Copulatory behavior and the initiation of pregnancy in California voles (Microtus californicus)

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 funtional luteal phase.