Humans have “concealed ovulation”. Concealed ovulation is the lack of any perceptible change in an adult female (for instance, a change in appearance or scent) when she is “in heat” and near ovulation.
Most mammals have overt (or “advertised”) ovulation. Some examples of such changes are swelling and redness of the genitalia in baboons. In contrast, the females of humans and a few other species have few external signs of fertility, making it difficult for the male to consciously deduct, by means of external signs only, whether or not a female is near ovulation.
While women can be taught to recognize their own level of fertility, whether men can detect fertility in women is highly debated. Several small studies have found that fertile women (compared to women in infertile portions of the menstrual cycle, or using hormonal contraception) appear more attractive to men. It has also been suggested that a woman’s voice may become more attractive to men during this time. Two small studies of monogamous human couples found that women initiated sex significantly more frequently when fertile, but male-initiated sex occurred at a constant rate, without regard to the woman’s phase of menstrual cycle. It may be that a woman’s awareness of men’s courtship signals increases during her highly fertile phase due to an enhanced olfactory awareness of chemicals specifically found in men’s body odor.
Analyses of data provided by the post-1998 U.S. Demographic and Health Surveys found no variation in the occurrence of coitus in the menstrual phases (except during menstruation itself). This is contrary to other studies, which have found female sexual desire and extra-pair copulations (“EPC’s”) to increase during the midfollicular to ovulatory phases (that is, the highly fertile phase).
These findings of differences in woman-initiated versus man-initiated sex are likely caused by the woman’s subconscious awareness of her ovulation cycle (because of hormone changes causing her to feel increased sexual desire), contrasting with the man’s inability to detect ovulation because of its being “hidden”.