Mare Expelled

Mare Expelled

A gelding might display stallionlike behavior toward a mare if she produces male-type hormones.

Photo: Thinkstock

Q. I have four horses—three mares and one gelding—that are pastured together. Three of them are 9 years old and one mare is 21 years old. The gelding and one mare were raised together, and when they were 3 years old I brought in the older mare. Last year my gelding decided to put the older mare out of the herd. He will run her to the point of both of them having to stop for air. He will not let her anywhere close to the other horses. Have you heard of this, and is there anything I can do to get them all happily together again?

Jeannine Key, via email

A. Yes, I do hear about this type of situation fairly frequently. Of course, each particular situation requires a detailed evaluation to understand enough to offer an informed opinion. Having said that, one explanation that comes immediately to mind is that your gelding’s behavior might represent residual stallionlike behavior. Many geldings under pasture herd conditions will show fairly strong stallionlike behavior, depending on the social conditions. And, for many reasons, a gelding’s behavior can change over time, as social stimulation and/or inhibitions change.

So why would your gelding develop stallionlike behavior so suddenly? And why would he select this one mare, even though he had not chased her in the past? One thought is that she has developed ovarian senescence—when an aged mare’s ovaries stop producing the cyclic hormones that turn their sexual attractiveness and receptivity on and off. Without the ovary’s progesterone turning off the mare’s sexual attractiveness, males might perceive her as continually in estrus. The chasing might represent the male trying to separate her from the herd for breeding when she is not exactly willing.

It would be interesting in this type of scenario to know if the aged target mare has undergone ovarian senescence. If that is the case, it would then be interesting to know whether the gelding’s guarding of the mare would diminish if she were treated with progesterone. Those questions are of academic interest and probably would not translate into a practical solution. Hormone treatments are expensive and time-consuming to administer. Simple separation is probably the most practical and effective method for situations such as yours. If the gelding can still see the mare, he might still run the fenceline. If he is in with the other mares, he might also try to keep them away from the area nearest the older mare.

Another possible scenario, again involving a hormone explanation, is that the older mare has started producing male sex hormones called androgens, and that the chasing is an effort to keep her away from his “other females,” as a harem stallion would repel competing males. Normal aged mares do not develop male-type hormones, but this can result from an ovarian tumor that produces testosterone at levels sufficient for a mare to be perceived by other horses as a stallion. If you want to explore that possibility, your veterinarian can help.

About the Author

Sue McDonnell, PhD, Certified AAB

Sue M. McDonnell, PhD, is a certified applied animal behaviorist and the founding head of the equine behavior program at the University of Pennsylvania's School of Veterinary Medicine. She is also the author of numerous books and articles about horse behavior and management.

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