Postimplantation pregnancy disruptions in meadow voles: relationship to variation in male sexual and aggressive behavior.

Previous research on Microtines indicates that the presence of new males may more effectively produce pregnancy disruptions than do pheromones alone. If the male's presence is important, behavioral differences among males may be related to the occurrence of disruptions. We observed female meadow voles (Microtus pennsylvanicus) interacting with new males twelve days after they had been paired with stud males. Behavioral interactions were recorded for one hour on each of Days 12-14. Timing of parturition and weight gain by Day 12 were used to assess whether a disruption had occurred. Females with disrupted pregnancies were frequently observed mating within an hour of being placed with the new male, whereas females that retained their original pregnancies rarely copulated with the new male. When pregnancies were disrupted, female-new male pairs fought more, but also engaged in more nonaggressive contact than when pregnancies were retained. Pup survival and male attendance of pups were lower when females retained litters, suggesting that females could successfully rear more pups if the original pregnancy was reabsorbed. However, relatively aggressive females paired with nonaggressive new males protected retained litters from new males. New males were tested for aggressive and pup care responses to an unrelated pup on the day before they were placed with the females. Males aggressive to unrelated pups in these tests were also more aggressive to females on Day 12. These data suggest that male aggressiveness may signal females when it would be advantageous to disrupt pregnancy.