1975) described a postimplantation disruption of pregnancy by a non-stud male as late as Day 17 of the 21\p=n-\23-daypregnancy, and they ascribed this prolonged susceptibility to factors associated with the occurrence of reflex ovulation in these voles. We therefore examined the effect of a 'strange' male on pregnancy in prairie and meadow voles, species in which ovulation is induced by copulation, and in prairie deermice which are spontaneous ovulators. MethodsThe animals of all three species (M. ochrogaster, M. pennsylvanicus and P. maniculatus) were maintained in temperature-controlled rooms on a reversed 16 h light : 8 h dark cycle with red light on at all times. Vaginal smears, examinations for pregnancy and counts of young were conducted in the second half of the light phase. Purina lab chow and water were freely available, and the voles received occasional supplements of lettuce and oats. Cages were cleaned once every 2 weeks.Laboratory-born, parous females of each species were paired with fertile, non-litter mate, conspecific males and daily vaginal smears were taken with a wire loop. A female was tentatively considered pregnant if (1) an oestrous vaginal smear (50% cornified cells) was followed by marked invasion of leucocytes lasting 13 days (Richmond & Conaway, 1969;Clulow & Mallory, 1970,1974Maddock, 1972), (2) spermatozoa were present in the vaginal smear, or (3) a copulatory plug was found in the vagina. In most cases, at least 2 of these criteria were met, and the day of sperm or plug presence or the last day of oestrus was called Day 1 of gestation. On Day 14 of gestation all females were examined manually to verify that they were pregnant, and any non-pregnant females were not further considered. The Day 14 pregnant females were assigned to one of 2 groups: in the control group, the females were returned to the cage of the stud male, and in the experimental group, the females were placed in the home cage of another fertile male. Daily vaginal smears were continued, and males with females of either group were removed the day before the projected parturition date for a litter sired by the first male. Litters were weaned on Day 21 except for some M. pennsylvanicus litters which were killed on Day 5. After a week of complete isolation, each female was again mated to the original male, but at Day 14 was allocated to the other group.Results and discussion M. ochrogaster. All 13 females adjudged pregnant on Day 14 and replaced with the stud male gave birth, with a mean of 4-5 ± 0-6 (S.E.M.) young, at the expected time, 22-8 + 0-2 days after mating. Most (77 %) of the females met the criteria for having mated on the 1 st or 2nd day after pairing. When exposed to a 'strange' male, 4 of 13 females showed vaginal oestrus 2-3 days later. Spermatozoa were found in the vaginal smears of 3 females which subsequently gave birth to 3-3 ± 0-9 young 24-3 ± 0-9 days later. No spermatozoa were found in the smears of the fourth female, and no litter was born.

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Six species of muroid rodents (Peromyscus californicus, P. eremicus, P. gossypinus, Mesocricetus auratus, Rattus norvegicus and R. rattus) were given artificlal vaginal-cervical stimulation in an attempt to induce pseudopregnancy. Some females of each species became pseudopregnant, but it was not clear that stimulation patterns mimicking the copulatory patterns of conspecific males were more effective than other patterns of stimulation. In the non-domesticated species (Peromyscus and R. rattus) a few females responded to a variety of stimulus conditions, and no species differences in responsiveness among the wild species were evident. The established laboratory rodents (R. norvegicus, M. auratusy were markedly more responsive to artificial stimulation, suggesting possible effects of domestication on fecundity and reactions to handling- Microtus ochrogaster, an induced ovulator, ovulated in response to vaginal-cervical stimulation only if one intromission from a male was also provided. No other differences in the responses of reflex and spontaneous ovulators were apparent.

The copulatory behavior of Microtus californicus and its effect on pregnancy initiation were examined in two experiments. In experiment 1 18 males and 18 females which had received exogenous hormones participated in 54 tests of copulatory behavior, each continued to a satiety criterion of 30 min with no copulations. The basic pattern involved no lock, intravaginal thrusting, ejaculation possible on a single insertion, and multiple ejaculation. Ejaculation frequency ranged from 1 to 5, with a mean of 2.2. In experiment 2, it was found that all females receiving satiety tests of copulatory behavior while in male-induced estrus ovulated and became pregnant. Whereas all 10 females receiving one ejaculation ovulated, only 60% became pregnant. Thus, it appears that copulation beyond one ejaculation functions in increasing the likelihood of pregnancy. By comparing different species of Microtus it is proposed that copulatory patterns in which males persist for many thrusts and ejaculations may have evolved in conjunction with ornate penile morphology, large litter sizes, and high stimulus requirements for the initiation of ovulation and a functional luteal phase.