These include:

Ovarian tumors – The most common ovarian tumors are granulosa cell tumors. About 40 to 50 percent of granulosa tumors contain theca cells, which produce large amounts of testosterone and can cause a mare to act like a stallion. Testosterone levels will remain high until the mare has the tumor surgically removed.

Pregnancy – Pregnant mares have elevated testosterone levels while their fetus produces gonads; the testosterone crosses the placenta into the mare's bloodstream and can affect her behavior. Mares begin to have increased testosterone levels around their fourth month of gestation and these levels peak around month seven. By the last trimester of pregnancy, as testosterone levels decrease, mares tend to become less aggressive.

Mares that aren't pregnant can experience testosterone fluctuations during their estrous cycles.
Steroids – Anabolic steroids can suppress estrus and increase stallion-like behavior, including aggression. These behaviors resolve once steroid use is terminated.

Pseudohermaphrodism – Though rare, pseudohermaphrodism in horses occurs when a horse looks like a mare, but has the genetic makeup of a male horse. These horses have testes instead of ovaries; they are cryptorchids that don't produce spermatozoa, but the testes produce enough testosterone to affect the horse. Horses that are pseudohermaphrodites may have female genitalia. Removing the testes will eliminate stallion-like behavior.

Read more here.

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Dr. Brian Nielsen has conducted multiple studies funded by the American Quarter Horse Association (AQHA) Foundation regarding bone mass and his findings have been consistent – and surprising. Though horses kept in stalls may be muscularly strong, their bones may be weak. Bone mass and strength can begin dissipating in as little as two weeks.

Though living outside 24/7 isn't a necessity to healthy bones, the ability to sprint is. A study of Nielsen's out of Michigan State University involved long yearlings kept on pasture that had normal bone growth. Half the horses were  put in stalls and walked on a walker for an hour every day. The other half remained living outside.

The researchers monitored the horses' bone mass and discovered that horses living in the pastures continued to increase cannon bone mass, but the stalled horses had an immediate drop in bone mass which persisted for the next three months.

The horses then began under-saddle training as 2-year-olds. During their two months of walking, trotting and cantering under saddle, their cannon bones were monitored. The exercise was not enough to cause an increase in stalled horses' bone mass. Bone mass in the stalled horses decreased to the point where it was less than when the study began.

More studies found that sprinting – short, intense running sessions – was key to healthy bone that increased in mass. Horses must have the ability to sprint on their own or while in training, or they will lose bone mass.

Sprinting exerts force on the bone and causes it to bend, encouraging the body to lay down more bone and make the bones stronger. If horses are not allowed to sprint and their bones are prevented from bending, the body will actually take up minerals, believing the bones are strong enough for the work being asked of them.

For horses that are engaged in high-impact activities like racing and jumping, keeping horses in stalls may contribute to injuries as the horses lose bone mass. Additional research shows that adult horses who are stalled also experience bone loss.

Read more at AQHA.

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