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Estrous Behavior in Mules
Published: September 09, 2019

For those of you with female mules, you probably have noticed that their behavior can change during their estrous or heat periods, just as it does in some horse mares.  Mules are a cross between a male donkey and a female horse. Female mules are commonly called mollies or mule mares and they actually have higher estrogen levels during their heat period than horses do.  Although there are very few studies on the heat cycle of mollies, it is known that they are sterile but still cycle normally.  With the advent of embryo transfer, mollies can even carry a fetus to term when a horse embryo is implanted in the uterus and can carry the foal to term, deliver the foal, and nurse the foal. 

A recent survey of mule owners revealed that the most common estrous behavioral change in mollies occurs when riding them due to frequent urination, tail whipping and performance issues.  The increased estrogen and decreased progesterone hormones during heat can lead to aggression, anxiety and irritability.  Also, some mollies will even try to steal foals from mares during the times they are in heat and will even try to steal calves from their mothers if in the same pasture with cows.  When these problems occur in mares, oftentimes they are treated with daily progesterone to keep them out of heat, which seems to be effective but is expensive.  There are other methods used in mares, but the most successful method is surgery to remove the ovaries.  This is not used in mares routinely because it prevents them from being bred in the future.  However, since mollies are sterile, breeding them is not a concern.  So, if you have a mare mule that is exhibiting behavioral problems when she is in heat, talk with your veterinarian about removing her ovaries to resolve the issue. 

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