What you see. The starting point for addressing any equine health related issue is your observation.


Aggressive or Stallion-Like Behavior in Mare


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.

  • 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.

your role


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.

your vet's role

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.
Questions Your Vet Might Ask:
  • When did you first notice the behavior?
  • What is the mare's age, breed and history?
  • When do you seem to notice this behavior?
  • What are the specific behaviors the mare is showing?
  • When was the last time you felt that the mare's behavior was "normal"?
  • What are the results of the Whole Horse Exam (WHE)?
  • Does the horse have a history of having been on the race track?
  • How old is the mare?
  • What is the mare's reproductive history?
  • Do you notice moodiness of your mare during the winter also?

Diagnoses Your Vet May Consider

The cause of the problem. These are conditions or ailments that are the cause of the observations you make.

Very Common
Less Common
more diagnoses

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.

Author: Doug Thal DVM Dipl. ABVP