Maybe not, according to researchers on a recent study … unless she’s acting like a stallion.

Lauren Huggins, VMD, MS, an associate at Select Breeders Services in Chesapeake City, Maryland, shared findings from her recent study on abnormal mare behavior while at the 2022 American Association of Equine Practitioners Convention, held Nov. 18-22 in San Antonio, Texas.

In her retrospective study as an equine theriogenology resident at the University of California, Davis, School of Veterinary Medicine, Huggins and her colleagues looked at 31,981 hormone samples submitted to the UC Davis endocrinology lab from 2011 to 2020. About 10% belonged to mares with a history containing the words “behave/behavior/behaving.”

Abnormal behaviors she said the mares’ owners reported included:

• Stallionlike behaviors, such as becoming studdish, vocalizing, or mounting other mares.
• Aggression, attacking other horses or humans, kicking, and biting.
• Estrous behaviors that were more pronounced or displayed at inappropriate times.
• Inconsistent or persistent estrous cycles.

The study, said Huggins, casts light on the assumptions horse owners often make about mare behavior and hormone levels, especially given that previous study results have suggested owners are satisfied after mares have bilateral ovariectomies (removal of both ovaries), even when preoperative testing did not reveal neoplasia (a tumor).

“So this kind of begs the question, is the procedure truly causing an improvement of these mares’ behavior, or is it merely causing a perception in the owners that, having done this procedure, this mare has now improved?” Huggins asked.

Diagnosing Granulosa Cell Tumors in Mares

Granulosa cell tumors (GCT) represent up to 85% of reproductive tract tumors in mares and are associated with abnormal behaviors similar to those described above. Typically, this diagnosis leads to ovarian removal; however, 20-30% of mares continue to show estrous behavior once both ovaries are gone.

A GCT diagnosis often begins with a behavior complaint followed by a multimodal diagnostic approach. The classic sign of GCT is finding one very large ovary and a small contralateral ovary on palpation, though sometimes the ovaries are normal-sized. The ovulation fossa (the concave side of the ovary) might be abnormal or obliterated. In most cases ultrasound reveals multicystic or honeycomblike structures.

Granulosa cell tumors can produce the hormones AMH (anti-Müllerian hormone), inhibin, and testosterone, making bloodwork an important diagnostic tool.

“So given that we know that a GCT can cause these abnormal behaviors that owners are complaining about and that they produce these hormones that we can readily assay, we designed the study to determine the incidence of abnormal behaviors in mares and their association with increased concentrations of these reproductive hormones,” Huggins explained.

Most of the Time, It’s Not the Hormones

The research team ran GCT panels—which test for AMH, inhibin, and testosterone—on samples from the 2,914 mares with a history of abnormal behavior.

“Overwhelmingly, 86% of these cases did not have a single hormone that was in this GCT-like range,” Huggins said. “Only 14% of these cases did.”

She and her colleagues also didn’t find a link between any given hormone and the behaviors.

Huggins said she personally read “every word” of the histories for all 3,000 samples with abnormal behavior and classified the behavior as stallionlike, aggression, estrus, or a catchall abnormal category.

“For all four hormones, when GCT-like values were present, stallionlike behavior was the most significantly reported abnormal behavior by owners,” Huggins said, calling it their most important finding.

Ultrasound and palpation results indicative of GCT correlated with the hormone levels, she added.