Elevated plasma testosterone concentrations during stallion-like sexual behavior in mares (Equus caballus).

This research investigates and finds a relation between high levels of testosterone in female horses, or mares, and particular sexual behaviors more typically seen in stallions (male horses). Notably, these behaviors, which include mounting, typically occur during a specific phase of the mare’s reproductive cycle.

Research Methods

• The researchers observed a large herd of mares over a period of three years in an attempt to understand whether there was a relationship between certain stallion-like behaviors within the herd and hormonal changes in the mares.
• The mares were separated into three different groups: the mares observed performing mounting behaviors; the mares that allowed the mounting; and control mares that were not involved in the behavior.
• A control mare would be selected based on being in a similar stage of their reproductive cycle to a mare from the mounting group.
• Blood samples from these mares were gathered and subsequently analyzed to assess the concentration of various hormones. Furthermore, an ultrasound of the mares’ reproductive tracts were performed.

Key Findings

• Over the three-year period, 15 instances of mounting behavior were recorded. Of these, the majority (13) happened during the follicular phase of the mare’s reproductive cycle.
• The timing of the mounting behavior in relation to the mare’s ovulation, the diameter of the largest follicles in the mare’s ovaries, and the number of follicles above or below a size of 20mm were found to be statistically equal across the three groups.
• However, the testosterone levels in the group of mares displaying mounting behavior were markedly higher than those in the other groups. The testosterone concentration in the mounting group was significantly higher than in the control group, and the difference approached a level of statistical significance when compared to the standing group.
• No significant differences were found in the levels of other hormones (androstenedione, estradiol, estrone, and progesterone) between the groups.

Conclusions

• The study concluded that mounting behavior amongst mares is rare. However, when it does occur, it generally happens during the follicular phase of the reproductive cycle, and correlates with elevated levels of the hormone testosterone.
• The researchers didn’t note any significant changes in the levels of other hormones, suggesting the behavior is particularly associated with testosterone concentrations.