Relationship of circadian temperature and activity rhythms in two rodent species

Despite the large number of publications concerning circadian body temperature in rodents, relatively little is known about phase relationships of the body temperature cycle relative to daily locomotor rhythms. Phase angle differences between core body temperature and locomotor activity were determined by a group of overlapping methods for 14 hamsters and 11 chipmunks under constant light (LL) and entrained (LD) photo schedules. The study compared the onset of the body temperature cycle with three separate components of daily locomotor activity: the onset of wheel running, the onset of total activity in a standard wheel cage, and the onset of total activity in a restricted cage, respectively. The temperature cycle showed a pronounced phase lead over onset of wheel-running activity in all hamsters and in the majority of chipmunks, but the phase lead was reduced when total activity was considered. With activity restriction, the onset of temperature rise and activity onset concurred. The body temperature onset in some species may serve as a valid phase point for indirect measurement of circadian timing, but the potential artifact of masking by heat generated from muscular activity should be kept clearly in mind.