Concealed Ovulation

Concealed ovulation or hidden estrus is the lack of distinctive signaling that the adult female of a species is "in heat" and near ovulation. These signals may include swelling and redness of the genitalia in baboons and bonobos Pan paniscus, pheromone release in the feline family, etc. By comparison, the females of humans and a few other species have few external signs of fecundity, making it difficult 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 (fertility awareness), 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 on hormonal contraception) appear more attractive. It has also been suggested that a woman's voice may become more attractive to males during this time. Two small studies of monogamous 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 female awareness of male courtship signals increases during her highly fertile phase in association with an enhanced olfactory awareness of chemicals specifically found in male body odour.

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 menses. Though this is contrary to other studies which found female sexual desire and extrapair copulations (EPCs) to increase during the midfollicular to ovulatory phases (that is, the highly fertile phase). These findings of differences in female-initiated sex versus male-initiated sex are likely caused by the female’s subconscious awareness of her ovulation cycle (because of hormone changes causing increased levels of sexual desire or lust in the female), and the male’s inability to detect ovulation because of its being “hidden”.

In 2008, researchers announced the discovery of hormones related to ovulation in human semen. They theorized that follicle stimulating hormone, luteinising hormone, and estradiol may encourage ovulation in women exposed to semen. These hormones are not found in the semen of chimpanzees, suggesting this phenomenon may be a male counter-strategy to concealed ovulation in human females. Other researchers are skeptical that the low levels of hormones found in semen could have any effect on ovulation. One group of authors has theorized that concealed ovulation and menstruation were key factors in the development of symbolic culture in early human society.