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