Horse Side Vet Guide ®

Equine Health Resource

Aggressive or Stallion-Like Behavior in Mare

Code Orange - Call Your Vet at Their First Available Office Hours

Code Yellow - Contact Your Vet at Your Convenience for an Appointment

Code Orange - Call Your Vet at Their First Available Office Hours

  • If this problem seems severe and has come on suddenly.

Code Yellow - Contact Your Vet at Your Convenience for an Appointment

  • If this seems mild or occasional and the horse seems normal otherwise.

Occasionally mares may show aggressive or stallion-like behavior. Abnormal hormone production may be the cause. Rarely, otherwise normal mares (with normal ovaries), can show stallion like behavior toward the end of their heat cycle.

Several abnormal conditions are known to cause this kind of behavior. The most common of these is an ovarian tumor (granulosa-thecal cell tumor) which produces testosterone and other hormones in excess. These hormones cause changes in the mare’s behavior. Some mares that have had anabolic steroids may behave this way too. This behavior is more common in mares that have been on the race track. In rare cases, mares are hermaphrodites and may have testicular tissue causing testosterone production.

WHAT TO DO

Assess the horse’s general health using the Whole Horse Exam (WHE), paying particular attention to changes in your mare’s general body appearance that might correspond to changes in behavior. Check the vulvar area for growths or abnormalities. Share your findings and concerns with your vet.

WHAT YOUR VET DOES

Your vet can evaluate the reproductive tract of a mare showing this behavior and perform necessary blood hormonal tests. Once those diagnostics are done, management plans can be considered. When evaluating these sorts of behaviors, always consider management and handling issues as these factors also affect behavior.

Author: Doug Thal DVM Dipl. ABVP

RELATED REFERENCES

Gastal MO, Gastal EL, Beg MA, et. al. Stallion-like behavior in mares: Review of incidence, characteristics, ovarian activity & role of testosterone. J Equine Vet Sc Sept 2007 27(9): 390-93.

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