Human females have concealed ovulation or hidden estrus. Most female animals show distinctive signs when they are "in heat". These include swelling and redness of the genitalia in baboons and bonobos, pheromone release in the feline family, etc. By comparison, human females have few external signs of fertility. It is difficult to tell, by means of external signs only, whether or not a woman is near ovulation. Humans are the only mammal to lack obvious, visible manifestations of fertility, although some argue that the extended estrus period of the bonobo (reproductive-age females are in heat for 75% of their menstrual cycle) has a similar effect to the lack of a "heat" in human females.
While women can be taught to recognize their own level of fertility (fertility awareness), whether men can detect fertility in women is highly debated. On one hand, 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. On the other hand, 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 in all phases of the menstrual cycle. One group of authors has theorized that concealed ovulation and menstruation were key factors in the development of symbolic culture in early human society.
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