Ontogeny of the rabbit's circadian rhythms without an external zeitgeber.

We have monitored parturition and different behavioral rhythms of does, the activity of suckling pups and behavioral rhythms of rabbits after weaning until adulthood. All animals were living in continuous light conditions (LL). As a consequence of the free-run of the does parturition, which takes place during the resting period of the animal, occurred at almost any time of the day. The once/day nursing was advanced during the first 7-10 days until it was in synchrony with the time of main activity. The pups significantly anticipated nursing by day 5: the 60 min bin prior to nursing exceeded the 24-h mean by 63 +/- 30%. The anticipatory activity (AA) was entrained by an artificial nursing schedule of T = 24 h, the 60 min bin prior to nursing exceeding the 24-h mean by 123 +/- 50%. Since AA persisted during a 48 h fast, its phase is controlled endogenously. At weaning behavioral rhythms of rabbits free-ran with a main tau < 24 h (average 23:35 +/- 17'), period length extending with increasing age. 7/10 animals had an additional component > 24 h (average: 24:24 +/- 16') which became increasingly dominant, the components < 24 h fading out within 30 days after weaning. Between days 340-350 a stable period length of 24:35 +/- 8' and a consolidation of the circadian rhythms was attained. These results confirm that during the first days of the rabbit's life the circadian system is controlled by a feeding entrainable oscillator, the light entrainable oscillators taking over control later.