Daily activity and body temperature rhythms do not change simultaneously with age in laboratory mice.

Daily rhythms of locomotor activity (AR) and body temperature (TR) were investigated in juvenile, adult, and senile female laboratory mice (5, 16, or 65 weeks old). All daily patterns were bimodal, with a main maximum in the dark and a secondary one immediately following lights on. The juvenile mice showed the highest magnitude of oscillation of the AR but the lowest magnitude of the TR; the magnitudes of the TR of adult and senile animals were not different, whereas those of AR in senile mice approached zero. For the AR, but not the TR, a phase advance with age was observed. The effect of locomotor activity on the body temperature was higher during the light time (minimum of motor activity) than during the dark time (maximum activity), and was least in juvenile mice. The calculated daily temperatures corresponding to zero activity gave rhythms that showed no age-dependent differences in daily mean or magnitude. This implies that the age-dependent changes of the TR were due mainly to masking effects.