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Human and bonobo females have concealed ovulation or hidden estrus. Most female animals show distinctive signs when they are "in heat". These include swelling and redness of the genitalia in baboons, pheromone release in the feline family, etc. By comparison, human females have few external signs of fertility. (In fact, the women themselves often do not know when they are fertile .1) It is difficult to tell, by means of external signs only, whether or not a woman is ovulating at the time.
Also unlike most other animals, human females are fertile all year rather than just 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, breastfeeding and low nutrition levels caused a natural fertility suppression.
Theories for causation
The most frequently cited theories about the origins of concealed ovulation as a reproductive strategy focus on the secondary function of sex as a reinforcement of social bondings.
Human children 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 another adult is encouraged to remain with the family unit.
The Engels theory
The Engels theory has been around since the 19th century and describes a hypothetical ancient family unit. Exactly what this family unit would have looked like remains uncertain. Modern concepts of monogamous marriage must not be assumed for early humans. The Engels theory proposes that the male's role in pregnancy was generally unknown until the beginning of the Neolithic Period . According to this theory, certain inferences are drawn about family structure:
- the concept of a 'father' did not yet exist; childrearing was done solely by mothers
- males were not known to have any capability to reproduce
- a person's mother would be regarded as his or her sole parent
- females were thus perceived to spontaneously generate life, as if acausally
- probably differing degrees of sexual bonding existed among mature clan members, depending on individual drives, including heterosexual and homosexual, lifelong and momentary, monogamous and polyamorous, much like the variation of sexual drives today
- evolving human clans share much in common with bonobo troops, likely including the bonobo's use of sex with numerous individuals to form social networks
An associated biological theory postulated that the only reproductive functions the male is specially adapted to are: producing spermatozoa, finding females of childbearing age and potential, and injecting spermatozoa into the female; and that there was no male adaptation for parenting or childrearing, as these were the specialization of the female alone. The Engels theory and its associated concepts such as this were once far more widely believed than it is today, although some theorists such as W. Fell still believe in the anthropological part of the Engels model. Anthropology and biology have several unresolved problems with it:
- If men did not know that they were involved in reproduction, then they were not associated with raising their offspring. This leaves the female to raise the children by herself. This would be extremely difficult in the indigenous environment for a group of human settlers to survive in this model, especially since many mothers would have several young children.
- That a man is necessary for conception would likely be soon discovered in certain situations; such as that a female does not produce offspring when separate from her collective group for extended periods of time.
- Men also appear to have adaptations that cannot be construed to serve any function other than active childrearing.
Theories for behavioral implications
According to one theory, in a hunting/gathering society without agriculture, the role of a male courting a female would play out as follows: the male's sexual interest in the female keeps him around her continuously. The male would give the female and her offspring (even if they 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, in "exchange" for her consent to sexual activity. Due to concealed ovulation, the male never knows when he can abandon sex with the female (without losing a chance to impregnate her and thus reproduce), and so does not keep the nutritious meat for himself until the female is fertile. This theory of reproductive mechanisms in a hunter/gatherer environment dates back to the late 19th century social scientists and is the reason that the "world's oldest profession" is so called.
Concealed ovulation is believed by some theorists 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 the principle would still work because 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 many human females show preferences for different types of males depending on whether or not they are within a few days of ovulation. Human females often prefer less "masculine"-seeming (lower-testosterone) males while not fertile and more "masculine" (higher-testosterone) males while fertile .
Males with visible signs of masculinity (higher testosterone as evidenced by more dominant behavior, a more toned and muscular physique, and pheromonal indicators directly corresponding to testosterone levels) supply "better" genes and a greater statistical chance for the long-term survival of the offspring, and are more likely to r-select. Lower-testosterone men tend to be better supporters for a woman and for children: they have the palaeolithic equivalent of high-paying computer jobs, while the higher-testosterone men are the race car drivers - risk-takers who often suffer premature death, making them unable to support the offspring to maturity. Less visibly masculine men are believed to be more likely to k-select and so are more likely to remain within the pair-bond, supporting offspring. K-selecting males would be involved in childrearing, actively raising their children. R-selecting males would in many cases not even be present to support their children. Their way of reproducing would focus on gaining access to fertile females and, ultimately, conceiving offspring. Rather than having a limited number of children but supporting and protecting each, the r-selecting father is present merely at conception, inseminating a female and having no association with her afterwards. If pregnancy results, the offspring would be cared for solely by its mother. R-selecting males would therefore have more offspring than their k-selecting counterparts. They would not engage in parenting; their contribution would be wholly genetic. An r-selector would not develop pair-bond relationships with his mates; upon finding a female of childbearing age, he would simply make direct advances toward the goal of sexual intercourse and, ultimately, causing her to become pregnant with his offspring.
