The Evolution of Human Estrus
Written by Ben Bunting: BA(Hons), PGCert.
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Estrus is a time in which a female is sexually receptive, and the hormones released by the ovaries exert their greatest influence on the behavior of the female. Animals in estrus exhibit a variety of physiologic changes as well as sexually receptive behaviors. Among these signs is the lordosis reflex, which elevates the hindquarters. Females in humans, however, do not show this consistent and noticeable signalling trait.
Biological oestrus
The biological oestrus of human females occurs before ovulation during the most fertile phase of the woman's cycle. The term derives from the Greek word oestro, meaning "botfly" and means "to thrill." By 1890, it had come to mean the sexual excitement of a woman during her heat. Today, the term is mostly used in a scientific context, with medical dictionaries trying to distill its meaning.
Oestrus is a distinctive sexuality characteristic in women, with reproductive benefits derived from the fertile phase of the cycle. This sexuality is functionally similar to that of other vertebrates. It debuted approximately 400 Myr ago. During this fertile phase, females are more attractive to males, which may have led to the evolution of a continuous sexuality cycle.
Estrus is interrupted by pregnancy and anestrus phases. A woman's estrous cycle lasts for seven to twenty-one days. The following seven to twenty-one days are the nonovulatory and ovulatory phases. The second phase, known as the non-pregnant diestrus, is characterized by a high level of circulating progesterone.
Females should also be more attracted to males during their oestrus than during other reproductive phases. This shift is moderated by the qualities of the primary partner. Females with primary partners lacking indicators of genetic fitness should be most attracted to extra-pair males during oestrus, while females with primary partners that possess these traits should show no or very minimal shift in female attraction to extra-pair males.
Evolutionary origins
The evolution of human oestrus has long been debated. There are some who believe that it disappeared as a result of an evolutionary change to protect females from men. However, a new study casts doubt on that assumption. A team of evolutionary scientists has used agent-based computational models to show that the loss of human oestrus may have evolved because of a desire to hide ovulation from other females.
Evolutionary studies have shown that humans are not the only species to lack a clear raging rump that shows the presence of oestrus. In fact, oestrus is found in all vertebrates. It debuted in a common ancestor 400 million years ago and was associated with female-specific reproductive capacity in jawed vertebrates. This suggests that the effects of oestrogen on female sexuality may be homologous between vertebrates. Of course, other reproductive hormones also play a role in female sexuality.
It is unclear whether female reproductive hormones differ in different species. However, most authors agree that all female reproductive hormones have similarities. Therefore, the term oestrus is used for reproductive hormones in all vertebrates. It is the most scientifically appropriate term for such a phenomenon.
Oestrous adaptations enable females to guarantee conception, even in unlikely circumstances. For instance, gravid tungara frogs must lay eggs quickly after maturation and accept sperm from low-quality males. However, they prefer sperm from a higher-quality male. This suggests that sexual selection favors males with adaptations to locate and reach fertile females.
Functions
Oestrus is a biological process that occurs during a woman's menstrual cycle. It is the time of her cycle when she is most fertile, and the sexual preference of women during this phase is enhanced. The functions of human oestrus are similar to those of other vertebrate species.
The discovery of human oestrus has potentially revolutionary implications for our understanding of human reproduction. It will require new theoretical frameworks that recognize and incorporate this new biological phenomenon. In addition, it will open up new research avenues. While the oestrus is a fundamental component of human reproduction, it is merely one of many hormones that determine female sexuality.
Oestrus is not only a hormonal process; it has many physiological and behavioral aspects. For example, women are more attractive to men when they are in their fertile phase, as their smell is more appealing. During this period, women are more attractive to men, and their tips will increase accordingly.
During the postpartum phase, the female will ovulate and produce the corpus luteum, a hormone responsible for fertilization. After a woman gives birth, her postpartum estrus will last between fourteen and twenty-four hours. The timing of estrus is essential for reproduction in mammals, and prolonged periods of estrus will increase the inefficiency of reproduction. For this reason, a woman may have to try several times before conception to maximize her chances of conceiving a child.
Effects
Women have a period of sexual excitement known as oestrus, which occurs just before and after the ovulation cycle. It is during this time when females are most receptive to mating. Although the exact mechanism responsible for these changes is unknown, several endocrine hormones have been implicated in its regulation. These hormones include oestrogen, progesterone, and testosterone. Other hormones such as prolactin and oxytocin also affect a woman's oestrus.
