Concealed Ovulation - Paternal Investment Hypothesis

Paternal Investment Hypothesis

The paternal investment hypothesis is strongly supported by many evolutionary biologists. Several hypotheses regarding human evolution integrate the idea that human females increasingly required supplemental paternal investment in their offspring. The shared reliance on this idea across several hypotheses concerning human evolution increases its significance in terms of this specific phenomenon.

This hypothesis suggests human females concealed ovulation to obtain male aid in rearing offspring. Schroder summarizes this hypothesis outlined in Alexander and Noonan’s 1979 paper. 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, more recent hypothesis is that concealed ovulation is an adaption for a promiscuous mating system similar to our closest ancestors bonobos and chimpanzees. The theory is that women have a concealed ovulation to lessen paternity certainty, to lessen the chances of infanticide (as a father is less likely to kill offspring that might be his), and to potentially increase the number of caretakers for their young (partible paternity). This is supported by the fact that all other mammals with concealed ovulation such as dolphins (gray langurs as well) are promiscuous, and that the only other apes that have multi-male communities like humans do are promiscuous apes. It is argued that effects like the Coolidge effect show that males do not seem to be naturally geared towards sexual mate guarding behavior of one female, which is evidence that sexual monogamy (though perhaps not social monogamy and/or pair bonding) was uncommon in early modern humans.