Effect of Multiple Short-Term Exposures of Pregnant Microtus ochrogaster to Strange Males

Bruce (1959, 1960) discovered that exposure of female laboratory mice to unfamiliar males within 24 h of coitus inhibited implantation and blocked pregnancy. Females experiencing pregnancy failure returned to estrus 4-5 days after the original mating (Bruce, 1960). This pre-implantation pregnancy block, or Bruce effect, was subsequently observed in wild house mice, Mus musculus (Chipman and Fox, 1966); deer mice, Peromyscus maniculatus (Eleftheriou et al., 1962); field voles, Microtus agrestis (Clulow and Clarke, 1968); meadow voles, M. pennsylvanicus (Clulow and Langford, 1971); and prairie voles, M. ochrogaster (Stehn and Richmond, 1975). Post-implantation termination of pregnancy following exposure to strange males has been found in P. maniculatus, M. pennsylvanicus, and M. ochrogaster (Kenney et al., 1977; Stehn and Richmond, 1975). The incidence of pregnancy failure was greater for M. ochrogaster than for the other two species (Kenney et al., 1977). In M. ochrogaster a reduced incidence of pregnancy termination was not apparent until more than 15 days of gestation had elapsed (Stehn and Richmond, 1975). In the above studies occurrence of pregnancy block or termination was determined by separating a pair that had mated and exposing the female to a strange male for prolonged periods ranging from one to several days. Pregnancy termination could occur naturally in pair-bonded species if a pregnant female lost her original mate and subsequently acquired a new one. Pregnancy failure might also occur in such species as the result of brief exposures to strange males. Chipman et al. (1966) found that multiple short-term exposures to a strange male were as effective as continuous exposure in causing pregnancy failure in recently inseminated female laboratory mice. The present study was designed to determine the effect of multiple short-term exposures to strange males on M. ochrogaster females at different stages of pregnancy (2, 5, and 10 days after mating). Since M. ochrogaster has a pair-bonded, monogamous mating system (Getz et al., 1981; Getz and Hofmann, 1986), the presence of the male during pregnancy might influence whether pregnancy block or termination occurred. Therefore, two experiments were performed; in one the females' mates were permanently removed prior to introduction of strange males; in the other, the mates were housed with the females throughout pregnancy. All experimental voles were laboratory reared from stock obtained near Urbana, IL; the animals had been in captivity for 4-5 generations. The breeding colony and experimental animals were maintained at 24?C under a 15L:9D light cycle. Voles were housed in 24 by 45 cm plastic breeding cages (except as noted below) with wood-chip bedding; Purina rabbit chow and water were provided ad libitum. Only nulliparous females were used because lactating females mated during post partum estrus could have a different susceptibility to pregnancy block due to the influence of prolactin (Bruce and Parkes, 1960, 1961; Mallory and Clulow, 1977). Virgin females were brought into estrus by the following procedure. After separation from their parents at 21 days of age, voles were housed in sibling groups; females do not come into estrus when maintained with siblings (Carter et al., 1980). At 45-50 days of age, males were removed and placed individually in 17 by 28 cm cages. Three days later a non-littermate female was placed in the cage of each male. Each pair remained together for approximately 5 h after which the males were moved to clean 24 by 45 cm cages, while the females were left in the soiled cages. On the following day each female was placed in the cage of the same male for 1 h and the pair observed to determine if they mated. If mating did not occur the pair was separated, the female remaining in the cage that had been occupied by the male. These pairs were placed together and observed for 1-h periods during the following 2-3 days. Vol. 68, No. 1 166