Sperm Storage in the Female Reproductive Tract: A Conserved Reproductive Strategy for Better Fertilization Success

In internal fertilizers including mammals, fertilization success depends on the timely arrival of sperm and egg at the site of fertilization. Males should transfer their spermatozoa to the female reproductive tract by copulation during or prior to ovulation in order to achieve this aim. However, such a collaborative mating behavior is often disconnected from the efficiency of the sperm-egg encounter, i.e., ovulation by females occurs independently from insemination by males in many species. To compensate for this time lag, females are capable of storing spermatozoa in their reproductive tracts until the eggs are ready to be fertilized. In avian species, simple tubular invaginations referred to as sperm storage tubules (SSTs) are located between the vagina and uterus as sperm storage sites. Spermatozoa, once ejaculated, migrate to and are thereafter stored in the lumen of the SSTs without loss of fertilizing capacity for up to 15 weeks at a body temperature of 41 °C. This is astonishing, because terminally differentiated cells that lack new protein synthesis are still capable of being functional for a long period at a high temperature; however, the actual mechanism has been an enigma for more than half a century. In this chapter, we will first describe the physiological importance and adoptive benefits of sperm storage in the female genital tract for successful fertilization in animals, and next, we will describe our recent findings in birds with regard to the specific mechanism of sperm uptake into the SST, sperm maintenance within it, and controlled release from it.