Dynamical model of aperiodic locomotor activity effects on mouse core body temperature removes transient perturbations from longitudinal temperature signals

Mammalian temperature changes across time due to multiple endogenous and exogenous factors including circadian rhythms, hormonal changes, and locomotor activity. These multiple factors make it difficult to disentangle each of their effects to understand their independent contributions. This is especially problematic due to the relatively high-amplitude, aperiodic heating effects of locomotor activity on core body temperature. These heating effects, combined with innate cooling effects back to core body temperature steady state, mean that locomotor activity can contribute apparent power to both circadian and ultradian rhythms in observed temperature data. We propose that the effect from locomotor activity to core body temperature is not simply the linear addition of circadian and ultradian oscillations, but rather a heating effect that can be offset by a cooling effect dependent on core temperature displacement from resting temperature. Since these effects appear to contribute power to independent rhythms in spectral analysis, in this work we develop an interpretable, parsimonious mathematical model of murine core body temperature that removes them in the time-domain. The model only depends on the initial observed core body temperature as well as minute-level locomotor activity data, making it robust to aperiodic mouse activity. We show that coefficients obtained after fitting the model to each mouse return physiologically relevant differences between sexes, as well as reflect directional changes within female mice between their non-estrous and estrous temperature data. We believe this work should be of use to researchers interested in how core body temperature dynamics change in response to experimental interventions, especially if locomotor activity may be affected as well.