Concealed ovulation

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 ovulating at the time. Humans are the only mammal to lack obvious, visible manifestations of ovulation, 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 ovulation (fertility awareness), whether men can detect ovulation in women is highly debated. At least one recent study has argued that men are more likely to initiate sex with fertile women, while another has found male-initiated sex to occur at a constant rate throughout 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.