by Lucía Améndola Saavedra, Postdoctoral Research Fellow, University of British Columbia
Standard barren housing systems for laboratory mice restrict the expression of natural behaviors and are associated with a high incidence of abnormal behavior. There is abundant evidence that environmental enrichment has a positive impact on affective states in mice, improving their welfare. However, there is considerable methodological variation between studies, such that simple conclusions on how to refine housing conditions are difficult to make. The aim of this systematic review, which was funded by an AWI Refinement Grant, was to identify the environmental characteristics that most consistently appear to improve affective states in mice.
Performing a literature search in two databases (MEDLINE and Web of Science), we identified studies on environmental enrichment in mice that assessed anxiety using three standard tests: the elevated plus maze, the open field test, and the light-dark box. After excluding studies that were not relevant (e.g., ones that considered the addition of nesting or bedding materials to be “enrichment”), we retained 25 studies for further assessment. Of these, 16 tested mice in the elevated plus maze. (The elevated plus maze involves an elevated four-way track in the shape of a plus symbol. Two of the arms are enclosed within walls, while two are exposed. A willingness to venture onto the exposed arms is viewed as an indicator of lower anxiety in mice.) Within these 16 studies, we examined characteristics of the study design, the animals, and the intervention and control conditions.
Thirteen of the studies showed a small to large positive effect of environmental enrichment (more time spent in the open arms: 60 percent of studies; more entries into the open arms: 90 percent of studies). Two studies showed a medium to large negative effect of environmental enrichment on anxiety. These studies, however, were carried out within the same research group and under a reversed light schedule, limiting our ability to compare the studies with others we reviewed.
All of the 11 studies showing large positive effects in the presence of environmental enrichment were carried out on male mice. Among these, 60 percent contained a running wheel, 90 percent contained some form of shelter (e.g., tubes, tunnels, boxes, boards connected at an angle, igloos, or cage shelves), and 80 percent contained other objects (e.g., ladders, plastic or wooden toys). In 70 percent of the studies with large positive effects, mice were introduced into enriched cages at 21 to 30 days of age, and in 60 percent, mice remained in the enriched cages between 30 and 60 days. Cage size was highly variable, providing between zero and 10 times more space than standard conditions. Only one used two-level cages with increased spatial complexity; the rest consisted of single-level cages.
The elevated plus maze data indicated that improvements to standard laboratory mouse housing ameliorate the negative effects of barren environments on mouse anxiety. We conclude that housing mice with additional shelters and other objects of interest, as well as beginning enrichment in the early stages of development, is beneficial to mouse welfare.