A Horse Health Puzzle: Mares Who Act Like Stallions

A Horse Health Puzzle: Mares Who Act Like Stallions

Mares occasionally exhibit behavioral traits that are more typical of stallions. There are several possible causes, including increase in blood testosterone levels.

close of a bay horse's eye (Credit: Remy Evans)

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By Dr. Patrick McCue

Mares occasionally exhibit behavioral traits that are more typical of stallions, and these are usually caused by an increase in blood testosterone levels. There are several possible causes.


Pregnant mares experience an increase in testosterone levels in their blood. Testosterone levels begin to rise by the fourth month of gestation and peak around the seventh month.

The source of testosterone is the fetus. During development, the gonads of male and female equine fetuses produce significant amounts of testosterone, which crosses the placenta into the mare’s blood stream.

An elevation in testosterone might result in behavioral changes, including aggression, herd dominance and teasing or mounting other mares. These adverse behaviors generally go away during the last trimester as testosterone levels decrease.

Non-pregnant mares experience fluctuations in testosterone levels during various phases of their estrous cycles. Testosterone levels increase along with estradiol as a mare comes into heat. Levels of both hormones peak near the day of ovulation and subsequently decline. Some mares respond to the increase in blood testosterone levels during estrus by exhibiting masculine behavior.

Ovarian Tumors

Granulosa cell tumors are the most common ovarian tumor of the mare. Approximately 40 to 50 percent of these tumors contain theca cells, which produce large quantities of testosterone.

Mares with granulose-theca cell tumors have high blood levels of testosterone and often exhibit stallion-like behavior. In fact, measurement of blood testosterone is used as a diagnostic marker for the presence of this type of tumor in mares. Concentrations of testosterone remain elevated until the tumor is surgically removed.


Mares receiving anabolic steroids to enhance athletic performance, or for other purposes, can exhibit behavioral changes. Synthetic testosterone derivatives can result in suppression of normal behavioral estrus, increased mounting behavior, increased aggression between horses, increased behaviors such as biting or kicking and stallion-like herding behavior. These behaviors resolve themselves as levels of exogenous androgens decline.


Some horses that look like mares are actually genetically stallions. A male pseudohermaphrodite is a horse that has the phenotypic appearance of a mare, but the genetics of a male and testes instead of ovaries.

The affected horses are cryptorchids that do not produce spermatozoa. However, the testes produce sufficient testosterone to cause masculine behavioral characteristics. The appearance of the external genitalia might range from that of a normal mare to somewhat ambiguous in character. Removing the testes of affected horses will decrease testosterone levels and eliminate adverse behavioral traits.

Dominant Mare Behavior

Horses are social animals, and it is common for one mare to assume a position of dominance and exhibit behavioral characteristics that are associated with their rank or position. Astute horse managers will often be able to tell when the social hierarchy has changed.

A dramatic alteration in the personality of a mare may be a result of a normal physiologic change in testosterone levels or result of a pathologic condition such as an ovarian tumor. If your mare exhibits behavioral changes such as these, consult your equine veterinarian.

Dr. Patrick M. McCue is the equine reproduction laboratory director at the College of Veterinary Medicine & Biomedical Sciences at Colorado State University and was a longtime contributor to The American Quarter Horse Journal.