Pregnant Personality

Q: I have a 7-year-old Morgan mare who is pregnant for the first time. This mare lost her dam during foaling and was orphaned for a few days prior to our farm obtaining a nurse mare. She has always been overly friendly and sweet; however, at several months in foal she has become aggressive, nippy, and significantly more active in her stall. When turned out she seems to "turn on" more than usual. Is there an answer to this change in personality?

Clare Simpson, via e-mail

A: Although it is not common, there is a certain percentage of pregnant mares that, like your mare, show aggressive and even male-type behavior during certain stages of pregnancy. In extreme cases, some might tease and mount other mares in the manner much as a stallion would do.

The most likely explanation for this circumstance is that during pregnancy all sorts of hormones are produced by the fetus and placenta. As these hormones are produced and metabolized, sometimes there can be androgens (male-type hormones) in fairly large quantities. Some mares appear to be quite sensitive to these hormones and show some transient aggressive and male-type behavior.

Another cause of aggressive behavior in a mare can be exposure to male hormones produced by an ovarian tumor. These can develop whether or not she is pregnant.

You mentioned that your mare was orphaned. Did you wonder if there was a connection? Assuming that she had a relatively normal life with her nurse mare after only a few days as an orphan, and that she has been fairly normal since then, I wouldn't expect there is a relationship between having been an orphan and this aggression that has developed during pregnancy.

You did say that she was overly friendly, which does go along with the orphan experience, but it doesn't sound like she has gone into the zone of the dangerously friendly orphan, interacting with you as if you were a horse, as some bottle-fed or excessively handled foals do.

It would be great to hear from any readers with large broodmare farms that might have had experience with orphans that grow up to be broodmares.

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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