Mares can get a bad rap for recalcitrant estrous behavior, but hormones and tumors could also be at play

Anormal mare should behave just like any other horse. So says Paula Hitzler, longtime farm manager at the Michigan State University Horse Teaching and Research Center, in Lansing.

Over her 35 years at the center, home to one of the nation’s oldest Arabian horse breeding programs, Hitzler has seen herds of as many as 125 horses. When The Horse spoke with Hitzler in early 2024, the herd size was 68. Though just 15 of those horses were broodmares, most of the horses kept at the farm are mares because colts are typically sold after they’re started under saddle.

“In my opinion, (with a) normal mare, you should not know by working with her if she’s a mare or a gelding,” Hitzler says. “She should just be a solid citizen.”

Even in estrus it’s rare for otherwise pleasant, healthy mares to display sour temperament, Hitzler says.

“I don’t think that very many mares are ‘mareish,’ ” Hitzler says. Behaviors such as being grouchy, showing signs of heat around people, or urinating while being groomed—“I don’t see very much of that.”

Rather than ignoring or blowing off grouchy behavior as a normal side effect of heat cycles, Hitzler sees “bad” behavior as a potential warning sign that something has gone awry in the mare’s body; maybe the mare’s hormones are out of whack, or maybe the mare has an ovarian tumor. And, of course, mares can experience other ailments, such as injuries, gastric ulcers, or infections. Authors of a 2023 case series on behavioral disorders in mares with ovarian disorders reported that veterinarians must exclude nonreproductive causes of unwanted behavior and reduced performance, such as urinary tract disorders (vaginitis, pneumovaginitis, cystitis, and urolithiasis) and low-grade musculoskeletal pain. Because owners mentioned poor rideability, vets must also rule out rider factors¹.

What Is ‘Normal’ Behavior for Mares During Estrus?

Healthy breeding-age mares spend 15 to 17 days out-of-heat and five to seven days in heat, spring to fall (some sources say 14 to 16 days). Signs that a mare is in heat include receptivity to a stallion (standing when he’s around, for example), frequent urination, raising the tail, and “winking” the vulva.

Whether you find that behavior annoying or problematic depends partly on your perspective and priorities.

If you’re looking to breed your mare, obvious signs she’s in heat make your job easier—so long as the signs of heat reflect reality. If the mare is showing signs of estrus but is not in heat, then timing insemination or breeding could prove tricky.

While Hitzler’s perspective is that grouchy, sour estrous behavior isn’t super common among healthy mares, there’s long-standing scientific literature that says many horse owners feel otherwise. For people who show or compete their mares, when estrous signs do include being difficult to ride or handle, that can be frustrating.

“I did have one grouchy mare we put on Regu-Mate (altrenogest) while we were riding her just to suppress estrous behavior so she wasn’t so grouchy,” Hitzler says.

The drug keeps mares from coming into heat, and it can be useful during competition season.

Evaluating the Mare

If you’re concerned about mareish behavior, pay attention to the frequency, note it on your calendar, and call your veterinarian.

After assessing behavior, practitioners typically examine the mare’s reproductive tract, via palpation and ultrasound, for any irregularities, checking the ovaries’ appearance, relevant structures during the breeding season, and hormone levels to determine her heat cycle stage.

Trouble Ovulating?

When a mare seems to be in heat for longer than expected, it could be she is having trouble ovulating.

It turned out that one of Hitzler’s few “grouchy” mares that pinned her ears during cinching and was irritated by rider leg pressure was in persistent estrus—basically, the ovarian follicle was still developing and had not released the oocyte.

“Upon evaluation using rectal palpation and ultrasound, a persistent follicle was observed,” Hitzler recalls. “Thankfully, a dose of hCG resolved the issue. Sometimes the persistent follicle will not respond to medications. We were thankful that the mare did not have an ovarian tumor.”           

Ovaries and Tumors

Indeed, ovarian tumors can cause behavioral issues, and vets can confirm these growths with ultrasound or hormone testing.

Removing an ovary when there’s a tumor is an obvious choice. Some owners remove ovaries based on behavioral problems even when a tumor has not yet been confirmed.

In the July 2023 study mentioned above, researchers at the Veterinary Teaching Hospital of the University of Teramo, in Italy, reviewed clinical records of mares with a history of problematic behavior that had one or both ovaries removed. Researchers followed up with the mare owners and found they reported improvement in the horses’ behavior. Researchers noted histological exams later revealed granulosa cell tumors (GCTs) or granulosa teca cell tumors (GTCTs) on most of the removed ovaries.

Complaints from the mares’ owners had involved behavior problems during riding.

“They really don’t appreciate when you just try to ride them,” says Giulia Guerri, DVM, PhD, a specialist in equine medicine and surgery, resident ECVDI (European College of Veterinary Diagnostic Imaging), and a researcher in the Equine Medicine and Surgery Section at the University of Teramo.

