Effects of Cage Enrichment on Behavior, Welfare and Outcome Variability in Female Mice

Abstract

The manner in which laboratory rodents are housed is driven by economics (minimal use of space and resources), ergonomics (ease of handling and visibility of animals), hygiene, and standardization (reduction of variation). This has resulted in housing conditions that lack sensory and motor stimulation and restrict the expression of species-typical behavior. In mice, such housing conditions have been associated with indicators of impaired welfare, including abnormal repetitive behavior (stereotypies, compulsive behavior), enhanced anxiety and stress reactivity, and thermal stress. However, due to concerns that more complex environmental conditions might increase variation in experimental results, there has been considerable resistance to the implementation of environmental enrichment beyond the provision of nesting material. Here, using 96 C57BL/6 and SWISS female mice, respectively, we systematically varied environmental enrichment across four levels spanning the range of common enrichment strategies: (1) bedding alone; (2) bedding + nesting material; (3) deeper bedding + nesting material + shelter + increased vertical space; and (4) semi-naturalistic conditions, including weekly changes of enrichment items. We studied how these different forms of environmental enrichment affected measures of animal welfare, including home-cage behavior (time-budget and stereotypic behavior), anxiety (open field behavior, elevated plus-maze behavior), growth (food and water intake, body mass), stress physiology (glucocorticoid metabolites in fecal boluses and adrenal mass), brain function (recurrent perseveration in a two-choice guessing task) and emotional valence (judgment bias). Our results highlight the difficulty in making general recommendations across common strains of mice and for selecting enrichment strategies within specific strains. Overall, the greatest benefit was observed in animals housed with the greatest degree of enrichment. Thus, in the super-enriched housing condition, stereotypic behavior, behavioral measures of anxiety, growth and stress physiology varied in a manner consistent with improved animal welfare compared to the other housing conditions with less enrichment. Similar to other studies, we found no evidence, in the measures assessed here, that environmental enrichment increased variation in experimental results.

Keywords: animal welfare; behavioral phenotypes; environmental enrichment; mice; variation.

Figure 1

Experimental timeline for a single batch of animals.

Figure 2

Proportion of intervals across observations (median and IQR) where mice were engaged with the housing environment.

Figure 3

Proportion of intervals across observations (median and IQR) where mice were engaged in stereotypic behavior in relation to housing condition. Note, the y-axis is truncated to 60% from 100% to aid with visual clarity, given the low levels of stereotypic behavior.

Figure 4

Proportion of intervals engaged in active, inactive and unseen behavior in relation to cage enrichment.

Figure 5

Proportion of intervals engaged in different forms of active behavior in relation to housing condition.

Figure 6

Variation in distance traveled and time-in-open arms (median and IQR) in relation to housing condition.

Figure 7

Variation in distance traveled, time-in-center and time-in-corners (mean ± SE) in relation to housing condition and day of testing.

Figure 8

Variation in body weight (mean ± SE) in relation to housing condition and week.

Figure 9

Variation in food and water intake (Mean ± SE) in relation to housing condition and week.

Figure 10

Variation in glucocorticoid metabolite concentrations (median and IQR) in relation to housing condition and time point of measurement.

Figure 11

Variation in adrenal mass (median and IQR) in relation to housing condition.

Figure 12

Variation in perseveration score (median and IQR) in relation to housing condition.

Figure 13

Variation in repetitions and alternations in relation to housing condition (median and IQR). The line represents the expected frequency of choices given a random search strategy.

Figure 14

Fitted regression lines showing go responses in relation to housing condition and strain.

Figure 15

Co-efficient of variation estimates by outcome measure, housing condition and strain.