One theory alleges that the best reproductive strategy for a female in the hunter/gatherer environment is a mixed strategy: to mate when fertile with an r-selecting, high-testosterone alpha male, but convince the beta male, by submission to frequent sexual intercourse throughout the remainder of her menstrual cycle, to remain in the pair-bond. This may include an evolutionary incentive to conceal the female's infidelity. The beta male, usually having less physical strength, would likely be unable to stop a high-testosterone, r-selecting male who is attempting to mate with the female. Rather, the beta male must use indirect means to prevent such occurrences. Such means would be directed against the female who is engaging in sexual acivity with the "intruding" male, rather than against the male himself. The major threat is that the beta male will discontinue his support of the female. Therefore she would have to keep him unaware of her activity with the r-selecting male. The r-selecting male must also attempt to keep him unaware of it, because of the risk of cessation of support to her as the mother would endanger his offspring, with which he is impregnating her. Thus, he inseminates her when the beta male is not present or aware. The offspring is believed by the beta male to be his own, but in actuality is from the r-selecting male. The r-selecting male has thus reproduced without making any investment in the care of the offspring.
There is some evidence that humans may have adapted a reactive tendency of newborns to be more likely to resemble their fathers more than their mothers in order to reinforce pair-bonding fidelity, to reinforce the paternal bond, or to preclude infanticide by a father unconvinced of his paternity .
Conclusion
This apparently "minor" change in humans from other primates, the concealing of fertility, may have major and wide-ranging effects on the composition of human society, encouraging pair-bonding, families and tribes, child nurturing by both parents, division of labor based on sex, taboos on infidelity and, according to some theories, also paving 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. Awareness of 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 typically involved in such pre-industrial warm-climate environments is a low total area cm2 of clothing over the average woman. This would allow the pheromone to drift or broadcast freely from the fertile female, signalling all males in proximity 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 of it by the pheromones put off by relatively uncovered fertile women. (The pheromone is secreted during the fertile part of a female's menstrual cycle, and especially during ovulation, by specialised apocrine glands located mostly in the woman's pubic region and the lower part or 'small' of her back (lordosis), as well as being sparsely throughout the body surface). 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 the phenomenon in which some husbands, who have been part of programs in which couples chart the cycle for purposes of "family planning" or trying to conceive, report eventually being able to perceive when their wives are fertile, while finding it difficult to articulate why.The lag time between menstruation 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.", or similar jokes intended as insults against the Roman Catholic Church. Some observational methods of fertility awareness, however, can have failure rates of less than 1% per year. While fertility is not intuitive to most women, the signs are simple enough almost any woman can be taught them.
Another externally detectable signal of female fertility is the observable set of changes in the mammae of the woman just before she ovulates. Observation of this fertility signal, however, is possible only with direct touch or very close proximity. Pheromonal release, however, can be picked up from a further distance, and through respiration. This characteristic means that pheromones could attract r-selecting males to an ovulating female. It would also be harder for the beta male (as described in the theory earlier) to prevent r-selecting males from inseminating the woman, because they can detect signs of her fertility. The pheromonal signal is also more accurate. Release of the pheromone from a woman reliably indicates that any spermatozoa injected into the woman's reproductive system at the time will likely have a chance to fertilise her ovum.
Also, the greatest incentive for the pair bond is agriculture. The parents need each other to sustain 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 inseminate a fertile woman and have no association with her after that; he must stay because he needs the woman's steady support. Therefore the man is present to raise the children that he fathers. In the hunter-gatherer model the woman was dependent on the man but not vice versa; agriculture balances the stick by making the man dependent on the woman as well. It also prevents any man from reproducing without having to invest himself in parenting. (Even today, rural societies that engage in agriculture tend to have more monogamy than extant hunting-gathering societies in the same regions of the world.)
Another unresolved issue is what used to be the signal, if there was a signal, for ovulation prior to the state of concealed ovulation in human evolutionary history. If there was a signal how did it disappear? A possible explanation is that it was signalled for longer and longer periods outside of the actual fertility period. Eventually the signal was active during the whole cycle, thus loosing its predictive effect.
Hence there are many objections to the theories 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 often to be the case with the ovulation-correspondent pheromone in studies done in many modern countries; the debate is whether this extends to indigenous environments, of which little research has been done. For reasons detailed earlier, it appears that in some indigenous societies the pheromonal indicator is easily perceived by all men in proximity, although very little study has been done. 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
"Concealed ovulation"