The estrous cycle of captive woodchucks (Marmota monax).

Changes in vaginal cytology were assessed and correlated with temporal changes in circulating concentrations of progesterone (P) and estradio!-17� (E,) during the breeding season (February to March) in a seasonally breeding rodent, the woodchuck (Marmota monax). Ten individually caged adult females, maintained under laboratory conditions for 3-11 mo, were studied. Vaginal smears were taken each morning for 2 consecutive months beginning 1 February 1990. Seven of 10 females exhibited readily identifiable estrus, characterized by a clear predominance (83%) of cornifled cells. The earliest estrous smear was recorded on 3 February and the latest on 12 March. These animals were monoestrous and remained in a prolonged estrous period during their brief breeding season. The average duration of estrus was 18.1 ± 2.1 days, ranging from 12-27 days. Levels of P and E2 were determined in serum samples taken before, during, and after estrus from 7 females who exhibited estrus. No changes in the circulating levels of P were apparent during the estrous cycle. However, there was a consistent pattern of estradlol secretion characterized by elevated levelsof E, before and during estrus, followed by a significant (p < 0.05) decline in E2 levels one week after the end of estrus. Elevated levels of E, preceded and coincided with maximal degree of vaginal cornification. Thus, the termination, but not the onset, of estrus in woodchucks reflected closely the temporal pattern of changes in serum E, levels during the breeding season.