Concealed ovulation
Whether other humans, potential reproductive partners in particular, can detect fertility in women through behavioral or invisible biological cues is highly debated.
[7][8] 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).
[11] One group of authors has theorized that concealed ovulation and menstruation were key factors in the development of symbolic culture in early human society.
[14] Several hypotheses regarding human evolution integrate the idea that women increasingly required supplemental paternal investment in their offspring.
Schröder[14] summarizes this hypothesis outlined in Alexander and Noonan's 1979 paper: if women no longer signaled the time of ovulation, men would be unable to detect the exact period in which they were fecund.
A similar hypothesis was proposed by Lovejoy in 1981 that argued that concealed ovulation, reduced canines and bipedalism evolved from a reproductive strategy where males provisioned food resources to his paired female and dependent offspring.
Therefore, the increased frequency of copulations due to concealed ovulation are thought to have played a role in fostering pair bonds in humans.
The offspring benefits from the supplemental investment, in the form of food and defense from the father, and receives the full attention and resources of the mother.
Another, more recent, hypothesis is that concealed ovulation is an adaptation in response to a promiscuous mating system, similar to that of our closest evolutionary relatives, bonobos and chimpanzees.
[20] This hypothesis suggests the adaptive advantage for women who had hidden estrus would be a reduction in the possibility of infanticide by men, as they would be unable to reliably identify, and kill, their rivals' offspring.
[14] This hypothesis is supported by recent studies of wild hanuman langurs, documenting concealed ovulation, and frequent matings with males outside their fertile ovulatory period.
Schröder[14] presents the idea of a "gradual diminution of mid-cycle estrus and concomitant continuous sexual receptivity in human women" because it facilitated orderly social relationships throughout the menstrual cycle by eliminating the periodic intensification of male–male aggressiveness in competition for mates.
Her usual sexual partner would have little reason to doubt her fidelity, because of the concealed ovulation, and would have high, albeit unfounded, paternity confidence in her offspring.
This would have caused humans to live in denser groups, and, in such a scenario, the long-distance sexual signaling provided by female genital swellings would have lost its function.
This hypothesis ultimately concludes that bipedalism, which was strongly selected for, caused the physiological changes and a loss of function of sexual signaling through female genital swelling, leading to the concealed ovulation we now observe.
[25] One of the strengths of this is derived from the other hypotheses' weaknesses – it is difficult to track the evolution of a behavior as it leaves no verifiable evidence in the form of bone or DNA.
