Nip, E., Adcock, A., Nazal, B. et al. 2019. Why are enriched mice nice? Investigating how environmental enrichment reduces agonism in female C57BL/6, DBA/2, and BALB/c mice. Applied Animal Behaviour Science 217, 73-82.

Compared to female mice raised in large, environmentally enriched (EE) cages, those from “shoebox” non-enriched (NE) cages demonstrate more stereotypic behaviour (SB) and depressive-like effects (i.e. learned helplessness; and inactive-but-awake behaviour [IBA], where a mouse simply stands still). Past research showed that NE females also display more agonistic behaviour (i.e. conflict/aggression) but why this occurs was unknown. We sought to replicate this phenomenon, and to explain it by testing several hypotheses: that NE mice are more agonistic than EE mice because 1) they have higher social interaction rates due to having less space and fewer things to do; 2) NE mice with high levels of SB are behaviourally inflexible and thus socially impaired (and additionally may be at risk of receiving agonism for this same reason); 3) NE mice who display depressive-like effects perceive others as threatening. Via home-cage scan-sampling, we observed behaviours (notably SB, IBA, social interaction rate and agonism) in 165 females (99 NE, 66 EE) of three co-housed strains (C57BL/6 s, BALB/cs, and DBA/2 s; 99 NE) over a 7 month period. EE mice were housed in 60 x 60 cm cages with running wheels, various nesting materials, climbing structures, and chewable items; while NE mice were housed in standard shoebox cages with nesting materials only. At 16 months of age, mice underwent Forced Swim Tests, in which they were placed in individual beakers of water and the duration of floating was measured (versus active swimming), since longer float times suggest higher learned helplessness. We successfully replicated past research: all variables (SB, IBA, social interaction rates, FST float times, and agonism) were significantly higher in NE mice than EE mice (P < 0.0001). Furthermore, NE mice were still more agonistic than EE mice after controlling for their higher social interaction rate (P < 0.0001). Thus, while social activity partially explained the housing effect on agonism, it could not fully explain it, suggesting that there may be intrinsic factors involved. However, we found no relationships between agonism and SB, IBA, and FST float times. Overall, this is the fourth study to show that NE mice are more agonistic than EE mice - further demonstrating the welfare benefits of providing enrichment. This is also the first study to examine potential explanations behind high agonism in NE subjects. We end by suggesting further hypotheses for future test in mice and other species.