by Megan Stoddard

Created on: October 24, 2010

All female mammals experience fertility cycles. In humans and most great apes, these cycles occur monthly. Many other mammals have annual or biannual cycles, becoming fertile only once or twice a year. Some go through fertility cycles every few months, or have their cycles triggered by the presence of a fertile male.

In humans, the fertility cycle is called a menstrual cycle. In most other mammals, it is called an estrous cycle. In great apes, it may be identified either way. The difference is how the cycles are counted and, to some extent, exactly what occurs.

An estrous cycle is the period from one ovulation to the next. Many mammals go into heat, also called estrus, around the time they ovulate. It is during that time that they are receptive to sexual activity and will seek out males to mate with. Among many mammal species, the females have no desire for sex except during heat, when they are fertile. Males may have no desire either except when in the presence of a fertile female.

Females in estrus show clear signs of fertility. Sometimes, these signs are highly visible. The rumps of female baboons become bright red during estrus. Cats and dogs behave quite differently during heat, as anyone who has ever lived with one not spayed can attest. All females in heat behave in ways that show they wish to mate. Many bleed a little from the vagina during this time. This is ovulatory bleeding, and it can happen to humans, too. When a human woman experiences this, it is usually like light period spotting.

Unlike many of our sisters in the animal kingdom, humans’ desire for sex isn’t restricted to fertile times, a trait we share with our close relatives the bonobos. Women and men alike usually desire sexual activity at any time of the month, not only when the woman is ovulating. However, studies have shown that many women experienced heightened sexual desires around the time of ovulation. Though there are no clearly visible signs of ovulation... women themselves usually do not know when they ovulate... humans experience estrous cycles. However, we call them by the name of their most visible component.

Fertility cycles in human women are called menstrual cycles. Though ovulation mostly passes invisibly, it is impossible for a woman not to notice when she has her period. This time of vaginal bleeding, which occurs once a month, usually lasts for close to a week. During menstruation, the woman sheds the lining of her uterus, which has built up over the previous month in anticipation of pregnancy. Menstruation typically does not happen if she is pregnant. The cycle is considered to start at the beginning of a menstrual period and end when the next one begins.

This is unique to humans. Most mammals reabsorb their uterine lining if they go through a fertility cycle without becoming pregnant. It is easy to tell when they are fertile, but not when their bodies are getting rid of unused uterine lining so as to build it up again in the next cycle. With humans, the opposite is true.

Some female great apes also shed the uterine lining, but they do not bleed as much as humans do. Their menstrual bleeding is more like estrous bleeding, typically very light. Only the female human bleeds enough to need sanitary protection.

Fertility cycles are part and parcel of life as a member of the animal kingdom. Yet menstruation as we know it is unique to humans. While great apes may also menstruate, and all mammals go through ovulatory cycles, most female mammals have very clear fertility signs. For that reason, their cycles are identified by the period of fertility, while ours are named after the opposite point in the cycle.

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