Effects of environmental enrichment on behavioral responses to novelty, learning, and memory, and the circadian rhythm in cortisol in growing pigs

Previously we showed that pigs reared in an enriched environment had higher baseline salivary cortisol concentrations during the light period than pig s reared under barren conditions. In the present experiment, it was investi gated whether these higher baseline salivary cortisol concentrations were a real difference in cortisol concentration or merely represented a phase di fference in circadian rhythm. The effects of different cortisol concentrati ons on the behavioral responses to novelty and learning and long-term memor y in a maze test were also studied in enriched and barren housed pigs. At 9 weeks of age enriched and barren housed pigs did not differ in baseline sa livary cortisol concentrations nor in circadian rhythm, but at 22 weeks of age barren housed pigs had a blunted circadian rhythm in salivary cortisol as compared to enriched housed pigs. The differences in baseline salivary c ortisol concentrations between enriched- and barren-housed pigs are age-dep endent, and become visible after 15 weeks of age. Enriched- and barren-hous ed piglets did not differ in time spent on exploration in the novel environ ment test. Barren-housed pigs had an impaired long-term memory in the maze test compared to enriched-housed pigs; however, no differences in learning abilities between enriched- and barren-housed pigs were found. Because blun ted circadian cortisol rhythms are often recorded during states of chronic stress in pigs and rats or during depression in humans, it is suggested tha t the blunted circadian rhythm in cortisol in barren-housed pigs similarily may reflect decreased welfare. (C) 2000 Elsevier Science Inc. All rights r eserved.