Evidence of pregnancy failure in the wild meadow vole, Microtus pennsylvanicus

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Abstract
Normal and blocked pregnancy were studied in the meadow vole M. pennsylvanicus (Ord) in the laboratory, on individuals bred from wild stock caught near Sudbury, Ontario [Canada]. The results were compared with those obtained from wild voles trapped from quadrats during the summers of 1969 and 1970. Criteria were found for distinguishing between individuals in which pregnancy had been blocked and those in which it had not, under laboratory conditions. Nursing females were not found to be susceptible to male-induced blockage and the 2nd set of corpora lutea in the ovaries was not observed to increase the rate of involution of the 1st set. Females were found to be susceptible to blockage on the 2nd and 5th days after coitus. Some indication of pregnancy failure in the wild was found and the incidence of this phenomonon appears to increase in populations with a higher density. Pregnancy failure may be one of the density-dependent factors that decreases reproduction and ultimately the recruitment of young animals to the population.