Strain-specific effects of cage enrichment in male laboratory mice (Mus musculus)

'Environmental enrichment' is often considered to improve captive animal we lfare. However, some studies using male mice, Mus musculus, indicate that i ncreasing cage complexity, increases aggression. Limited evidence suggests that enrichment differs in ifs effects on behaviour and physiology between strains; but behaviour also differs between strains in non-enriched environ ments. Differ ences in enrichment type, evaluation methods, and strains use d, have caused difficulty in interpreting the efficacy of environmental enr ichment in improving welfare. Using enrichment suitable for commercial labo ratories (nesting material and a Perspex tunnel), we physiological response s among males of six strains housed in iron-enriched standard polypropylene cages with those housed in 'enriched' cages. Outbred ICR(CD-1) and TO mice , and inbred BALB/c mice were more aggressive than C57BL/6, CBA/Ca and DBA/ 2 mice, which exhibited low levels of aggression typical of most inbred str ains. Enrichment did not significantly affect aggression levels. Animals in enriched cages spent more time investigating the internal cage environment , eating and drinking, and in stereotypic behaviour patterns, although leve ls differed between strains. The greatest increase in stereotypy levels (ba r-related stereotypies,pies) with enrichment was found in DBA/2 mice. Highe r testosterone levels were maintained over the study, period ill mice house d in enriched cages, and in more aggressive strains. IgG levels were also h igher in mice housed in enriched cages, and in the outbred strains ICR(CD-1 ) and TO compared with inbred strains. The relationship between aggression, testosterone and 'enrichment' suggests, that increasing complexity in labo ratory cages may increase a naturally, selected territorial response in som e strains. The implications for strain-specific welfare are discussed.