Roaming in a Land of Milk and Honey: Life Trajectories and Metabolic Rate of Female Inbred Mice Living in a Semi Naturalistic Environment

SIMPLE SUMMARY: Different forms of environmental enrichment are used to increase the wellbeing of laboratory animals. These forms include extending the available cage space, housing a large group of animals within the same unit and adding stimulating physical objects. The semi naturalistic environment (SNE) used in this study implements all of these enhancements. However, there is debate as to whether such variation in housing standards increases the variability of experimental data. Indeed, it has been shown that mice living in the SNE developed individual differences in activity and behavioral parameters. Here, we investigated whether housing in the SNE enhances individual differences in aged animals and whether these differences are reflected in certain physiological parameters. These aspects were considered to assess the suitability of the SNE as a reference system in future studies. We found that the individual-level activity patterns of the animals stabilized during the housing period in the SNE. These behavioral characteristics did not correlate with the measured physiological parameters. Considering the variance of the measured data, which is comparable to the literature, the SNE seems to be a suitable system for studies comparing different housing systems in terms of animal welfare. ABSTRACT: Despite tremendous efforts at standardization, the results of scientific studies can vary greatly, especially when considering animal research. It is important to emphasize that consistent different personality-like traits emerge and accumulate over time in laboratory mice despite genetic and environmental standardization. To understand to what extent variability can unfold over time, we conducted a long-term study using inbred mice living in an exceptionally complex environment comprising an area of 4.6 m(2) spread over five levels. In this semi-naturalistic environment (SNE) the activity and spatial distribution of 20 female C57Bl/6J was recorded by radio-frequency identification (RFID). All individuals were monitored from an age of 11 months to 22 months and their individual pattern of spatial movement in time is described as roaming entropy. Overall, we detected an increase of diversification in roaming behavior over time with stabilizing activity patterns at the individual level. However, spontaneous behavior of the animals as well as physiological parameters did not correlate with cumulative roaming entropy. Moreover, the amount of variability did not exceed the literature data derived from mice living in restricted conventional laboratory conditions. We conclude that even taking quantum leaps towards improving animal welfare does not inevitably mean a setback in terms of data quality.