title: Environmental enrichment during rearing alters corticosterone levels, thymocyte numbers, and aggression in female BALB/c mice.
abstract: The goal of environmental enrichment for laboratory animals is to improve welfare, but some enrichment practices may affect research in unintended ways or even be harmful to the animals themselves. We previously found that mice raised at a commercial vendor then given multiple enrichment  devices upon arrival at our facilities experienced thymic atrophy and greater variation in measured parameters than did their unenriched counterparts, suggesting that enrichment conditions affected corticosteroid expression in mice. The current study verified and expanded these results, examining  120 female BALB/c mice raised with or without nesting material at a commercial vendor (n = 60 per group) and allocated (n = 20 per group) to receive no enrichment, nesting material, or 'superenrichment' on arrival at our facilities. Nesting material provided prior to weaning  was associated with higher levels of urinary corticosteroid, whereas superenrichment and nesting material during the adult period both led to increased thymic atrophy. Paradoxically, mice that never received enrichment, despite having the lowest corticosterone levels and least thymic atrophy,  had increased tail wounds resulting from aggressive interactions. Therefore, enrichment devices that are as seemingly innocuous as nesting material, even if only provided in the preweaning period, may lead to significant, lasting changes in behavioral, physical, or immunologic measures with  the potential to alter research outcomes.
authors.affiliation: Department of Molecular and Comparative Pathobiology, Johns Hopkins University School of Medicine, Baltimore, MD, USA. ehutchi8@jhmi.edu
authors.forename: Eric K
authors.initials: EK
authors.lastname: Hutchinson
authors.forename: Anne C
authors.initials: AC
authors.lastname: Avery
authors.forename: Sue
authors.initials: S
authors.lastname: Vandewoude
mesh_terms: D000374:Aggression / Q000502:physiology*; D000704:Analysis of Variance; D000822:Animal Husbandry / Q000379:methods*; D000827:Animal Welfare; D000818:Animals; D001835:Body Weight; D003345:Corticosterone / Q000652:urine*; D004777:Environment; D005260:Female; D051379:Mice; D008807:Mice, Inbred BALB C / Q000502:physiology* / Q000652:urine; D013239:Stereotyped Behavior / Q000502:physiology; D060168:Thymocytes / Q000166:cytology*
chemical_list: D003345:Corticosterone