Concealed ovulation
From Freepedia
Human females, almost uniquely among animals (with the notable exception of bonobos), have concealed ovulation. Most female animals show distinctive signs when they are "in heat". These include swelling and redness of the genitalia in baboons, pheremone release in the feline family, etc. By comparison, human females have few external signs of fertility. (In fact, the women themselves often do not consciously know when they are fertile.1)
Also unlike most other animals, human females are fertile all year rather than just be fertile at certain times of the year. Unlike other animals which are fertile all year, scientists do not believe that humans evolved this trait in order to allow greater numbers of offspring to be produced. In a hunter-gatherer environment, a human female can only support about one offspring every four years. Until the beginnings of agriculture, breast feeding and low nutrition levels caused a natural fertility suppression.
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Theory for causation
Current theories about the origins of concealed ovulation as a reproductive strategy focus on the secondary benefits of sex as a reinforcement of pair-bonding. Human children remain children and require significant care far longer than most animals. The stresses of child-bearing and child-rearing create a need for paternal support during the process. Concealed ovulation is believed to be a mechanism by which the male is encouraged to remain with the family unit.
Theories for behavioral implications
According to one theory,in a hunting/gathering society without agriculture, the following scenario would play out: the male's desire for sex keeps him around the female continuously. The male will barter for her sexual favours by giving the female and her offspring (even those that are not his own offspring) foods that are more difficult for the female to acquire alone such as the highly concentrated protein and fat in the meat of large animals. Due to concealed ovulation, the male never knows when he can safely abandon sex with the female, keeping the nutritious meat for himself until the female is fertile. Furthermore, because fertility is concealed, the male cannot safely allow other males to copulate with "his" female as those other males might win the lottery of the female's fertile days and one of them might become the father of her next offspring. This theory of reproductive mechanisms in a hunter/gatherer environment dates back to the late 19th century scientists and is the reason that the "world's oldest profession" is so called.
Concealed ovulation is believed to influence the adaptation of male reproductive strategies and drives as well. If the female is fertile all the time (from his perspective), the male must be interested in sex all the time. Neither partner knows when sexual activity will result in progeny. The male may not consciously want offspring but those males not wanting sex will not produce offspring to pass that lack of desire on to.
Concealed ovulation is also thought to enable differential strategies in female mate selection. Several studies have shown that human females show preferences for different types of males depending on whether or not they're fertile. Human females generally prefer less masculine-seeming men while not fertile and more masculine men while fertile.
Males with visible signs of masculinity (higher testosterone levels evidenced by more dominant behavior and a more toned and muscular physique) men supply "better" genes and a stronger chance for the long-term survival of the offspring. Less visibly masculine men tend to be better providers for a woman and her offspring: they have the paleolithic equivalent of high-paying computer jobs, while the masculine men are the race car drivers - risk-takers who often suffer premature death, making them unable to provide for the offspring to maturity. Less visibly masculine men are believed to have fewer reproductive options and so are more likely to remain within the pair-bond, getting sex and providing for offspring.
One theory concludes that the best reproductive strategy for a female in the hunter/gatherer environment is a mixed strategy: mate when fertile with an alpha male, but convince the beta male, by frequent sex when not fertile, to remain in the pair-bond. This may include an evolutionary incentive to conceal the "infidelity". There is some evidence that humans may have adapted a reactive tendency of newborns to resemble their fathers more than their mothers in order to reinforce pair-bonding fidelity, to reinforce the paternal bond, or to forestall infanticide by a father unconvinced of his paternity.
Conclusion
This apparently "minor" change in humans from other primates, the concealing of fertility, has major and wide-ranging effects on the composition of human society, paving the way for pair-bonding, families and tribes, child nurturing by both parents, division of labor based on sex, taboos on fidelity and, according to some theories, also paves the way for jealously, lying, infidelity, and fratricide.
Footnotes
Discussion of the claim that women do not know when they are fertile
The claim that many women do not know when they are fertile is disputed. The relationship between menstrual cycles and fertility has been documented even in aboriginal societies. Some also dispute this claim on the theory that pheromone sensitivity is high enough to reach conscious levels in pre-industrial societies because of increased and continuous proximity, absence of chemical agents used in personal hygiene, and in some cases a warm/tropical or subtropical climate (since warmer air carries suspended substances more readily). Also involved in such pre-industrial warm-climate environments is the lower total area cm2 of clothing over the average woman. This would allow the pheromone to drift freely from the fertile female, signalling all nearby males that she is ready to be impregnated. This would imply that rather than having no way of knowing when ovulation occurs, men would be alerted by the pheromones put off by relatively uncovered fertile women. In developed societies today in most cases, the intensity of pheromonal output from an ovulating woman would be only enough to influence someone extremely and consistently close to her, such as her husband. Recent Mexican and U.S. research has been studying phenomena in which husbands report being able to "just tell" or perceive when their wives are fertile.
The lag time between the menstrual cycle and fertility complicates the other objection, that menstruation is the marker signal (a visible sign). As evidence of the difficulty of relating menstrual cycles with fertility, variations on the following joke have been found in many cultures: "What is the medical term for a woman who relies only on the rhythm method for birth control? A mother." (The rhythm method is also occasionally known as the Vatican method.) Also, the coition itself may accelerate ovulation.
Also, the greatest incentive for the pair bond is agriculture. The parents need each other to support the agricultural establishment. This would mean that they stay with each other, and consequently raise their children together. A man in an agricultural society generally cannot simply stay with a woman during her ovulation and then pursue another, because he needs the woman's steady support. Therefore the man is present to raise the children that he fathers.
Hence there are many objections to the theory elaborated upon at the beginning of this page. Another explanation is that relatively concealed ovulation (if it even really exists in the indigenous environment) is actually due to the pair bonding scheme and not the other way around. In an exclusive pair bonding situation it would not be desirable to advertise one's fertility to men other than one's partner. Signals of ovulation would be of benefit only if they could be discerned solely by the female's one male partner. Such appears to often be the case with the ovulation pheromone in many modern countries; the debate is whether this extends to indigenous environments. For reasons detailed earlier, it appears that in some indigenous societies the pheromonal indicator is easily perceived by all men in proximity. In that case a man would be able to take advantage of this reproductive opportunity by making the woman pregnant with his offspring. The likelihood that a man would have the inclination to do that, and that she would allow him to, are matters for further research and debate. It is unlikely that a large majority of the scientific community will agree on these disputes anytime soon.
External links
a theory of concealed ovulation explaining it more prosaically, and not as a social adaptation
