Loss of estrus: Evolution of Concealed Ovulation in Humans.

Complex mating Rituals of Chimps

Many researchers have studied the quantitative differences between human reproductive cycles and those of other primates. Although there are some potential physical signals that females emit during the time of fertility, they lack the level of manifestation that their closest relatives have. This raises the important evolutionary question of why. Not only do humans hide the fertility phase from conspeices, but also from themselves. On the other hand, many non-human females do not act subtly when they are telling other males it’s time to conceive. Researchers hypothesized various potential advantages of being members of the small group of mammalian world who practice “concealed ovulation.” While looking at the evolution of physical characteristics, environmental, and cultural factors that have influenced the behavior of human sexual signaling, social construct of monogamy is the most effective explanation for such behavioral change .Researchers found that concealed ovulation increased emerged monogamy and intensified parental care from males.
First of all, one of the greatest misconceptions that many people have while understanding evolution of human ovulation is that humans lack the complete and total manifestation of fertility phase. We fail to acknowledge the fact that when we try to dissect the problem of “concealed ovulation” we are just looking at physical advertisement of fertility. Humans may not physically manifest their state of fertility through visual cues like many other primates, but ovulation in humans is not completely concealed. Humans engage in more intense sexual activity and are more flirtatious in the proliferate and ovulatory phases. The intensity of perceptivity changes across the cycle of menstruation (Powlowski, 1999).
A 2004, study done by Roberts et al suggests that female attractiveness increases during the fertile phase of the menstrual cycle. This study showed that when men and women looked at the face of females of ovulatory phase and leutal phase, they found the ovulatory phase photographs to be more attractive. The study also suggested that the nature of the facial characteristics’ differentiated during the different phases including the difference in the lip color, size, papillary dilation and skin color tone. (Roberts et al, 2004). It also emphasized on the possible advantages of facial attractiveness around the time of ovulation because it enlarges pool of potential mates and extra-pair partners. This endergonic change of behavior attracts males and increases the level of conception. Hence, this shows that only physical and advertised physical manifestation of ovulation is lacked by humans and since these behavioral changes are usually not controlled through rational deliberation, humans perceive them to be missing.
In addition, a study done by Scutt and Manning 1996 on symmetry and ovulation in human women suggests that change in symmetry in soft tissues occurs during the reproductive cycle of humans. They were able to find that asymmetry was lowest in the day of ovulation and the changes occurred within subjects from -2 days to -1. With high degree of receptivity the traits were measured since the independent variable were easily and reliably accessible. The subjects’ data were recorded on days 12 to 16 where they measured ovulation using real time ultra sonography and scanned each subject. The uterus and ovaries were examined, and the results of the subjects were then compared. 27 subjects were ovulating, two subjects had ovulated twice and two subjects did not ovulate. The results were compared by looking at the end data and the data collected at day 0. Changes in soft tissues were reported. The changes in such tissues may suggest that the women are ovulating. “These changes are probably hormone related” (Scutt and Manning, 1996). This shows that there might actually be some physical changes associated with ovulation.
Also, there are two possible hypotheses made regarding the change in advertisement of ovulation in human species. There have been various speculations about the presence of physical manifestation of visual cues in prehominines. Hardy and Whitten hold the perspective that pink swelling is absent in the majority of the primate species and the characteristics are specific to Pan (Powlowski, 1999) and this leads to the assumption that the occurrence of such manifestation occurred after the evolution of Pan continued after the divergence from homo lineage. The other aspect proposed by Powlowski, is the idea that concealed ovulation in homo might just be the side effect of bipedalism, a change in vision and environment. It has been argued that human erect position changed the position and visibility of the female’s genitals. Hence, the manifestation through advertisement became useless (Tanner 1981). Adding to the previous argument, the change in spine also changed the line of vision. While before males could constantly see female’s genitals, the evolution of bipedalism changed this. Another considerable aspect is the environmental changes. The new Savannah environment required many animals to conserve water resource and evaporation from swelling would have been a great disadvantage (Powlowski,1999).Hence, these explanations of such behavior might be due to the change in environment and development of hominines and could simply just be the byproduct of human evolution.
