In all animals except for humans, the desire for sex is timed to correspond to the female’s ovulation. Our inclination to have sex at any time during the month or year is peculiarly human; it separates us from the rest of the animal kingdom and is, reproductively, extremely inefficient. There is only a very brief period of time, just a matter of days during each month, in which intercourse is likely to lead to pregnancy. The timing of sex is thus very important if a species is to have a very high fertility rate. If intercourse doesn’t take place around the time of ovulation, it is less likely to lead to a pregnancy. In most animals other than humans, intercourse only occurs at this precise time, when it is likely to result in pregnancy.

Animals go through what is called an estrous cycle, or heat, whereas humans go through a menstrual cycle. In the normal cycle of any animal, as the follicle (or follicles) destined to ovulate begins to grow and prepares the egg for maturation and ultimately fertilization, it produces tremendous quantities of the female hormone estrogen from its granulosa cells. (This high production of estrogen from the developing follicle begins in the human on the first day of menstruation.) In all animals except for humans, this increasing estrogen level chemically triggers the female to desire sex. When this happens, any farmer or animal trainer knows the animal is “going into heat.” Whether it’s a cow, moose, pig, or rat, the female assumes a peculiarly hunched body position, indicating she is ready for sex. Only at that time in the cycle will she allow a male to get near her. Thus, most animals have sex only when it is likely to result in fertilization and a baby.

Humans, who go through a menstrual cycle, have the same increase in estrogen prior to ovulation. The difference between humans and all other animals is that in humans, the female is just as apt to accept the male in a sexual act at any time during the month. Most of the time, the human female has no idea whether she is having sex at a time of the month that is likely to lead to pregnancy.

Sexual desire in the human is much more complex and is not driven by the female hormone estrogen. In fact, in the human female, sexual drive comes from the male hormone testosterone, the same hormone that is constantly turning on the sexual drive in the male. This is unique in the animal kingdom. Of course, the female level of male hormone is about one-tenth the male level, explaining the lower chemical sexual drive in the female than in the male. The small amount of testosterone that the female makes, however, is quite adequate for her to become willing and interested in being a sexual partner. The amount of testosterone produced by the male is fairly constant from day to day and from year to year. This is true in the female also, although there is a slight increase just around the time of ovulation. Thus, we are similar to our animal friends in that human female-initiated sex is more common around the time of ovulation. But this phenomenon can only be noticed in a small way in large population studies. Usually couples have sex fairly randomly throughout the month, whereas animals only have sex on a few very specific days, when the female’s estrogen level triggers her interest just prior to ovulation.