Methods
Female C57/bl mice were used in a parallel group study where the effects of environmental enrichment as well as single or group housing were investigated. The 24 mice had a body weight of 16 - 17 g at arrival and were after one week randomly placed single in Macrolon type I cages (8 mice) or in groups of 4 mice in Macrolon type IV cages (16 mice). Half of the cages were provided with objects and nesting material. Water and food intake, body weight , and the ratio of urine corticosterone/creatine were measured weekly. Urine was collected by the method described by Dahlborn et al, 1996. The mice were subjected to two behavioural tests (aluminium foil test, petri dish test) several times during the study.

Results
Single housed mice had a higher food and water intake (p<0.01) than group housed mice and their body weight was higher (p<0.05). The only difference found between enriched and standard cages was a lower water intake in single housed enriched mice. Urine corticosterone varied both within the treatments and within the individual mouse. The only over all significant effect found in single housed mice was the ratio of urine corticosterone/creatine, which was lower in enriched compared to standard housing. When the mice were placed in the petri dish, there was not a time difference in their leaving the dish. However, the enriched mice were faster in re-entering into the petri dish, this was also true for the group housed compared to single housed animals.

Conclusions

In this study more differences were found between single and group housed mice compared to mice kept in enriched or standard cages. The behavioural tests showed that the enriched mice were more alert than mice from standard cages. The overall effect of this study showed that the housing conditions will influence the experimental model and the welfare of the mouse.