BASIC BOOKS, JUNE 2013 Most women are taught that conception only occurs if coitus takes place within a so-called “fertile window” around ovulation, usually halfway through a menstrual cycle. This underlies the “rhythm method” and related birth-control techniques that include monitoring a woman’s basal body temperature or mucus secreted by glands in her cervix. It has long been held that the window lasts only 3 days, allowing 2 days for survival of ejaculated sperm and at most a day for egg survival after ovulation.
But pregnancy lengths in higher primates (monkeys, apes, and humans) seemingly vary twice as much as in other mammals. Why?
It is well known that female bats can store sperm for months between copulation and fertilization. But sperm storage has rarely been considered for primates. Intervals between copulation and birth vary more if sperm are stored in the female’s body. It has also been shown that human sperm can survive for at least 5 days in the uterus, but no attempt has been made to identify an upper limit. I explore the implications of possible sperm storage and other aspects of the science of sex in my new book, How We Do It: The Evolution and Future of Human Reproduction.
In most mammals, females mate ...
