Variability of wheel running behavior in mice is dependent on housing, sex, and genetic background

Environmental enrichment (EE) has been increasingly used to enhance and support the physiological fitness and natural behavior of research animals. For mice, which are known to be highly active nocturnal animals that live in large social groups, the introduction of running wheels is a particularly promising approach in EE. When wheels are introduced to their home cage, mice increase their nocturnal locomotion activity by orders of magnitude. However, little is known about how mice share such a readily adopted EE. Here, we studied wheel running in single-housed and group-housed mice. We hypothesized that group-housed mice will compete over the scarce resource of a running wheel leading to a change in overall wheel usage compared to single-housed mice that have exclusive wheel access. We measured wheel locomotion in two strains of single-housed and group-housed mice over multiple 24h periods at sub-second temporal resolution using a custom-designed data acquisition system. We observed both sex- and housing-specific differences in wheel usage. In single-housed C57Bl/6 mice, mice ran at consistent speeds and females ran larger distances. In group housing, periods of slow locomotion speeds as well as speed transitions emerged with fewer breaks in wheel usage. This group effect was less pronounced in a genetic mouse model (parvalbumin-Cre on C57Bl/6 background). Our results demonstrate a change in wheel usage during group housing which supports competition over EE resources while enhancing overall locomotion in mice.