Oestrus is a reproductive hormone in female mammals that has evolved to enhance their sexuality. Although the hormones differ from species to species, women exhibit the same functional characteristics. In addition, human oestrus and its effects on female sexuality are often homologous in most other vertebrate species.
Although the human oestrous period is accompanied by a wide range of physiological changes, females have evolved to conceal their reproductive state from males. This reduces the intensity of fertility cues and makes females more likely to reproduce with a male that can successfully provide sperm. The cost of breeding and protecting the offspring limits female reproduction under such circumstances.
The importance of endogenous hormones in human reproductive behavior is still debated. Most studies of the menstrual cycle and sexual behavior focus on Western women living in industrialized societies. However, these studies are not a complete picture of the biology of human female sexuality.
Relationships with men
Oestrus is a period during a woman's cycle during which she is fertile. This fertile period lasts from several days prior to ovulation to the day of ovulation. The human female should be more attractive to men during this period. It is thought that the benefits of genetics and evolutionary pressures shape her preferences during this time.
Studies have shown that women dress more provocatively and attractively near ovulation. However, this shift in sexual receptivity is not restricted to men. Lesbian women also showed an increased interest in sexual contact with women during the ovulatory phase. These shifts in sexual receptivity suggest that a woman is in oestrus.
The discovery of oestrus in women has potentially revolutionary implications for the study of human mating. A new theoretical framework is needed to take this information into account. Some authors have proposed conjectures about the role of oestrus in human relationships, but this research is not definitive.
During the fertile phase, women prefer masculine faces and symmetrical men. A recent study found that men's social displays may also affect women's attraction to men. Using videotapes of men, the study investigated whether these changes in behavior affected women's preferences for men. The results showed that women who were in the fertile phase were attracted to men who were physically attractive, arrogant, intrasexually confrontational, intelligent, and kind.
Evolutionary implications
The reproductive hormones that women release during their fertile phase of the menstrual cycle are known as oestrus, and are found in all vertebrates. This process has been thought to have originated in the common ancestor of all vertebrates (400 Myr ago), and its receptors may be homologous among vertebrates. While oestrogen is not the only reproductive hormone that women secrete, it plays an important role in the development of female sexuality and sexual behavior.
The evolutionary implications of the human oestrus remain unclear. It is possible that the distant vertebrate ancestors of women possessed oestrus, but the components of oestrus may have been altered by selection post-biparental investment in hominins. The reproductive benefits of oestrus are partly obtained through extra-pair copulations (EPC). However, primary investing partners vary in genetic fitness, and females may derive some of the benefits of EPC from unfit primary investing partners.
In other animals, males can detect females' fertility status. This means that females have evolved a means to hide their fertility status. However, in humans, they have been able to conceal ovulation from males, which may have helped them secure male partners and raise children. This concealment of the reproductive status of females may be a result of positive selection on both the male and the female.
The evolution of oestrus is also related to reproductive benefits and the quality of offspring. For example, a female's oestrus affects the quality of her eggs. Women's sexual desire changes during their oestrus. This makes females more attractive to extra-pair men.
Conclusion
Human estrus is considered a unique trait, but this trait was not always present in the most recent evolution of primates. While most other primates have visual signs of ovulation, humans are receptive to mating only during a small part of their cycle. The evolution of this trait has been linked to important changes in social structures and reproductive behavior.
Some animals, such as marmosets, have distinct estrus phases and limit mating behaviour to that stage. However, chimpanzees and gorillas do not display a behavioural pattern that can be directly related to the concept of oestrus. In fact, their extended sexuality is a reflection of material and adaptive benefits.
Human oestrus is an important reproductive trait in women. It has been theorized that women have an innate preference for good-looking males during their fertile phases of their menstrual cycle. This behavior may be a result of selection following biparental investment in hominins. In addition to the reproductive benefits of oestrus, females may also benefit from extra-pair copulations (EPC).
Despite the fact that females have a distinct oestrus period, extra-pair paternity is still very low. In some areas, it is as low as 5%. However, this low rate does not mean that sexually antagonistic coevolution has not occurred. Males may have evolved counter-adaptations that are costly and ineffective, maintaining low EPP rates.
