The effects of intromission frequency on successful pregnancy in the female rat.

The copulatory pattern of several species of rodents including the rat, mouse, hamster, and deermouse (Peromyscus) includes a series of mounts during which the male briefly inserts the penis into the vagina but withdraws without ejaculating. Ejaculation occurs during the last insertion of a series, when intromission is maintained for a longer period of time. The function of pre-ejaculatory intromissions has previously been investigated from the standpoint of the male. Beach l 3 postulated that each intromission of the male rat contributed an increment toward the attainment of an ejaculatory threshold; a series of studies developing this model have subsequently been carried out.4-IO Another function of the multiple intromission pattern, however, may apply to the female. A variety of neurogenic factors are known to trigger endocrine events connected with reproductive functions. ll, 12 For example, females of some species (rabbit, cat, ferret, and mink) are induced ovulators: their ovaries do not release eggs unless there has been copulation by the male, glass rod insertion, or some other form of stimulation to the vagina and cervix. 13, 14 The neurogenic stimuli activate the hypothalamo-hypophyseal system causing the release of luteinizing hormone (LH) , which in tum leads to ovulation. In addition, there are species in which the female normally produces nonfunctional corpora lutea but upon appropriate stimulation will maintain a prolonged luteal phase, pregnancy, or pseudopregnancy.14-16 This prolongation induces progestational changes in the uterine endometrium and delays the subsequent estrus. Stimulation in this case leads to a change in the female's endocrine status-presumably an extended secretion of prolactin (LTH). Thus, in both induced ovulation and pseudopregnancy, vaginal stimulat.ion can playa decisive role in initiating a neuroendocrine reflex. Finally, Beach17 has proposed the hypothesis that the female rat's secretion of progesterone, and hence the probability that the progestational changes necessary for implantation will take place after mating, is dependent upon stimulation derived from repeated insertions by the male during copulation. Based on this hypothesis, one of the present authors, Wilson, performed a pilot study. He found that 5 of 6 females receiving 4 or more intromissions prior to ejaculation became pregnant, whereas only 1 of 6 receiving less than 4 intromissions became pregnant. The purpose of the present study was to confirm and extend these findings demonstrating a correlation between the multiple penile intromissions of the male rat and the maintenance of a successful pregnancy in the female. Materials and Methods.-Twenty male and 20 female Long-Evans rats obtained from Diablo laboratories served as 8's. They were 130 days old at the beginning of the experimental tests. All animals had continuous access to food and water. Observation cages were cylindrical, with 24-inch glass sides and 3D-inch diameter wooden floors covered with wood shavings. Responses were scored with an Esterline-Angus events recorder.