in both tests are classified as either R or NR pigs. On average eighty percent of a population can be classified by this test. R and NR pigs show consistent behavioural, physiological and immunological differences when tested in various challenge test in later life. The R pigs are more sympathetically 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/withdrawal). In intensive husbandry, breeding sows are housed individually and often tethered. After long term tethering these sows show signs of chronic stress; overreaction of the sympathetic nervous system, hypercortisolaemia and disturbed behaviour. The most common disturbed behaviour found in tethered sows is stereotyped behaviour. Most sows develop stereotypies within 1 month after first tethering. Again great differences are found in the amount of stereotypies shown between sows. Some sows 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 chronic stress of tethering as was shown by a decrease in heart rate during bouts of stereotyped behaviour. In this view stereotypies help the animal 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 phase of chronic stress when the animal is striving to regain control. When chronic stress persists stereotypies get dissociated from their effect on the sympatho-adreno-medullary system and the animal loses control.