Guerri was one of the authors of the study who cited “increased sensitivity at both flanks, general problems when ridden, and unwillingness to engage the hindquarters when ridden” as just a few of the problems common to the mares that had ovaries surgically removed.

When the veterinarians found tumors, “common behaviors were also biting or kicking other horses, kicking humans, bucking at work, pulling away from contact when asked to engage the hindquarters, and unwillingness to engage.”

Keep in mind that affected ovaries can become quite large. Normal ovaries are only a couple of inches in length, but a tumor can make them the size of a soccer ball or bigger.

“You can imagine that it’s also a problem during the riding,” Guerri explains. At that size, the ovaries pose a mechanical issue. “After the removal, you see really quick improvement (in) the behavior.”

Tumors can also disrupt hormones.

“The tumor involves specific cells of the ovary that are hormonally active,” Guerri says. “These cells can overproduce hormones (usually inhibin, testosterone, and anti-Müllerian hormone) leading to hormonal imbalances that cause ‘misbehavior.’ ”

In the case series study Guerri and her co-authors wrote, “The best option to diagnose GCT/GTCTs before surgical removal is a hormonal assay to measure the blood concentration of anti-Müllerian hormone. This is secreted by the granulosa cells in the ovary and has recently gained popularity as a potential biomarker for GCT/GTCTs.”

In an Equine Veterinary Journal study², University of California, Davis, researchers reported they were less likely to attribute bad mareish behavior to hormone problems: “Of the abnormal behaviors, stallionlike behavior was the only one that was found to have significant association with increased concentrations of the tested hormones,” the authors wrote.

UTIs and Kidney Infections

Urinary tract and bladder infections in mares can lead to frequent urination, posturing, and perineal irritation, often mistaken for estrous behaviors. Veterinarians typically examine and then treat affected mares with antibiotics to resolve these infections. 

Ridden Mares vs. Broodmares

Riding or exercising your mare can provide additional opportunities to detect that something’s bothering her. Grooming, tacking up, or riding all potentially involve touching the mare’s flanks. As Guerri points out, that pressure might be unpleasant.

Sport horse mares are common in these studies of estrus, ovaries, hormones, and how they impact mare behavior, she says. Returning a mare to productive under-saddle work is a frequent concern.

For owners of broodmares that exhibit problem behavior, there is hope; in the case series articles the authors reported that mares with just one ovary removed were able to get pregnant.

If you’re no longer riding your mare—whether she’s retired to pasture-puff status or is a broodmare—you’ll have to rely on other methods to detect unusual estrous behavior.

Entering estrus in the winter months would be one clue, Hitzler points out. You can also keep an eye out for how she behaves around geldings. Consider shifting turnout arrangements if your mare is sometimes receptive to geldings on a shared fence line. Horses could get hurt when a mare is considerably less enthused about a male horse’s attentions.

“You have to understand behavior,” Hitzler explains. “You have to understand whether this behavior is going to be dangerous for the horse.”

Reading ear, head, neck, and tail expression can help identify problems, she adds.

Other approaches, such as keeping track of food consumption, body condition score, and herd dynamics, might also provide insight.

Your Mare is Unique      

Mares are individuals. Some are spicy by nature while others are personable and sweet. Familiarize yourself with your mare’s habits and demeanor.

Hitzler recommends watching your mare’s facial expressions for clues about her mood or health.

So many people look at horses’ legs, shoulders, and feet while working with them on the ground.

“Look at their faces,” Hitzler urges. “Their faces tell you an awful lot, whether they’re content, whether they’re worried, whether they’re afraid. And then that also coordinates with the ears and neck height and body posture and tail. So, they all go together. But I think a lot of people forget that there’s a lot of facial expressions that go on with horses.”

Take-Home Message

If your mare is acting sour, there might be a health-related reason. Normal estrous behaviors include receptivity to a stallion, frequent urination, raising the tail, and winking the vulva. When estrous signs include sensitivity on the flanks, poor behavior under saddle, tail swishing, and squealing, you might consider chatting with your veterinarian.

References

1. Straticò. P.; Hattab, J.; Guerri, G.; Carluccio, A.; Bandera, L.; Celani, G.; Marruchella, G.; Varasano, V.; Petrizzi, L. Behavioral Disorders in Mares with Ovarian Disorders, Outcome after Laparoscopic Ovariectomy: A Case Series. Vet Sci. 2023 Jul 25;10(8):483. doi: 10.3390/vetsci10080483. PMID: 37624270; PMCID: PMC10458155.

2. Huggins, L. Norris, J. Conley, A. Dini, P. Abnormal mare behaviour is rarely associated with changes in hormonal markers of granulosa cell tumours: A retrospective study. Equine Vet J. 2023. doi.org/10.1111/evj.13967.