What is need of replacement of oestrous cycle by menstrual cycle in the course of primate evolution?
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Answer:
The menstrual cycle has evolved relatively recently and is quite rare even among primates. Instead, most mammalian females have cycles of estrus. Distinguishing characteristics of the menstrual cycle have traditionally included: copious menses, concealed ovulation and extended sexual receptivity. The menstrual cycle consists of several days of menses, which is followed by the follicular phase, ovulation and then the luteal phase. The luteal phase, which is not present in many estrous cycles, is energetically more demanding than the follicular phase. Indeed, it has been suggested that a benefit of menses is that it helps the female conserve energy, by stopping the energetically costly build-up of an endometrium which cannot be fully re-absorbed. Bleeding is associated with reproductive cycles other than the menstrual cycle, but copious bleeding as a result of the sloughing off of the endometrium is unique to the menstrual cycle. In addition to the luteal phase and menses, humans and a few other species have extended sexual receptivity. Females with estrous cycles typically have a narrow window of sexual receptivity and in several species, the act of mating reduces that window. This is unlike the extended sexual receptivity present during the entire menstrual cycle. Along with extended sexual receptivity, "concealed ovulation" has traditionally been thought to be a defining characteristic of a true menstrual cycle.
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However, species vary significantly in the detailed functioning. One difference is that animals that have estrous cycles resorb the endometrium if conception does not occur during that cycle. Animals that have menstrual cycles shed the endometrium through menstruation instead. Another difference is sexual activity. In species with estrous cycles, females are generally only sexually active during the estrus (oestrus) phase of their cycle. This is also referred to as being "in heat." In contrast, females of species with menstrual cycles can be sexually active at any time in their cycle, even when they are not about to ovulate.
Humans have menstrual cycles rather than estrous cycles. They, unlike most other species, have concealed ovulation, a lack of obvious external signs to signal estral receptivity at ovulation (i.e., the ability to become pregnant). There are, however, subtle signs to which human males may favorably respond, including changes in a woman's scent[1] and facial appearance.[2] Some research also suggests that women tend to have more sexual thoughts and are more prone to sexual activity right before ovulation.[3][4] Animals with estrous cycles often have unmistakable outward displays of receptivity, ranging from engorged and colorful genitals to behavioral changes like mating calls.