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, 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 tell, by means of external signs only, whether or not a female is near ovulation.

Several hypotheses regarding human evolution integrate the idea that human females increasingly required supplemental paternal investment in their offspring. This suggests human females concealed ovulation to obtain male aid in rearing offspring. If human females no longer signaled the time of ovulation, males would be unable to detect the exact period in which they were fecund. This led to a change in their mating strategy; rather than seeking multiple female partners and mating with them hoping they were fecund during that period, males instead chose to mate with a particular female multiple times throughout her menstrual cycle. A mating would be successful in resulting in conception when it occurred during ovulation, and thus, frequent matings, necessitated by the effects of concealed ovulation, would be most successful.

Continuous female sexual receptivity suggests human sexuality is not solely defined by reproduction; a large part of it revolves around conjugal love and communication between partners. Copulations between partners while the female is pregnant or in the infertile period of her menstrual cycle do not achieve the base purpose of sex – conception – but do strengthen the bond between these partners. Therefore, the increased copulations because of concealed ovulation are thought to have played a role in fostering pair bonds in humans.

The pair bond would be very advantageous to the reproductive fitness of both partners throughout the period of pregnancy, lactation, and rearing of offspring. Pregnancy and lactation require vast amounts of energy on the part of the female, necessitating a large amount of energy intake in the form of food. However, during these periods, the female’s foraging ability would be greatly hindered because of constraints placed upon her by the pregnancy itself or the amount of time tending to or minding the offspring. Supplemental male investment in the mother and her offspring is advantageous to all parties. While the male is supplementing the female’s limited foraging intakes, she can devote the necessary time to the care of her offspring. 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. Through this shared parental investment, both male and female would increase their offspring’s chances for survival, thereby increasing their reproductive fitness. This increased reproductive fitness is the key to natural selection favoring the establishment of pair bonds in humans, and since pair bonds are thought to have been strengthened by concealed ovulation, this must have been under selective pressure.

Another hypothesis suggests the adaptive advantage for females who had hidden estrus would be a reduction in the possibility of infanticide by males, as they would be unable to selectively kill their rivals’ offspring. This hypothesis shows support in the recent studies on wild Hanuman langurs that display concealed ovulation and frequent matings with males outside their fertile ovulatory period. Concealed ovulation is thought to be used by females to confuse paternity and thus reduce infanticide. Since males determine paternity and thus decide on whether to kill the female’s child based on his previous matings with that female, a female’s promiscuous matings in conjunction with concealed ovulation would lead a male to believe there is still a possibility for that child to be his own.

Another hypothesis holds that estrus became hidden after monogamous relationships became the norm in Homo erectus because concealed ovulation allowed the female to mate with genetically superior males and thus gain the benefit of their genes for her offspring while still maintaining the pair bond with her lesser male partner. The bonded male would have little reason to doubt her fidelity because of the concealed ovulation and would have high, albeit unfounded, paternity confidence in his offspring. His confidence would encourage him to invest in the child even though it was not his own. Again, the idea of paternal investment being vital to the offspring’s survival, and thus vital to the male and female partner’s fitness, is a central.

Finally, another theory presents the importance of bipedalism to the mechanics and necessity of ovulation signaling. The more-open savannah environment inhabited by early humans (as made available by bipedalism) brought greater danger from predators. This would have caused humans to live in more-dense groups, and, in such a scenario, the long-distance sexual signaling provided by female genital swellings would have lost its function. Concealed ovulation is thus argued to be a loss of function evolutionary change rather than an adaptation. Thermoregulatory systems were also modified in humans with the move to the savannah to conserve water. It is thought that female genital swellings would have incurred added cost because of ineffective evaporation of water from the area. Further, the change to bipedalism in early hominins changed both the position of female genitals and the line of vision of males. Since males could no longer constantly see the female genitals, swelling of them during estrus as a mode of signaling would have become useless.

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