Cultural and environmental factors may also play a vital role in explaining the emergence of distinct qualities of humans. Comparative anatomy might be supportive in explaining the reasons behind such behavior in Homo. It has been seen that in hunter-gatherers, the level of testosterone in males is lower than that of western males, and the level of hormone production in females is relatively low in such societies as well. Therefore, they engage more in sexual behavior only in the phases of proliferate and ovulatory phases when the hormone levels rise. The linkage between the levels of hormones in the blood directly correlates to the frequency of sexual activity and the time of ovulation, since females tend to have higher levels of steroids in their blood streams. (Powlowski, 1999) Thus, the economy and the change of life style might have influenced some of the indicators of human sexuality. However, this doesn’t tell us that hunter gathers are more efficient in detecting the time of ovulation in females.
To further this explanation, Marlowe 2003 conducted an experiment to detect whether or not ovulation is concealed in human females if they live in natural conditions with minimal hygiene. It is true that selection will always favor men, who can most often detect ovulatory phases of females. The optimum goal of specie is to transfer genes to another generation. If the males can detect such cues, then they can better gain attractiveness from ovulating female (Marlowe, 2003.) Since understanding this phenomenon is challenging for males in non-literate small-scale societies, so Marlowe chose to investigate the Hazda hunter-gather society and their knowledge about the time of the ovulation and conception. “Fewer than expected said “during midcycle” and more than expected said “right after menses” (Marlowe, 2003). This show, that, males in different males have no knowledge about detecting the time of ovulation.
The more important question that needs to be addressed is why did human emerge out and became distinct from that of other primates? Even though the nucleotide sequence data suggests that homo and pan are sister taxa, they differ significantly in manifestation of ovulation. Animals, like Pan, do go through what is called estrus period, where the females have a strong proceptive behavior and where she accepts male’s copulation. This is more prominent in animals like rodents, carnivores and lemuroids (Powlowski,1999). Primates operate in a different fashion. Males usually have the dominance and they engage in what is known as the beach receptivity (Iron 1979), when males initiate the copulation. Therefore many primates have visual cues for the time of the ovulation. In many animals it is not difficult to know when the female is ready to conceive. Female baboons (P.paniscus) have pink swellings that indicate to males that she is ovulating. Our closest relative chimpanzees (Pan troglodytes) are known for the extensive amount of swelling during the peak of ovulation (Knobil). Female baboons only engage in copulation at estrus, where she emerges from a month of abstinence and has sex for up to 100 times (Diamond).Homo, on the other hand, has no such visual cues that indicate to males that the females are ovulating. This leads to so-called females’ constant receptivity or prolonged sexual receptivity with causes non-reproductive mating. (Moller & Tullberg, 1991)
Field data suggest that non-human primates engage in copulation, when the level of estrogen and testosterone are in their peak, which is during proliferate and ovulatory phases of menstrual cycle (Powlowski,1999) .Human females engage in continuous receptivity, as in they participate in sexual interactions, at any given time of the month. Due to this aspect of human nature, most humans have sex that has no biological value. “Not only do we engage in sex at wrong time of the cycle we continue to have sex during pregnancy and menopause, when we know that fertilization is impossible” (Diamond, 1993).
Moreover, a study done by Tullberg and Moller studied the temporal differences between various mating system and the disappearance of visual ovulatory signs with the major motive to detect whether or not the nature of concealed ovulation is an adaptive where they found that humans are much more tightly coupled with monogamy than any other mating systems. The Alexander and Noonan theory, is also reinforced here, as this is primarily seen due to the effect of monogamy makes the parental security more viable.
There are many assumptions regarding concealed ovulation and its influence on male parental care. One of the biggest reasons for this development is due to the importance of parental cares for helpless infants. Human infants are extremely helpless and therefore, require a lot of support. The two most prominent theories that facilitate the previous theory are the father at home and many fathers theory. The father at home was a development of University of Michigan professor Richard Alexander and his graduate student named Katherine Noonan. This states that concealed ovulation evolved hand in hand with monogamy with a major motive to “force” men to stay with a particular female. It is used by females as a strategy to make males invest in parental care. If females conceal their ovulation than the males will want to stay home and consort relationships long enough to impregnate her. He copulates various times during the month in hopes that he will impregnate her and also wishes to guard her from other males, who might access her at the wrong time. Since he stays home and engages in copulation throughout the month, when he finally impregnates her, he can be confident on the paternity of the child and hence will contribute towards the child. (Schroder, 1993)
On the other hand, the many fathers theory developed by University of California’s anthropologist, Sarah Hardy females copulate with many males at a given time and no male can be confident about the paternity .Hence; numerous males become more and more attached to the infant since they recognize that they might have contributed towards its formation. Hardy stated that signs of ovulation conceal in humans due to nonmonogamous context and males tend to stick around due to confused paternity issues rather than established one.(Tullberg and Moller,1991).This assumes the multi-male mating since they help the infants with protection. (Diamond, 1999).Hence, the nature of concealed ovulation indicates that there might be some social construct for emergence of this trait in humans. If males don’t know when females are ovulating, they cannot understand when to stay with the females. On the other hand, if they knew that the females when she is not ovulating, they might wander to find females who are ovulating. Thus, men don’t engage in parental care because most of their time is going to be invested in attracting mates.
