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Age-related changes in plasma oestrogen concentration, behavioural responsiveness to oestrogen, and reproductive success in female gray-tailed voles, Microtus canicaudus

Summary. Age-related increases in the incidence of vaginal cornification were associated with increases in the reproductive success of female gray-tailed voles previously isolated from males. The pregnancy rate of females first paired with males at 30–50 days of age was significantly lower than that of females first paired at 90–120 or 150–200 days of age. The improvement was due to increases in propensities to display receptive behaviours and decreases in the incidence of sterile matings. Although plasma oestrogen concentrations increased with age and were higher in receptive than unreceptive females, plasma oestrogen values alone did not account completely for differences in receptive behaviours amongst females of different ages. Females ovariectomized at 30–50 days of age rarely displayed receptive behaviours when treated with large doses of oestradiol benzoate, whereas nearly all females similarly treated were receptive if ovariectomies were performed between 150 and 200 days of age. The receptivity rate of females ovariectomized between 90 and 150 days was intermediate between the rates of the other two groups.