Q: My two 18-year-old Quarter Horse geldings have lived in the same paddock since they were weanlings. One has always been the boss, but has only shown aggressiveness during feeding time by pinning his ears. Recently, however, he actually attacked my other gelding, leaving two fairly severe bites in his withers area. I separated them overnight, then put them back together the following day—but it happened again. Why would my gelding’s aggressiveness escalate so quickly? Is there anything I can do to re-establish a peaceful relationship between my horses?
Margaret Gates, Illinois
A: When horses in a longterm relationship suddenly have a significant behavioral change toward one another, it can be a red flag that something has changed with one of the horses.
First, schedule an appointment with your vet for a thorough examination of both horses to rule out any physical problems that might have triggered your gelding’s sudden aggression. These could include lameness, a vision impairment, or a metabolic abnormality.
If your “victim” gelding has recently lost condition, your “attack” gelding may perceive he’s more likely to win power struggles. Or, if your attacking gelding has lost condition, he may feel less dominant over the other—and the more submissive gelding may not respond to ear pinning. Thus, your boss gelding may have resorted to more drastic aggressive action.
The location of the victim’s bite injuries offers clues about the dynamics between your horses. It may indicate that the victim no longer runs away or submits when the attacker threatens—another symptom that the “boss” horse may have lost standing. Pay especially close attention to your boss horse in order to detect whether anything’s amiss.
Withers bites are often characteristic of stallion fights. If you’ve recently added a mare to an adjacent pasture, your geldings may have adopted stallion-like behaviors toward one another. It’s also possible your geldings have developed sexual behavior between each other, for no apparent reason.
Be on the lookout for other stallion-like behaviors in your geldings such as “marking”—urinating on their own or other horses’ urine, and defecating on manure (called a “stud pile”).
Stallions also commonly exhibit the “flehmen” response, or lip curl, in which they smell something of interest, then throw up their heads and curl their upper lips. This gesture enables a particular smell to stimulate receptors in their brains that release sex hormones.
If you observe any of these stallion-like behaviors in your geldings, especially with increased frequency, you might be able to minimize aggressiveness by administering the hormone progesterone. This is available as an oral preparation, called Regu-Mate®, administered daily. Caveat: This hormonal preparation can be dangerous, as it can be absorbed through the skin. Use only vinyl or plastic non-porous gloves when administering (avoid latex— its porous nature allows the substance to be absorbed through your skin). Ask your vet for assistance if you’re considering this treatment option.
It’s also possible your aggressive gelding is developing Cushing’s disease, a hormonal abnormality caused by a pituitary tumor in the brain. Your vet can perform tests to diagnose this condition.
After evaluating and considering the above factors, reintroduce your geldings across a fence to determine if they’re safe to house together.
Make sure the fence is secure, with no exposed wires or sharp edges. If your geldings interact peacefully with the fence between them, consider reuniting them— only at grain feeding—for a period of time. Start by tying or holding them far apart from one another during feeding. If all goes well, gradually move them closer together. You’ll be rewarding them with grain in each other’s presence, while preventing a fight by keeping them separate from one another.
If they seem to be getting along, turn them loose together, but be prepared to separate them quickly if they become agitated or aggressive toward one another.
If the aggressiveness continues, however, maintain them separately to avoid serious injury.