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 vulva in baboons and bonobos, and pheromone release in the feline family. In contrast, the females of humans and a few other species[1] 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.
Human females[edit]
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.[2][3] It has also been suggested that a woman's voice may become more attractive to men during this time.[4] 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.[5] It may be that a woman's awareness of men's courtship signals[6] increases during her highly fertile phase due to an enhanced olfactory awareness of chemicals specifically found in men's body odor.[7][8]
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).[9] 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).[10] 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.[11] One group of authors has theorized that concealed ovulation and menstruation were key factors in the development of symbolic culture in early human society.[12][13]
As a side effect of bipedalism[edit]
Pawlowski[25] presents the importance of bipedalism to the mechanics and necessity of ovulation signaling. The more open savannah environment inhabited by early humans brought greater danger from predators. This would have caused humans to live in denser groups, and, in such a scenario, the long-distance sexual signaling provided by female genital swellings would have lost its function. Concealed ovulation is thus argued to be a loss of function evolutionary change rather than an adaptation. Thermoregulatory systems were also modified in humans with the move to the savannah to conserve water. It is thought that female genital swellings would have incurred added cost because of ineffective evaporation of water from the area. Pawlowski continues by saying the change to bipedalism in early hominins changed both the position of female genitals and the line of vision of males. Since males could no longer constantly see the female genitals, swelling of them during estrus as a mode of signaling would have become useless. Also, anogenital swelling at each ovulatory period may have interfered with the mechanics of bipedal locomotion, and selection may have favored females who were less hindered by this occurrence. This hypothesis ultimately concludes that bipedalism, which was strongly selected for, caused the physiological changes and a loss of function of sexual signaling through female genital swelling, leading to the concealed ovulation we now observe.
Pawlowski's paper offers views that differ from the other hypotheses regarding concealed ovulation in that it pinpoints physiological changes in early humans as the cause of concealed ovulation rather than social or behavioral ones.[25] One of the strengths of this is derived from the other hypotheses' weaknesses – it is difficult to track the evolution of a behavior as it leaves no verifiable evidence in the form of bone or DNA. However, the fact that the hanuman langurs also display some concealed ovulation and that it is not directly caused by a physiological change to bipedalism may suggest bipedalism was not, at least, the sole cause of concealed ovulation in humans. As stated earlier, it is possible for many elements of different hypotheses to be true regarding the selective pressures for concealed ovulation in humans.