Allali KE, Achaâban MR, Bothorel B, Piro M, Bouâouda H, Allouchi ME, Ouassat M, Malan A, Pévet P. Entrainment of the circadian clock by daily ambient temperature cycles in the camel (Camelus dromedarius). Am J Physiol Regul Integr Comp Physiol 304: R1044 -R1052, 2013. First published March 13, 2013 doi:10.1152/ajpregu.00466.2012.-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 T a cycle, proving that the daily T a 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 T a cycle in only 2 (or 3) of 7 camels: in these specific conditions a systematic effect of T a on the clock could not be evidenced. In conclusion, depending on the experimental conditions (DD vs. LD), the daily T a cycle can either act as a zeitgeber or not.camel; circadian clock; body temperature; daily ambient temperature; melatonin; nonphotic entrainment RHYTHMICITY in physiological processes is a fundamental property of all living organisms (32). A number of biological functions display daily and seasonal variations in a way to anticipate and adapt to the upcoming cycling changes in environment (light, temperature, food availability, etc.). In mammals, the circadian clock, located in the suprachiasmatic nuclei of the hypothalamus (SCN), is central for these adaptive processes. This clock is a strong autonomous oscillator cycling with a period close to 24 h under constant conditions (23) and entrained by environmental cues to an exact period of 24 h. Thus SCN play a pivotal role to control numerous circadian biological rhythms such as those of body temperature (T b ), melatonin, or behavioral features. In all mammals studied, the light-dark cycle is the most powerful synchronizer (zeitgeber) of the master clock (for a review see Ref. 15). T b rhythm represents a robust output of the clock, widely used in clinical research to determine pro...