Because stallions are usually kept only for breeding purposes, many horse owners have no experience with them or their sex-linked behaviors. And we certainly don’t expect male displays from our geldings, having had them castrated in large part to eliminate those very behaviors. But geldings do sometimes act like stallions, and it can cause inconvenience, frustration, and even injury to themselves and others. Here we’ll look at some specific behaviors natural to stallions but not expected or wanted in a gelding. We’ll also give you some tips on how to manage or eliminate stallion-like behavior in geldings. 

What Causes ‘Studdy’ Behavior?

When we castrate a male horse, we remove his testes, the source of the male hormone testosterone. Unfortunately, though, some of the greatest effects of testosterone occur long before castration—because colts in utero have very high testosterone levels. The mare’s pregnancy hormones stimulate his gonads, too, so the fetus’ testes are pumping out a lot of male hormones, called androgens.

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These androgens act on his brain to masculinize it. Males have a much larger sexually dimorphic nucleus—that is, the part of the brain that differs from males to females. So the result of this early influence is that many geldings still behave like stallions, exhibiting behaviors such as showing the flehmen response (top lip curled up), trying to breed mares, fighting with other geldings, acting aggressively with people, attacking foals, and/or herding mares.

Interestingly, many of the geldings that exhibit stallion-like behavior are in their teen years. We aren’t sure why these senior geldings act this way more than younger geldings do. One theory suggests a tumor on these geldings’ pituitary glands excretes hormones that may stimulate the stallion-like behavior; another theory suggests geldings increase in confidence and social rank with age, so are more overt in their behaviors overall.

Horse being aggressive to another horse.

“Studdy” geldings may drive other geldings away when mares are also present.

In rare circumstances, geldings aren’t completely gelded. This sometimes happens when the horse is cryptorchid, a condition in which one testis fails to drop into the scrotum. A horse with this condition will often have some physical traits of a stallion, such as a thickened neck.

At the time of gelding, the veterinarian performing the procedure will know that one testicle has been retained. If the horse changes owners and that information doesn’t get passed along, blood tests can determine if he’s cryptorchid. If he is and the owner desires corrective measures, a surgeon will have to search in the abdomen for the retained testicle.

Identifying Stallion Behaviors

Any of the following stallion-oriented actions, or a combination of them, may be part of a gelding’s behaviors. Some are more troublesome or dangerous than others, but if you spot one or more of them in your gelding, you may want to have him tested for cryptorchidism.

Flehmen response. The stallion raises his head, curls his upper lip back, and inhales. This posture helps better transfer scents to a structure known as the vomeronasal organ, a scent-detecting structure inside the nasal cavity.

The flehmen is often seen when a stallion smells a mare’s urine or is simply in the presence of a mare in estrus. Horses of either sex will display the flehmen response under various circumstances, but it’s extremely common among stallions near mares.

Though this action is harmless in geldings and all horses, it may accompany more troublesome behaviors.

Horses checking each other out.

Don’t let “studdy” geldings have even fence-line access to mares.

Fecal marking. Leaving “stud piles” of manure is another harmless stallion-like behavior. Some geldings will defecate not only on their own piles of feces but on mares’ piles as well, just as a stallion will in the wild—his way of making other horses aware of his presence.

Masturbating. Stallions commonly rub their erect penises against their bellies, and geldings may show this behavior, too. Some will even make a thrusting motion.

Working with the horse’s feet can trigger masturbation, as the areas of the brain that record sensory information from the feet are close to those recording information from the genitals. In addition, farriers—and especially their leather aprons—may smell like other horses, including mares in heat. Anyone working under a horse must be alert to the possibility of this behavior.

Mounting mares. This may be the first behavior that comes to mind when you imagine geldings acting like stallions. Beyond inconvenience for horse owners, it can be quite dangerous on several fronts. It can injure nearby humans, the targeted mare (and even potentially an embryo in the early stages of a pregnancy), and the gelding himself, especially if the mare is unreceptive and kicks him.

Guarding or herding mares. This is one of the more common stallion-like behaviors among geldings. An example is a gelding watching over female pasture-mates and refusing to allow anyone to remove them from the pasture by continuously herding the mares out of reach. The gelding may even display the “snaking” motion stallions use, with the head low and ears flattened back, which appears quite aggressive.

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Fighting with other geldings. Just as a stallion in the wild would feel the need to fight off other males to protect his mares, a gelding may fight with other geldings to keep them away from “his” mares, injuring any horse he picks a fight with as well as himself. He can be distinguished from a generally bad-tempered gelding by the fact that he won’t fight with mares, only other geldings, making it a sexually linked behavior.

Attacking foals. In the wild, stallions often kill foals that aren’t their own, and geldings that act like stallions have been known to engage in this behavior (though it’s rare, as geldings seldom have access to foals).

Acting aggressively with people. Stallions can act aggressively toward humans, as can geldings that exhibit too much male sexual behavior. Any person working with or near a gelding like this may be at risk of being bitten, charged, or struck at with a foreleg, especially if mares are present.

When It’s a Mare That’s ‘Studdy’
If a mare exhibits stallion-like behavior, she should be checked by a veterinarian for a granulosa cell tumor on one of her ovaries. Such tumors produce hormones, some of which are androgenic (male) and can cause a mare to display the flehmen response, attempt to mount other mares, fight with other horses, and so on. The ovary containing the tumor can be removed surgically, which should return the mare to normal behavior.

Managing Troublesome Behaviors

Trouble from “studdy” geldings can arise at the barn, in the pasture, in a horse trailer, or at a competitive event, causing potential harm to the geldings themselves, other horses, and people. The two basic approaches to dealing with such geldings are stable-management techniques and medication.

Smart management is the simplest way to prevent dangerous behaviors. We know that some of the stallion-like behaviors, such as the flehmen response and fecal marking, are harmless. The more dangerous behaviors, though, may require changes in your horsekeeping arrangements and routines.

To keep a gelding from mounting mares, herding or guarding them, or fighting with other geldings, pasture him away from mares, even disallowing fence-line contact with them if possible. Without access to mares, the gelding will also be far less likely to act aggressively toward humans.

Be careful when adding new horses to the property, especially mares, as that sort of social disruption can stimulate an otherwise well-behaved gelding to act like a stallion.

In barns, keep the gelding away from mares, as well. If his stall neighbor is a mare, consider moving him to another stall. Stay mindful of his tendencies as you use community crossties, wash racks, and turnout pens, striving always to avoid close proximity to mares.

If for some reason you’re unable to keep the gelding away from mares, you may choose to have a veterinarian run tests to determine if he’s cryptorchid. If he is, you may choose surgery to remove the remaining testicle. Once that’s done, his testosterone levels will drop.

If, as in most cases, the gelding is not cryptorchid, you may choose to have your veterinarian prescribe drugs to reduce your gelding’s anxiety when he’s separated from “his” mares. Sometimes the female hormone progesterone reduces male-like behavior. The drug cyproheptadine acts as an anti-androgen and may also help.

Using one or a combination of these techniques should help solve your issues with a “studdy” gelding, keeping him and everyone around him safe.

Book: From the Horse's Point of View: A Guide to Understanding Horse Behavior and Language with Tips to Help You Communicate More Effectively with Your Horse

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