Horses do not menstruate like humans. Mares do not shed their uterine lining, which is characteristic bleeding. Instead, they undergo an estrous cycle, also known as being “in heat.” This cycle prepares the mare’s body for breeding and pregnancy through hormonal and physiological changes.
The Equine Estrous Cycle
The equine estrous cycle, lasting approximately 21 to 22 days, is the mare’s reproductive rhythm during her breeding season. Mares are seasonally polyestrous, cycling during periods of increased daylight from late spring to early fall. The entire cycle is divided into two main phases: estrus and diestrus.
Estrus, also known as “heat,” is when the mare is receptive to breeding with a stallion. This phase typically lasts between 2 and 8 days, though it can be longer in early spring. During estrus, follicles on the mare’s ovaries enlarge, producing estrogen, which causes the mare to display receptivity. One dominant follicle usually matures to 30 mm or more before ovulation, the release of the egg.
Following ovulation, the mare enters diestrus, when she is not receptive to a stallion. This phase is more consistent in length, lasting about 14 to 16 days. After the egg is released, the ruptured follicle transforms into a corpus luteum, which produces progesterone. Progesterone prepares the uterus for potential pregnancy and prevents the mare from showing signs of heat. If pregnancy does not occur, the corpus luteum regresses around day 14 of diestrus, triggering a decrease in progesterone and allowing a new cycle to begin.
Key Differences from Menstruation
A primary distinction between the equine estrous cycle and human menstruation lies in the uterine lining. In humans, if pregnancy does not occur, the thickened uterine lining is shed, resulting in menstrual bleeding. Mares do not shed their uterine lining. Their endometrium, the inner lining of the uterus, thickens for pregnancy but is reabsorbed if conception does not occur.
Another difference is the timing of sexual receptivity. Mares are receptive to breeding only during the estrus phase, when they are fertile and close to ovulation. In contrast, human females can be sexually active throughout their menstrual cycle, though fertility is confined to a specific window. The mare’s “heat” behaviors directly signal her readiness to conceive.
Uterine preparation also differs. While both cycles prepare the uterus for a potential embryo, the mare’s system reabsorbs uterine tissue if pregnancy is not established, avoiding outward shedding. This physiological distinction explains why mares do not experience a “period” with visible bleeding.
Recognizing a Mare’s Cycle
Observing a mare’s behavior provides clear indicators of estrus. Common signs include frequent urination, often in small amounts, sometimes accompanied by squatting. Mares may also “wink” their vulva, a rhythmic opening and closing of the vulvar lips. Tail raising or holding the tail to one side is another behavioral cue.
Mares in heat often show increased interest in stallions or geldings, sometimes seeking them out. They may adopt a wide stance with their hind legs and round their hindquarters, indicating receptivity. Behavioral changes can also include irritability, distractibility, or increased sensitivity, sometimes with squealing. While these signs are common, their intensity can vary among individual mares.