Oestrus, copulation, and related aspects of reproduction in female eastern chipmunks, Tamias striatus (Rodentia: Sciuridae)

Weekly livetrapping in a southeastern Ontario woodlot throughout 2 years of aboveground activity by Tamias striatus lysteri, plus supplementary data, provided information on changes in the external genitalia of female chipmunks before, during, and alter oestrus. The main stages are described and tentatively named. Oestrus was marked by a deep pink, greatly distended vulva and eventually by an elongated aperture. Our finding that oestrus lasted at least 3 days and up to 10 days is new. Post-oestrus, in which the vulva was less distended, persisted throughout half or most of gestation. There was much variability among individuals in the duration of oestrus and related stages. Copulatory behavior is described. Most observations of oestrus and copulation in Canada relate to the summer breeding season.In our local population, breeding occurred in both spring and summer, 1972, but only in spring, 1973. There was considerable variability among individual females of breeding age regarding participation in one or both breeding seasons in 1972. Among females breeding in the summer of 1972, those that had bred in the spring came into oestrus near the time when their spring young emerged from their burrows, and later than those that had not bred in the spring.