Effects of Environmental Enrichment on Males of a Docile Inbred Strain of Mice

Environmental enrichment is intended to improve the welfare of laboratory animals. However, regarding male mice, numerous studies indicate an increase in aggressive behavior due to cage structuring. On the one hand, this might be a problem concerning animal welfare. On the other hand, enrichment is though to hamper environmental standardization and to increase variability of data. Furthermore, increasing fights, arousal, and/or injury in enriched housed animals might superimpose other (positive) environmental effects on behavior and physiology. Therefore, the present study investigated effects of environmental enrichment on behavioral, endocrinological, and immunological parameters in male mice of the docile inbred strain ABG. From weaning until day 77+/-3 of life, animals were kept in stable sibling groups of four under three different housing conditions: (A) nonstructured Makrolon type III laboratory cages ("standard housing"=S); (B) equivalent laboratory cages that were enriched with a box and scaffolding ("enriched housing"=E); and (C) spacious terrariums that were structured richly ("super-enriched housing"=SE). No differences in agonistic behavior, levels of plasma corticosterone (CORT), and activities of adrenal tyrosine hydroxylase (TH) existed among S-, E-, and SE-housed ABG males. Play behavior and general activity increased significantly with increasing enrichment. Concerning immunological parameters, males of both forms of enriched housing showed significantly lower percentages of CD4 and CD8 cells compared to S-housed mice. However, regarding the ratio of CD4/CD8 cells, IL-2, IL-4, IL-10, IFN-gamma, IgG1, and IgG2a, no significant housing-dependent differences were found. Enrichment did neither hamper standardization nor negatively influence the variability of physiological parameters. In summary, using a docile strain of mice revealed the positive effects of environmental enrichment also on male mice. The lack of adverse effects on behavior, physiology, standardization, and variability of data defuses these arguments against providing docile male mice with enrichment.