A Novel Postcopulatory Block of Reproduction in White-Footed Mice1

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Abstract
Female white-footed mice (Peromyscus leucopus) fail to produce offspring when paired with a male from weaning until 150 days of age if an adult female or her odor is also present. The present study delineates more clearly which stage of the young female''s reproductive cycle is inhibited by the chemosignal of the older female. Age at vaginal opening and first estrus are delayed by the presence of the older female, but only for about 10 days. The presence of a male counteracts this delay. Thus, the basis for the block is not a delay in puberty. The older female''s presence does not influence the number of estrous cycles experienced during the 30 days following first estrus, nor does it influence the number of corpora luteal found at autopsy. The presence of sperm in the vaginae of young females indicates that they were copulating. Likewise, examination of embyros 2 and 3 days after copulation reveals normal developmental progress. However, implantation does not occur in young females that have been exposed to an adult female. Thus, the block occurs either during the final stages of embryo transport or in relation to the implantation process itself.