The oestrous cycle is the reproductive cycle governing biological changes in most female mammals that are not primates. This cycle prepares the female body for mating and potential conception. It is a sequence of hormonal and physical changes that repeats unless interrupted by pregnancy. The term “oestrus” refers to the specific, limited period when the female is sexually receptive, commonly known as being “in heat.”
The Phases of the Oestrous Cycle
The oestrous cycle is divided into four sequential phases driven by fluctuating hormone levels. The cycle begins with the proestrus phase, characterized by the regression of the previous cycle’s corpus luteum and the start of new follicle development in the ovaries. During this period, oestrogen begins to rise, causing the uterine lining, the endometrium, to proliferate in preparation for a fertilized egg.
The next phase is oestrus, or “heat,” the period of peak sexual receptivity that coincides with the egg’s maturation and release. Oestrogen levels are highest during this time, triggering a surge of luteinizing hormone that leads to ovulation in most species. Behavioural changes are pronounced, as the female actively seeks and accepts the male for mating.
Following ovulation, the cycle moves into the metestrus phase, where the ruptured ovarian follicle transforms into the corpus luteum. This structure begins to secrete progesterone, which is necessary for maintaining a potential pregnancy. The final stage is diestrus, where the corpus luteum is fully functional and progesterone levels dominate. Progesterone causes the uterine lining to become secretory, providing a nutrient-rich environment for a developing embryo.
If fertilization does not occur, the corpus luteum eventually regresses, leading to a sharp drop in progesterone that initiates the next proestrus phase. In some species, particularly seasonal breeders, the diestrus phase is followed by anestrus, a prolonged period of reproductive inactivity. Anestrus is often triggered by environmental factors like a lack of daylight, ensuring the animal does not give birth during unfavorable times of the year.
Variations in Mammalian Oestrous Cycles
The frequency and timing of the oestrous cycle vary significantly across different mammalian species, reflecting their specific ecological niches and reproductive strategies. Animals are categorized based on how often they cycle, with three primary patterns observed.
Monoestrous mammals experience only a single oestrous cycle per year, with a clearly defined breeding season. Examples include dogs, foxes, bears, and wolves, which have one period of heat followed by a long anestrus phase. This strategy ensures that offspring are born at the most optimal time, such as spring, when food availability is highest.
Polyestrous animals cycle continuously throughout the year if they do not become pregnant. Domestic livestock like cows and pigs fall into this group, as their reproductive activity is not limited by seasonal changes. This constant cycling allows for multiple reproductive opportunities annually, which is beneficial in stable or domesticated settings.
The third pattern is seasonally polyestrous, where animals experience multiple cycles only during specific times of the year. These animals are divided into “long-day” breeders, such as horses and hamsters, that cycle when daylight hours increase in spring and summer. They also include “short-day” breeders, like sheep and goats, that cycle as daylight hours shorten in autumn and winter. The amount of light perceived influences hormone production, linking the reproductive cycle to the most favorable season for raising young.
Oestrous Versus the Menstrual Cycle
The oestrous cycle, found in most non-primate mammals, is often confused with the menstrual cycle, which occurs in humans and some other primates. A fundamental distinction lies in the fate of the uterine lining, or endometrium, if conception does not take place. In the oestrous cycle, the endometrium is completely reabsorbed by the body at the end of the cycle, preventing any external discharge of tissue.
In contrast, the menstrual cycle is defined by the shedding of the endometrium when pregnancy does not occur, resulting in visible external bleeding, or menstruation. This difference means that visible bleeding is generally absent or minimal in most non-primate mammals, with a few exceptions like the domestic dog.
Another significant difference is the timing and duration of sexual receptivity. In an animal with an oestrous cycle, the female is receptive to mating only during the brief oestrus phase, or “heat,” a period of heightened reproductive hormones. This short window of receptivity signals that mating should occur because ovulation is imminent.
The human menstrual cycle does not include a defined, limited period of “heat,” as sexual receptivity is not strictly confined to the time of ovulation. The hormonal shifts occur internally without the overt, species-wide behavioral changes that signal the receptive period in oestrous mammals. While the underlying hormonal control involves the interplay of oestrogen and progesterone, the outward manifestation and the handling of the uterine lining are markedly different.