Stallionlike Behavior in Mares: The Role of Adrenal Glands (AAEP 2010)

Stallions are commonly known to be feisty, fresh, and sometimes difficult to handle, largely attributable to the testosterone coursing through their bodies. But when mares begin to display aggressive or stallionlike behavior, the reason for the atypical behavior can be less obvious.

At the 2010 American Association of Equine Practitioners Convention, held Dec. 4-8 in Baltimore, Md., Monica Morganti, DVM, aa resident in veterinary medicine at the University of California, Davis, presented a study in which she examined the role the adrenal glands play in causing mares to exhibit stallionlike behavior.

The stallionlike behaviors that mares sometimes exhibit include stallionlike vocalizations, aggressive attitudes towards handlers and other horses, and regular performance of the flehman response. These mares may also have an elevated testosterone level.

Along with a team of colleagues, Morganti set out to determine if the adrenal glands have an effect on the testosterone level in mares that display stallionlike behavior.

The team tested levels of blood serum cortisol, a substance primarily produced by the adrenal gland, in 24 control mares and 29 mares with elevated testosterone levels. All of the mares studied had a history of displaying aggressive or stallionlike behavior. Before testing, Morganti and her colleagues ensured none of the mares were pregnant, and they ruled out granulose-theca cell tumors in the mares (a type of ovarian tumor and the most common cause of stallionlike behavior in mares).

The team analyzed their results and found that mares with elevated testosterone levels also had higher serum levels of cortisol. They also noted that there was a moderate correlation between serum levels of cortisol and testosterone in both groups of mares, meaning that the control mares had lower levels of both substances.

According to Morganti, the data collected suggest that the adrenal gland (specifically the adrenal cortex) could increase both testosterone and cortisol levels in mares, leading to aggressive or stallionlike behavior. There are no treatment options currently available. Morganti also added that stress in mares could add to the aggressive behavior.

About the Author

Erica Larson, News Editor

Erica Larson, news editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in eventing with her OTTB, Dorado.

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