Mares aren’t inherently mean, but they do experience hormonal cycles, physical discomfort, and social pressures that geldings simply don’t have. Much of what riders interpret as “attitude” has a biological explanation, and some of it turns out to be human bias. Understanding what’s actually going on can change how you work with a mare and, in many cases, resolve the behavior entirely.
The Estrous Cycle Changes Everything
Mares cycle roughly every 21 days during breeding season, with about 5 to 6 days of active heat (estrus) per cycle. What surprises most people is that mares are actually better behaved during heat, not worse. Research tracking mare behavior across cycle phases found that negative behaviors toward other horses were significantly less frequent during estrus. Mares in heat were calmer, stood quietly for longer periods, and were more willing to seek contact with humans.
The difficult phase is dioestrus, the roughly two weeks between heat periods. During dioestrus, mares become more restless, more physically active, and more aggressive toward other horses. Typical dioestrus behaviors include squealing, striking, and kicking, particularly when approached. So when people say their mare is “moody,” they’re often seeing the behavioral swing between these two phases rather than a personality flaw. The mare isn’t choosing to be difficult. Her hormones are shifting her entire behavioral baseline every few weeks in ways that geldings never experience.
Physical Pain Mimics Bad Behavior
A mare that pins her ears when you tighten the girth, kicks at your leg under saddle, or refuses to engage her hindquarters may not be expressing attitude. She may be in pain. Reproductive organs can generate visceral pain that gets “referred” to the skin over the flanks, abdomen, groin, lower back, and the base of the tail. This referred pain creates genuine hypersensitivity in those areas, meaning normal touch or pressure from a saddle, rider’s leg, or grooming brush can feel amplified or outright painful.
In a clinical study of 14 mares with performance problems like kicking against the rider’s legs, bolting, or refusing to move forward, the issues either occurred permanently or worsened during estrus. The mares weren’t being defiant. Stimulation of skin near an area of internal inflammation was triggering pain responses. Tail swishing, frequent urination, and hypersensitivity across the hindquarters are hallmark signs of this kind of discomfort, and they overlap almost perfectly with what many riders call “mare-ish” behavior.
Ovarian problems amplify this further. In a study of sport mares with ovarian disorders, 91% showed increased flank sensitivity and 82% had general riding problems. Some displayed outright aggression toward people or other horses. Back pain related to ovarian sensitivity is well documented in mares, and enlarged or inflamed ovaries can cause abdominal discomfort that produces behaviors easily mistaken for a bad temperament.
Ovarian Tumors Can Cause Extreme Behavior
A small but significant number of mares develop granulosa cell tumors, a type of ovarian growth found in roughly 0.6% of mares. These tumors produce excess testosterone, and the result is dramatic: 58% of affected mares display stallion-like behavior, including mounting other horses, aggression, and territorial displays that look nothing like typical mare behavior. Another third stop cycling altogether, while about 8% show persistent, nonstop signs of heat.
The behavioral change is directly tied to hormone overproduction by the tumor. In one study, elevated testosterone levels dropped back to normal after surgical removal in nearly 73% of cases. If a mare’s behavior shifts suddenly toward aggression or she starts acting like a stallion, a veterinary exam of her ovaries is a reasonable step.
Human Bias Plays a Bigger Role Than You Think
A University of Sydney study surveyed over 1,200 experienced horse people and found something striking: riders apply human gender stereotypes to horses. When given the choice between a mare, gelding, or stallion for a specific task, and told all three were equally competent, more than 70% chose the gelding. Mares were rated as less reliable, less predictable, and less desirable than castrated males.
This bias creates a self-fulfilling prophecy. As one of the researchers put it, if you’ve grown up believing mares are moody or fiery, you approach them accordingly, ride them differently, and the horse responds differently in return. When a gelding disobeys a cue, riders tend to assume the horse didn’t understand and try to retrain the response. When a mare disobeys the same cue, riders are more likely to interpret it as a “bad attitude” and respond with correction or punishment. Over time, this pattern can make a mare more defensive and reactive, reinforcing the very stereotype that started the cycle.
The researchers compared this to the longstanding belief that chestnut (red) horses are inherently more difficult. There’s no evidence for that claim either, but the belief persists and shapes how people handle those horses.
Herd Behavior Is More Complex Than “Bossy Mare”
The popular idea that every horse herd has a dominant “lead mare” who bosses everyone around may not be accurate. Field observations of feral horse herds found that only stallions displayed herding behavior, where one horse drives others ahead of it. When it came to departure behavior, where one horse leaves and others follow, mares of both high and low social rank initiated movement. Leadership appeared to be a shared, situational duty rather than a fixed role held by one dominant female.
This matters because the “alpha mare” concept often gets used to explain why mares seem pushy or controlling. In reality, what looks like bossiness in a domestic setting may just be a horse responding to its environment, not fulfilling some ingrained dominance role. Training methods built around “becoming the lead mare” may be based on a dynamic that doesn’t universally exist in horse herds.
What Actually Helps
For mares whose behavior is clearly tied to their cycle, synthetic progesterone given daily for about two weeks can suppress estrus-related behavior changes. In controlled studies, 100% of treated mares showed a predictable estrus response after withdrawal, and ovulation rates exceeded 91%. This approach is commonly used in competition mares to stabilize behavior during show season. It doesn’t fix the problem permanently, but it removes the hormonal swings for as long as the mare is on it.
Nutritional factors can also play a role in overall irritability. Magnesium supplementation has shown measurable effects on nervous system reactivity in horses. In one study, magnesium alone reduced a pain-related behavior by 52%, and magnesium combined with boron reduced it by 64%. Magnesium helps block certain nerve receptors and inhibits calcium channels that, when overactive, bring nerves closer to their firing threshold. A mare that’s deficient in magnesium may genuinely have a lower tolerance for stimulation, making her seem touchy or reactive when she’s actually dealing with a nutritional gap.
Beyond supplements and hormonal management, the most effective change is often the simplest: ruling out pain. A thorough exam of the ovaries, back, and flank sensitivity can identify problems that no amount of training will fix. Many mares labeled as “difficult” for years turn out to have treatable physical conditions. Once the pain is resolved, the “mean” behavior often disappears with it.