The Contribution of Environmental Enrichment to Phenotypic Variation in Mice and Rats

Abstract

The reproducibility and translation of neuroscience research is assumed to be undermined by introducing environmental complexity and heterogeneity. Rearing laboratory animals with minimal (if any) environmental stimulation is thought to control for biological variability but may not adequately test the robustness of our animal models. Standard laboratory housing is associated with reduced demonstrations of species typical behaviors and changes in neurophysiology that may impact the translation of research results. Modest increases in environmental enrichment (EE) mitigate against insults used to induce animal models of disease, directly calling into question the translatability of our work. This may in part underlie the disconnect between preclinical and clinical research findings. Enhancing environmental stimulation for our model organisms promotes ethological natural behaviors but may simultaneously increase phenotypic trait variability. To test this assumption, we conducted a systematic review and evaluated coefficients of variation (CVs) between EE and standard housed mice and rats. Given findings of suboptimal reporting of animal laboratory housing conditions, we also developed a methodological reporting table for enrichment use in neuroscience research. Our data show that animals housed in EE were not more variable than those in standard housing. Therefore, environmental heterogeneity introduced into the laboratory, in the form of enrichment, does not compromise data integrity. Overall, human life is complicated, and by embracing such nuanced complexity into our laboratories, we may paradoxically improve on the rigor and reproducibility of our research.

Keywords: animal welfare; coefficient of variation; environmental heterogeneity; phenotypic variability; sex differences; translation.

Figure 1.

Prisma flow diagram.

Figure 2.

Descriptive analysis of common EE use in research. a, Picture of a classic EE cage set-up for rodents. b, Proportion per 100,000 citations of PubMed articles returned when searching “environmental enrichment.” Graph depicts articles published between 1998 and 2019. It includes both primary and secondary sources (an update from Simpson and Kelly, 2011). Graphs depict the (c) general species and settings and (d) primary sex studied using EE between January 2013 and September 2018. The selected articles used in this study are primarily from the areas of neuroscience and animal welfare (Extended Data Figs. 2-1, 2-2).

Figure 3.

Descriptive analysis of common EE methodology. All descriptive data are from rat and mice studies where animals are housed in a classic EE design. Data outline the (a) frequency of types of EE devices used, in addition to the percentage of EE studies using (b) running wheels, or a particular (c) age of EE onset, (d) duration of EE housing, and general/social structure of (e) EE and (f) control groups. Data derived from a total of 681 research articles published between January 2013 and September 2018.

Figure 4.

“Coefficients of variation” for all studies where control and EE mice or rats were directly compared. All data presented as the overall trait variance and further separated into subcategories of behavior and physiology as well as seven specific trait measures for (a, c) naive/untreated and (b, d) treated/manipulated animals (mean ± SEM). Each data point represents a single control or EE measure from a single experiment along with the mean for each respective trait. Coefficient of variance was calculated as the standard deviation divided by the mean for each data point. e, Histogram of distribution of CV ratios (EE CV/EE CV + control CV) collapsed across naive and treated/manipulated mice and rats. To determine whether the variance from the mean was normally distributed for the different traits, we evaluated the CV ratios (p values from Extended Data Figs. 4-1, 4-2, 4-3, 4-4, 4-5, 4-6, 4-7, 4-8, 4-9, 4-10, 4-11, 4-12, 4-13, 4-14, 4-15, 4-16, 4-17, 4-18, 4-19, 4-20, 4-21, 4-22, 4-23, 4-24, 4-25, 4-26). A value of 0.5 (black dotted line) indicates that EE and control animals are similar. Values to the right suggest that EE animals are more variable than controls.