In mammals the light-dark (LD) cycle is known to be the major cue to synchronize the circadian clock. In arid and desert areas, the camel (Camelus dromedarius) is exposed to extreme environmental conditions. Since wide oscillations of ambient temperature (Ta) are a major factor in this environment, we wondered whether cyclic Ta fluctuations might contribute to synchronization of circadian rhythms. The rhythm of body temperature (Tb) was selected as output of the circadian clock. After having verified that Tb is synchronized by the LD and free runs in continuous darkness (DD), we submitted the animals to daily cycles of Ta in LL and in DD. In both cases, the Tb rhythm was entrained to the cycle of Ta. On a 12-h phase shift of the Ta cycle, the mean phase shift of the Tb cycle ranged from a few hours in LD (1 h by cosinor, 4 h from curve peaks) to 7-8 h in LL and 12 h in DD. These results may reflect either true synchronization of the central clock by Ta daily cycles or possibly a passive effect of Ta on Tb. To resolve the ambiguity, melatonin rhythmicity was used as another output of the clock. In DD melatonin rhythms were also entrained by the Ta cycle, proving that the daily Ta cycle is able to entrain the circadian clock of the camel similar to photoperiod. By contrast, in the presence of a LD cycle the rhythm of melatonin was modified by the Ta cycle in only 2 (or 3) of 7 camels: in these specific conditions a systematic effect of Ta on the clock could not be evidenced. In conclusion, depending on the experimental conditions (DD vs. LD), the daily Ta cycle can either act as a zeitgeber or not.
Keywords: body temperature; camel; circadian clock; daily ambient temperature; melatonin; nonphotic entrainment.