Teaser Mare Genes Impact Stallion Testosterone, Semen

Research shows that the amount of effort a stallion invests in his ejaculates, referred to as ejaculate economics, can vary based on his attraction to the mare he is intended to breed and the teaser mare. Surprisingly, that attraction can be affected by the genetics of both the mare and stallion.

What makes a mare attractive to a stallion, driving him to put forth his best effort? According to a recent study*, stallions can alter the characteristics of their semen, including both the individual sperm and the seminal fluid in which the sperm are delivered, based on the stage of estrus and a group of genes called the major histocompatibility complex (MHC).

The MHC theory suggests that animals with dissimilar MHCs will be more compatible. “Based on research in over 20 vertebrate species, including horses, the researchers found that matings between animals with MHC-dissimilar genotypes reduce inbreeding and, in turn, reduce the production of low-quality offspring likely to be created with inbreeding,” explained Laura Petroski, B.V.M.S., a veterinarian for Kentucky Equine Research.

According to the research, stallions can detect a mare’s MHC via the olfactory system, primarily the vomeronasal organ, while performing the Flehmen response, a behavior in which horses curl back their upper lip, exposing their incisors.

Previous studies show that stallions have higher testosterone levels and ejaculated more sperm after an eight-week exposure period to MHC-dissimilar mares compared to MHC-similar ones. Whether or not these same responses can occur quickly, after only a brief exposure with a teaser mare, for example, remains unknown.

To examine this issue, Jeannerat and colleagues exposed stallions to MHC-similar and MHC-dissimilar teaser mares for short periods of time: 15 seconds to the teaser mare’s fresh feces (an important source of social cues and information for horses) and then 30 seconds to the mare’s head from a distance of 2 meters (6.5 feet). Ejaculates were then immediately collected using a phantom mare and artificial vagina, and evaluated. Serum testosterone was also measured.

“Even this short exposure to a MHC-dissimilar teaser mare increased serum testosterone and sperm numbers in the ejaculate,” Petroski noted.

The researchers concluded that “males can quickly adjust ejaculate quality relative to a female’s MHC, and this male reaction to the social environment can also affect important characteristics of cold-stored semen.”

Petroski added, “Of course, the overall health and nutritional status of a stallion are also vital to his breeding success. Kentucky Equine Research developed several products to maximize the health of the stallion.”

For example, EO•3 is a marine-derived oil rich in omega-3 fatty acids, including DHA and EPA. Omega-3 fatty acids are known to provide multiple benefits to stallions, contributing to improved semen quality, such as increased sperm motility and mobility. In addition, the antioxidant Nano•E provides a highly bioavailable natural vitamin E to horses through a unique nanodispersion delivery system.

In addition to EO•3 and Nano•E, Australian horse owners should look for Preserve, an antioxidant product that contains natural vitamin E, organic selenium, and vitamin C.

*Jeannerat, E., E. Marti, C. Berney, et al. Stallion semen quality depends on MHC matching to teaser mare. Molecular Ecology. In press.