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

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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 stallion-like behavior. Many geldings under pasture herd conditions will show fairly strong stallion-like 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 stallion-like 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