Ontogeny of a circadian rhythm in body temperature in newborn lambs reared independently of maternal time cues.

To investigate the ontogeny of a circadian rhythm in body temperature, we performed 24-h temperature measurements at six postnatal ages in ten newborn lambs reared independently of their ewes. At 18 to 48 h of age, a significant time-of-day variation in body temperature was found, the variation achieving a peak between 5.00 pm and 6.00 pm. Over the next two weeks, this time-of-day variation disappeared, reappearing at 18 days of age, although the phase had shifted such that the peak temperature was achieved between 8.00 am and 9.00 am. At 26 days of age, the time-of-day variation in body temperature was still present, maintaining the same phase relationship as at 18 days of age. It is possible that the significant time-of-day variation observed in the two oldest groups is due to the emergence of a discernable circadian rhythm in body temperature entrained to the light-dark cycle during the second or third week of life, whereas the time-of-day variation observed in the 18 to 48-h old is due to residual effects of entrainment to maternal zeitgebers. We speculate that the lack of any observable time-of-day variation in the 4, 6 and 11 days-old lambs may denote the presence of no rhythm or possibly a free-running rhythm in body temperature in subjects not yet able to entrain to photic zeitgebers.