Concealed ovulation

Concealed ovulation or hidden estrus in a species is the lack of any perceptible change in an adult female (for instance, a change in appearance or scent) when she is fertile and near ovulation. Some examples of perceptible changes are swelling and redness of the genitalia in baboons and bonobos, and pheromone release in the feline family. In contrast, the females of humans and a few other species that undergo hidden estrus have few external signs of fecundity, making it difficult for a mate to consciously deduce, by means of external signs only, whether or not a female is near ovulation. Concealed ovulation or hidden estrus in a species is the lack of any perceptible change in an adult female (for instance, a change in appearance or scent) when she is fertile and near ovulation. Some examples of perceptible changes are swelling and redness of the genitalia in baboons and bonobos, and pheromone release in the feline family. In contrast, the females of humans and a few other species that undergo hidden estrus have few external signs of fecundity, making it difficult for a mate to consciously deduce, by means of external signs only, whether or not a female is near ovulation. In humans, an adult woman's fertility peaks for a few days during each roughly monthly cycle. The frequency and length of fertility (the time when a woman can become pregnant) is highly variable between women, and can slightly change for each woman over the course of her lifespan. Humans are considered to have concealed ovulation because there is no outward physiological sign, either to a woman herself or to others, that ovulation, or biological fertility, is occurring. Knowledge of the fertility cycle, learned through experience or from educational sources, can allow a woman to estimate her own level of fertility at a given time (fertility awareness). Whether other humans, potential reproductive partners in particular, can detect fertility in women through behavioral or invisible biological cues is highly debated. Scientists and laypersons are interested in this question because it has implications for human social behavior, and could theoretically offer biological explanations for some human sexual behavior. However, the science here is weak, due to a relatively small number of studies. Several small studies have found that fertile women appear more attractive to men than women during infertile portions of her menstrual cycle, or women using hormonal contraception. It has also been suggested that a woman's voice may become more attractive to men during this time. Two small studies of monogamous human couples found that women initiated sex significantly more frequently when fertile, but male-initiated sex occurred at a constant rate, without regard to the woman's phase of menstrual cycle. It may be that a woman's awareness of men's courtship signals increases during her highly fertile phase due to an enhanced olfactory awareness of chemicals specifically found in men's body odor. Analyses of data provided by the post-1998 U.S. Demographic and Health Surveys found no variation in the occurrence of coitus in the menstrual phases (except during menstruation itself). This is contrary to other studies, which have found female sexual desire and extra-pair copulations (EPCs) to increase during the midfollicular to ovulatory phases (that is, the highly fertile phase). These findings of differences in woman-initiated versus man-initiated sex are likely caused by the woman's subconscious awareness of her ovulation cycle (because of hormonal changes causing her to feel increased sexual desire), contrasting with the man's inability to detect ovulation because of its being 'hidden'. In 2008, researchers announced the discovery in human semen of hormones usually found in ovulating women. They theorized that follicle stimulating hormone, luteinising hormone, and estradiol may encourage ovulation in women exposed to semen. These hormones are not found in the semen of chimpanzees, suggesting this phenomenon may be a human male counter-strategy to concealed ovulation in human females. Other researchers are skeptical that the low levels of hormones found in semen could have any effect on ovulation. One group of authors has theorized that concealed ovulation and menstruation were key factors in the development of symbolic culture in early human society. Evolutionary psychologists have advanced a number of different possible explanations for concealed ovulation. Some posit that the lack of signaling in some species is a trait retained from evolutionary ancestors, not something that existed previously and later disappeared. If signaling is supposed to have existed and was lost, then it could have been merely due to reduced adaptive importance and lessened selection, or due to direct adaptive advantages for the concealment of ovulation. Yet another possibility (regarding humans specifically) is that while highly specific signaling of ovulation is absent, human female anatomy evolved to mimic permanent signaling of fertility. The paternal investment hypothesis is strongly supported by many evolutionary biologists. Several hypotheses regarding human evolution integrate the idea that women 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 that women concealed ovulation to obtain men's aid in rearing offspring. Schoroder summarizes this hypothesis outlined in Alexander and Noonan's 1979 paper: if women no longer signaled the time of ovulation, men would be unable to detect the exact period in which they were fecund. This led to a change in men's mating strategy: rather than mating with multiple women in the hope that some of them, at least, were fecund during that period, men instead chose to mate with a particular woman repeatedly 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 evolutionarily successful. A similar hypothesis was proposed by Lovejoy in 1981 that argued that concealed ovulation, reduced canines and bipedalism evolved from a reproductive strategy where males provisioned food resources to his paired female and dependent offspring. 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 woman is pregnant or in the infertile period of her menstrual cycle do not achieve conception, but do strengthen the bond between these partners. Therefore, the increased frequency of copulations due to concealed ovulation are thought to have played a role in fostering pair bonds in humans.