Estrus

Estrus, from the Greek word oistros meaning “sting” or “frenzy,” refers to the time in a female mammal’s sexual cycle when they accept a male and mate. The physiological and behavioral events of estrus are complex and intertwined. For successful mating, it is often necessary that the male and female cooperate. Estrus ensures that both sexes are impelled to mate at the same time—when the female’s body is ready for a pregnancy.

During the days prior to estrus, ova in follicles ripen rapidly within the ovary. Estrogens, produced by cells in the follicle walls, increase rapidly in this phase, causing not only follicular growth but a thickening of the vagina epithelium and the various signs of “coming into heat,” and then of estrus itself. In most species, the ova spontaneously burst from their follicles at the peak of estrus and enter the oviduct, which leads to the uterus, ready to join with sperm. Females of several species, however, including the rabbit and the cat, require the added stimulus of mating for the release of their ova. Once the ova are released, the ruptured follicles begin secreting progesterone in place of estrogen; this hormone prepares the uterus to support a pregnancy. This internal process is regulated through feedback connections between the ovaries and the hypothalamus in the brain. The hormonal effects which make females eager for mating also originate largely in the hypothalamus, sensitizing the nervous system to respond strongly to the presence of and stimulation by their male counterparts.

Variations Between and Within Species

External estrus signals vary greatly by species. In every case, they are largely produced by hormonal influences and are keyed to evoke specific sexual responses from the male of the species. Such signals may be visual, behavioral, auditory, or scent-based, or may draw on senses imperceptible to human observers.

Visual signals are often found among social mammals. These have been most observed in primates. Chimpanzee and bonobo females show a spectacular pink swelling of their genitals during estrus, which recurs every five weeks if pregnancy does not occur. Certain monkeys develop an estrus flush on their faces as well as their buttocks.

Scent and sound signals can alert males who are not initially within sight range of the receptive female. As such, they are effective for species that do not live in groups, although these signals are not confined to such species. Urine markings are among the most common scent signals. A female black rhino in estrus leaves a long trail of scent posts, which the male follows. Other scent signals are partly airborne, like those of domestic cats and dogs. Mating calls are given by many females, such as the female gibbon’s ascending call, which is then answered by the male.

Behavioral changes during estrus are almost universal, as both sexes concentrate on the quest for one or several partners. The initial stages, which may suggest female coyness to an observer, are part of the courtship process. For example, the female cheetah leads several males on a headlong run across the plain, finally selecting one with whom to mate. Pet owners notice a restlessness in their dogs and cats; female cats in heat are especially likely to roam. The penultimate female signal in many species is lordosis, an arched-back posture which allows the male to mount. Mammalian species are either monestrous—having a single estrus period a year—or polyestrous—with several estrus periods recurring annually. For example, dogs are monestrous, while ground squirrels are polyestrous. The latter situation is more common.

Environmental changes can cause variations in these patterns. Such factors include climate shifts, changes in available light, nutrition, and the presence of a new male. In monestrous mammals, the estrus period usually falls at a time when resulting births will take place in the optimal season for young animals to thrive and grow. Domestication may disrupt this pattern. For instance, a female dog typically comes into heat twice a year with no regard to seasons. Her ancestor, the wolf, has one estrus period a year, sometime between January and April, depending upon latitude.

The length of estrus also varies by species: it lasts four to nine days in the mare and only fourteen hours in the rat. In a few species, such as the ferret, estrus lasts for several weeks if copulation has not occurred. There are also infrequent phenomena such as “silent heats,” in which the animal ovulates, but the usual external signs are absent.

Estrus, Evolution, and Ethology

Sexual reproduction provides genetic diversity and adaptability to a species. However, to proceed successfully, it requires a complex series of events. Estrus is central in this process. It communicates to males a female’s readiness at the same time that it implies that female to mate and primes her body for pregnancy. Nature plays many variations on this basic theme, each connecting the species’ estrous cycle to its environment and its whole life cycle.

Because the survival of young mammals requires maternal care, and often that of the father or other adult animals as well, estrus patterns are significant for social structure. In some species, estrus may help secure a pair bond, a polygynous herd, or bonds within a troupe or pack. It can also cause disruption, as when males battle for access to estrous females or a new male takes over a harem and kills the young, hastening their mothers’ next heat.

Humans have known of estrus since the first domesticated animals. Charles Darwin’s On the Origin of Species (1859) credits estrous females’ mate choices with being an evolutionary mechanism, giving many examples. Ethologists’ field studies have recently described many details about species-specific estrus behavior, as well as its role in life cycles and social behavior.

However, direct estrus observations are intricate and sometimes hard to interpret, especially in wildlife. While traditional methods of detecting estrus, like back pressure tests or physical changes visible to the naked eye, are relatively useful, they are time and labor-intensive. Modern technologies continue to emerge that improve scientific understanding of indicators and detection of estrus. For example, scratch cards or pressure-sensitive or touch-sensitive radiotelemetric transmitters attached to a female’s back can detect mounting activity. The scratch cards change color if the animal has been mounted or the transmitters notify the researchers. In cows, monitoring the progesterone levels in the female’s milk indicates her stage in the reproductive cycle. Ear tags and pedometers can monitor activity, which may change when the animal is in estrus, though this is species-dependent. More complex methods involve artificial intelligence, machine learning, and advanced algorithms that use data collected from wearable technology and data taken from animals, such as temperature, to indicate estrus. Using this combination of approaches improves accuracy. These technologies are essential for farmers and conservationists aiming to increase the population of a species.

Principal Terms

Copulation: Mating; the insertion of the male’s penis into the female’s vagina to fertilize her ova

Estrogens: A group of female sex hormones which regulate the estrous cycle

Estrus Cycle: The cycle of females’ bodily changes related to reproductive potential

Ova: Eggs released from the females’ ovaries at the height of estrus

Progesterone: A female sex hormone produced chiefly in the latter half of the cycle

Bibliography

Gentry, Roger L. Behavior and Ecology of the Northern Fur Seal. Princeton UP, 1998.

Hill, Mark. “Estrous Cycle.” Embryology, 24 Dec. 2019, embryology.med.unsw.edu.au/embryology/index.php/Estrous‗Cycle. Accessed 25 June 2023.

Hrdy, Sarah Blaffer. “Why Women Lost Estrus.” Science ‘83, vol. 4, 1983, pp. 72-77.

Kevles, Bettyann. Females of the Species. Harvard UP, 1986.

Sharifuzzaman, M., et al. "Technological Tools and Artificial Intelligence in Estrus Detection of Sows—A Comprehensive Review." Animals, vol. 14, no. 3, 2024, p. 471. doi.org/10.3390/ani14030471.

Small, M. F., and F. B. M. de Wall. “What’s Love Got to Do with It?” Discover, vol. 13, no. 6, 1992, pp. 46-51.

Young, J. Z. The Life of Mammals: Their Anatomy and Physiology. 2nd ed., Clarendon Press, 1975.