'Studdy' BEHAVIOR IN GELDINGS
Horse and Rider|Winter 2020
Why do some geldings act like stallions? Such behavior can cause trouble in the barn or out in pasture. Learn what to watch for and tips for safely managing a “studdy” gelding.
KATHERINE HOUPT

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.

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.

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.

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

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.

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