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Concealed OvulationHuman females, almost uniquely among animals, 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 ___ (more examples requested). By comparison, human females have few external signs of fertility. (In fact, the women themselves usually 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. 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 The male's desire for sex keeps him around the female continuously. The male will barter for sex by giving the female and her offspring foods that are more difficult for the female to acquire alone such as the highly concentrated protein and fat in meat. By concealing ovulation, the male never knows when he can safely forego the sex, 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. Concealed ovulation is believed to influence the evolution of male reproductive strategies and drives as well. If the female is fertile all the time, the male must be interested in sex all the time. Neither knows when sex 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 with in the pair-bond, getting sex and providing for offspring. One theory concludes that the best reproductive strategy for a woman 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 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 pheremone sensitivity is high enough to reach conscious levels in pre-industrial societies because of increased and continuous proximity and because of lower standards of personal hygiene. The lag time between the menstrual cycle and fertility complicates this objection. 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? Mother." External links a theory of concealed ovulation explaining it more prosaically, and not as a social adaptation
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