It is generally appropriate to house naturally gregarious animals in compatible social groups unless there are scientific or welfare reasons not to do so (National Health and Medical Research Council 2004; Canadian Council on Animal Care 1993; Council of Europe 2006; NRC 1996). Social housing can activate stress responses involving the HPA axis in rats, but when a wider range of measures is taken into account, overall, social housing is neither stressful nor harmful (Hurst et al. 1997, 1998). For example, even macaques fitted with a cranial implant could be paired with another compatible macaque without it inflicting damage to the device or interfering with the research goals (Roberts and Platt 2005). Furthermore, a considerable body of evidence indicates that housing naturally sociable animals (e.g., rats, mice, dogs, primates) in solitary conditions can result in stress and harm (Baker 1996; Eaton et al. 1994; Hetts 1991; Hubrecht 1995; Novak 2003; Patterson-Kane 2002; Sharp and Lawson 2003;Van Loo et al. 2004). Even cats, which are not particularly gregarious, can benefit from). social housing (Council of Europe 2006). It is therefore important to provide thorough scientific rationale for solitary housing.

Disruption of established social groups, pairing (for additional information see Appendix), or the introduction of animals to larger preformed units are all potential causes of aggression or stress. As a husbandry refinement, therefore, social groups should be established early, and disruption of established groups should be minimal, as demonstrated in studies of mice, rabbits, and cats (Bradshaw and Hall 1999; Jennings et al. 1998; Morton et al. 1993; Sharp et al. 2002b). Close cooperation with the supplier or breeder may be necessary to promote group formation and ensure minimal disruption of group dynamics. Adequate socialization to both humans and conspecifics at an early age may also help prevent subsequent stress and distress (Council of Europe 2006).

Animals housed in social groups generally need adequate space as well as objects in their enclosure to allow them to modulate their social interactions. However, some structures can actually trigger aggression, as shown in certain strains of male mice (Haemisch and Gartner 1994). Because competition for resources often triggers aggression, the provision of sufficient or separate feeding devices for some species (e.g., dogs, cats, pigs) can help minimize the risk of fights during feeding. For other species, such as mice and marmosets, that regulate social interactions through olfactory markings, appropriate cage changing and cleaning routines can minimize social disruption. For example, decreasing the frequency of cage cleaning or leaving some older bedding can help maintain tolerance between familiar male mice (Hurst et al. 1993) and transferring nesting material between cages can positively influence several stress-related physiological

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