Mares Choose Genetic Diversity in a Mate

When it comes to forming couples, do opposites really attract? Or does it boil down to chemistry? New research in Switzerland suggests that, at least for horses, both of these phenomena play into the process of selecting mates. The reactions of mares at different stages of their estrus cycles to various stallions were investigated and compared to the "chemistry"--or more specifically, the major histocompatibility complex (MHC), proteins within a set of genes that are also involved in the immune system--of both the mares and the stallions. What the researchers found was that the more a stallion's MHC varied from her own, the more that mare found him attractive, according to Charles Meuwly, DVM, MSc, researcher for the Swiss National Stud in Avenches and primary author of the study.

Mares choosing

Stallions wait in stalls behind blinded walls in an experimental setting where a mare will select a preferred stallion without being able to see him.
Mares choosing

A small square was left open in each blinded wall so the mare could smell the stallion behind the wall.
Mares choosing

A mare walks through the corridor between blinded stalls in an experimental setting in which she will select a preferred stallion without visual cues.

"MHC, or genes that are related to MHC, seem to play an important role in natural mating," Meuwly said during the presentation of his results at the fifth annual Swiss equine research day, held in Avenches on April 30. "The choice of a partner with very different MHC could mean more heterogeneity and thus more immunological advantages for the offspring."

Nineteen mares were tested both in and out of estrus, Meuwly said. They were allowed to walk freely in a hallway between stalls of six stallions. The amount of time each mare spent at each stallion was recorded in order to see which stallion she preferred. Afterward, the preferred stallion was removed and the test begun again. This was repeated until only two stallions were left and a "ranking order" of stallions could be established for each mare. Once this order was established, Meuwly compared it to the analysis of equine leukocyte antigens I and II, for the evaluation of MHC, for each horse.

In additional tests, the stallions were hidden from view by walls on the stall doors. However, through a small hole in the wall, the mares were still able to smell each stallion.

"Regardless of visual contact, the mares made considerably different choices between when they were in heat and when they were not, with a strong tendency to prefer stallions with significantly different MHC from themselves when they were in heat," Meuwly said.

The results could contribute to new approaches to equine breeding research, including fertility and social and sexual behavior, he said.

The study benefited from contributions by the Institute for Reproductive Biology and Unit for Reproductive Medicine at the University of Veterinary Medicine in Hanover, Germany, and by the Department of Clinical Research, Vetsuisse Faculty, at the University of Berne, Switzerland.

More from the Swiss Equine Research Day:

About the Author

Christa Lesté-Lasserre, MA

Christa Lesté-Lasserre is a freelance writer based in France. A native of Dallas, Texas, Lesté-Lasserre grew up riding Quarter Horses, Appaloosas, and Shetland Ponies. She holds a master’s degree in English, specializing in creative writing, from the University of Mississippi in Oxford and earned a bachelor's in journalism and creative writing with a minor in sciences from Baylor University in Waco, Texas. She currently keeps her two Trakehners at home near Paris. Follow Lesté-Lasserre on Twitter @christalestelas.

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