A Biological Clock Controlling the Estrous Cycle of the Hamster1

Onset of estrus was observed every 4 days in the same hamsters exposed either to 24-hr light:dark cycles (LD 16:8) or to constant light (LL). Regression analysis indicated that only LL hamsters maintained estrous cycle periods significantly greater than 96 hr (p >0.05). These “circa-quadridian” (CQ) rhythms indicate that an endogenous timing mechanism capable of entrainment by 24-hr LD cycles (the Zeitgeber) controls the estrous cycle. Pituitary gonadotropin release and ovulation maintained constant time relationships to onset of estrus in both LD and LL. Pregnancy and pseudopregnancy reset the CQ rhythm to an earlier hr. However, spontaneous extensions from a 4- to 5-day estrous cycle followed by a day of vaginal cornification did not cause a phase shift in the CQ rhythm. This indicates that the endogenous timer functions with circadian rather than CQ rhythmicity. In LL, periods of the endogenous circadian timer ranged from 23.84 to 24.39 hr. Most values are significantly greater than 24 hr (p > 0.05). In LD, timer periods ranged from 23.96 to 24.03 hr. No value is significantly greater than 24 hr. (Endocrinology 88: 1368, 1971)