On the other hand, the Cuckoldry hypothesis proposed by Bornshoof and Thornhill, argue that monogamy established concealed ovulation. They suggested that Australopithacus aferensis practiced polydry and since the men had too many dependent offspring, they converted to monogamy in Homo erectus. Women wanted the best genes and females who practice concealed ovulation minimized the potential mates hence the males could be confident in their paternity. (Schronder, 1993)
It has been seen that more monogamous creatures practice concealed evolution. Humans, however, are not completely monogamous; because we don’t really practice complete concealment of ovulation. The extensive amount of advertisement of ovulation is found in more multi-male bands where females have to compete for males (Iron 1979). In humans, mostly females are choosy while males tend to be competitive. It has been suggested that concealment in ovulation is advantageous for females and they have high ranking males available because they are already committed .Even low ranking females can gain some parental care for their offspring, (Iron 1979) hence this enforced monogamy.
Human reproductive features are qualitatively different than that of many other animals. Even though we do have some changes in behavior during the estrus period, we don’t manifest our fertility phase to the world. This unique feature of human behavior has led us to become more and more monogamous and has acquired males to generate more parental care. This has caused males to cooperate and not engage in infanticide. Human infants are extremely dependent which might be a reason why females conceal their fertility. Also, there are many explanations that need more research. Even though there has been data supporting that human fertility behavior might just be a byproduct of human evolution, there needs to be more work done in that field. The change in sexual signaling has much to do with humans’ social construct. More research can be done on how the changes might be effective on defining the “serial monogamous” behavior of humans. Also, we can also scrutinize the claim made by Worthman 1978, that different environmental and economical factors might be responsible for the differences in frequency of copulations in westerns and hunter-gathers.
Humans, tend to be more cryptic about ovulation than their closest relatives. It is obvious that human females are cryptic from males for reasons of monogamy and parental investment but human females conceal ovulation from themselves as well. One of the biggest and most supported hypothesis for this human behavior suggested by Burley is that humans are more intelligent than any other creature, and since they usually make conscious decisions based on self-preservation, they understand the pain and fear of death caused by childbirth (Freeman &Wong, 1991). This would lead human females to never copulate when they are fertile. The conscious reasoning between copulation and pregnancy might be a negative effect for human females.
Humans have concealed ovulation but do provide some clues about their fertility status through unconscious sexual signaling. This might be one of the most prominent reasons why humans tend to be serial-monogamist. Researchers have formulated various hypotheses on possible mechanisms and evolutionary reasons for such behavioral change. Bipedalism and environmental changes might provide some biological basis for how humans adopted this characteristic. Evolution of monogamy from polygamy might be the most prominent reason for this. Even though all “father at home”,“many fathers’ and cuckoldry theories make good points, there isn’t a right answer. These different characteristics might have existed in human behavior at different times.
References
Diamond, J. (1993). Sex and female agenda. Discovery, 14(9)
Freeman, A.L, & Wong, H.Y. (1195). Ethology and Sociobiology, 16, 531-533
Havlicek, J., Roberts, S.C., and Fleg, J. 2005. Women’s preference for dominant male odour: effects of menstrual cycle and relationship status. Biol. Letters. 1:256-259.
Iron W.(1979).Evolutionary biology and human social behavior.California Duxbury Press.
Knobil, E. (n.d.). Debate on Concealed Ovulation. Encyclopedia of reproduction. Texas: Academic press Marlowe, F. (2004). Is Human ovulation concealed? Evidence from conception beliefs in a hunter gatherer society. Springer Science, 33(5), 427-432.
Powlowski, B. (1999). Loss of oestrus and concealed ovulation in human evolution: The case against sexual selection hypothesis. Current Anthropology, 40(3), 257-275.
Pagel, M. (2002). Reproduction. Encyclopedia of evolution. Oxford University Press.
Scutt, D, & Manning, J.T. (1996). Symmetry and ovulation in women. Human Reproduction, 11(11), 2477-
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Schroder, I. (1993). Concealed ovulation and clandestine copulation: a female contribution to human evolution. Ethnology and Sociobiology, 14, 381-389.
Tullberg, BS, & Moller, AP. (1993). The Relationship between concealed ovulation and mating systems in athropoid primates:a phylogenetic analysis. The american naturalist, 141(1)

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One Response to “Loss of estrus: Evolution of Concealed Ovulation in Humans.”

  1. Dni Plodne says:

    Hallo. Thanks for your post, I learn a lot.
    We are trying to have a baby and I have a two questions:
    1) What are the common early pregnancy Symptoms?
    2) How can I calculate my due date?
    I would really appreciate for answers. Thanks in advance.