We conducted field and laboratory experiments with the well-studied monogamous prairie vole, Microtus ochrogaster, to distinguish among three hypotheses for the failure of females that lose their mates to bond with a new male ("the widow effect"). The reproductive value hypothesis predicts that males prefer young to older females because they potentially have a longer reproductive lifespan. The mate rejection hypothesis predicts that females will prevent repairing by aggressively deterring males that might harm their current offspring. The misdirected paternal care hypothesis assumes that females will mate during postpartum estrus and thus will be pregnant and/or nursing young throughout the breeding season; males will avoid pairing with older females to avoid providing care to unrelated offspring and/or because of a delay to the next breeding opportunity. Males associated and bred more with older than young females, allowing us to reject the reproductive value hypothesis. Our results were consistent with the male rejection hypothesis in that females were aggressive toward unfamiliar males. Our results were most consistent with the misdirected paternal care hypothesis in that once females started breeding, they continued to become pregnant and nurse young throughout the study period. Thus, our findings suggest that the potential of misdirected paternal care and delayed mating opportunity in conjunction with the aggressive behavior of females toward unfamiliar males are likely explanations for the lack of repairing for widow females.