Evolutionary Aspects
Sex in private distinguishes humans from bonobos, chimpanzees, and gorillas. Testis and penis size are related to family structure: monogamy or promiscuity, harem, in human, chimpanzee, and gorilla, respectively (see The Third Chimpanzee and “Why is Sex Fun?” by Jared Diamond). Involvement of the father in education, concealed ovulation, and menopause in women, are quite unique to our species, at least when compared to other hominins. Concealed (or “hidden”) ovulation means that the phase of fertility is not detectable in humans, whereas chimpanzees advertise ovulation by an obvious swelling of the genitals. Women can be partly aware of their ovulation, along the menstrual phases, but men are essentially unable to detect ovulation in women. Most primates have semi-concealed ovulation; thus, one can think that the common ancestor had semi-concealed ovulation, that he transmitted to gorillas, but that later evolved into concealed ovulation in humans and advertised ovulation in chimpanzee (see "Why is Sex Fun?").
Read more about this topic: Human Sexuality
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