Run Far, Gain Weight, Repeat: Seasonal Cycles in Locomotor Activity and Body Mass of Laboratory Mice

Abstract

Seasonal rhythms are widely documented in wild organisms across diverse species. Seasonality in laboratory rodents, however, remains an intriguing phenomenon, considering that laboratory conditions are supposed to be constant. Here, I study seasonality in locomotor activity and body mass measurements taken as part of a long-term artificial selection experiment involving ~36,002 house mice (Hsd:ICR) spanning 78 generations across 23 years. The selection experiment consists of four control (C) lines, and four replicates of selected (S) lines bred specifically for high voluntary wheel running. Continuous wavelet transforms (CWT) revealed strong circannual rhythmicity with inverted 12-month cycles in both wheel-running distance and body mass, although amplitude declined in the last 5 years. I applied generalized additive mixed models to test group differences between sex, line type (S vs C), and laboratory location (University of Wisconsin–Madison, where the experiment started and ran through 31 generations, versus University of California, Riverside) in running distance, duration, speed, and body mass (measured before and after 6 days of wheel running). Running distance showed greater amplitude in S than C lines and in males than females. Running duration and body mass cycles were stronger in Wisconsin than Riverside, again with males showing higher amplitudes than females. For running speed, S lines displayed more seasonality than C lines. These results unambiguously demonstrate seasonal cycles in behaviour and morphology under controlled laboratory conditions, without, however, being able to provide a definitive explanation (i.e., endogenous circannual mechanisms maintained across generations versus subtle environmental cues). This thesis contributes to the understanding of seasonal cycles and stresses the importance of considering them to improve reproducibility and reliability of research using laboratory mice.