A Block to Pregnancy in the Mouse Caused by Proximity of Strange Males

Summary. Pregnancy and pseudopregnancy both failed in a high proportion of mice when the recently mated female was housed with or near strange males, particularly males of a different strain, or when she was housed with a castrated male. In these circumstances, the female returned to oestrus 4 to 5 days after the original mating and implantation did not take place. The use of genetically marked test males of a different strain from the stud males showed that superfoetation did not occur. By contrast, pregnancy was not blocked when the female was returned to her own stud male after being separated from him for 24 hr, or when she was kept in the presence of other females. The evidence suggests that the presence of other females may even help towards stabilizing a pregnancy. The histology of the ovaries from females having blocked pregnancies showed a varying degree of pituitary-gonadotrophin stimulation. The fate of the blastocyst from the first mating remains as yet undetermined.