Biology of Reproduction, Vol 20, 540-544, Copyright © 1979 by Society for the Study of Reproduction

Relative Efficiency of Sperm Transport and Number of Sperm Ejaculated in the Female Rat

¹ Department of Psychology, University of Pennsylvania, Philadelphia, Pennsylvania 19174

We examined whether the percentage of sperm which enters the uterus of the rat is related to the number of sperm which the male ejaculated. After receiving 1 ejaculation, female rats were examined for the presence of sperm in the vagina and uterus and were dissected in order to measure the position of the vaginal plug in the vaginal-cervical junction. There were 3 main results. First, the greater the total number of sperm which were recovered from the female’s entire reporductive tract, the greater was the percentage of those sperm which had reached the uterus. Second, more females with higher total sperm counts than females with lower total sperm counts had vaginal plugs lodged tightly in the vaginal-cervical junction. Third, the more the vaginal plug extended into the vaginal-cervical junction, the greater was the percentage of the total sperm which was found in the uterus. However, this latter relationship held only in females having more than 200 x 10⁵ total sperm, since some females having fewer than 200 x 10⁵ total sperm also had tightly fitting vaginal plugs but had poor sperm transport. When low numbers of sperm are ejaculated, sperm transport may be adversely affected by 1) the frequent failure of the vaginal plug to lodge in the vaginal-cervical junction and by 2) the failure of even those plugs which do lodge tightly in the vaginal-cervical junction to promote sperm transport. Conversely, the ejaculation of large numbers of sperm may facilitate the transport of sperm into the uterus.