Winnicker, C. 2013. Challenges and opportunities of implementing EU standards for social housing of rodents. Enrichment Record 15, 3-4.

The EU Directive on the protection of animals used for scientific purposes specifically states: Animals, except those which are naturally solitary, shall be socially housed in stable groups of compatible individuals. Even in cases where single housing is allowed, the duration shall be limited to the minimum period necessary and visual, auditory, olfactory and/or tactile contact shall be maintained. This is a more stringent stand than previously, where rats and mice should be group-housed. Effectively, the new Directive requires social housing for social species as a default condition. ... There are a couple of excellent caveats that are specifically stated in the European guidelines regarding space allowances. One is that in long-term studies, if space allowances per individual animal fall below those indicated, priority shall be given to maintaining stable social structures. In other words, rather than break up the socially stable group because the animals have outgrown their space allowance, the physiologic and psychological benefits of remaining in an established, stable social group outweigh engineering standards. This appropriately acknowledges that space allocation is not as important as the influence of a stable social group on the animal. The second caveat is that weaned stock can be maintained at higher housing densities for the short period between weaning and sale, provided that the animals are housed in larger enclosures with adequate enrichment, and these housing conditions do not cause any welfare deficit such as increased levels of aggression, morbidity or mortality, stereotypes and other behavioral deficits, weight loss, or other physiological or behavioral stress responses.