Rearing piglets in a poor environment: developmental aspects of social stress in pigs

Effects of rearing conditions on the development of social relationships were investigated in female pigs. For that purpose, one group of piglets was reared in a poor environment (i.e., a commercially used indoor 4.2-m2 standard farrowing crate) and the other in an enriched environment (i.e., 0.5-ha outdoor pasture with half-open farrowing crates). After weaning, all piglets were housed in pairs under standard conditions. Dominant and subordinate individuals were distinguished within pairs and social relationships between members of a pair were studied in adulthood. The results show that pairs reared in a poor environment behave more aggressively. The subordinates of these pairs also develop symptoms indicative of chronic social stress exposure: i.e., they show a delayed onset of puberty, a smaller daily weight gain, and elevated basal cortisol levels that are also higher 5 h after an acute restraint stressor (tethering stress). No such deviations were found in subordinates reared in an enriched environment. It is concluded that rearing piglets in a poor environment may facilitate the development of social stress in adult (subordinate) pigs. Two underlying mechanisms may be proposed: a) deteriorated social skills lead to increased social stress, or b) a failure to cope with stressors in general, and the stress of being subordinate in particular, occurs in animals reared under poor conditions.