horse breeding

Who run the horse breeding world?

Mares … at least in some ways.

The results of a new study suggest a mare’s attraction to a stallion—specifically, to his body odors—affects pregnancy success rate.

“Our study indicates that the female organism has a high level of control over reproduction in two ways: through the choice of partner and through the spontaneous resorption of an early embryo, even before gestation can be detected,” said Dominik Burger, DVM, a scientist at the Swiss Institute of Equine Medicine (University of Bern and Agroscope), in Avenches.

A mare’s partner choice is influenced by the way the stallion smells, Burger said. That’s partly because of the major histocompatibility complex (MHC), a set of proteins within certain genes that are also involved in the immune system. The MHC modifies body odor—and females can pick up on that. True to the age-old adage that opposites attract, females in heat tend to show clear preferences for males with MHC that differs from their own.

In their latest study, Burger and fellow researcher Claus Wedekind, PhD, of the University of Lausanne, found that that preference expands even to pregnancy success. Mares exposed to stallions of dissimilar MHC for the period just before insemination until two days afterward were about 20% more likely to be pregnant 14 days post-breeding than those exposed to stallions of similar MHC.

“We found that social stimuli depending on the MHC at the time of ovulation can have a real influence on the success of the gestation,” Burger said.

Study Design

The researchers exposed 191 Warmblood mares, housed in individual stalls in a barn with up to six other mares, to one of several teaser stallions around the time of insemination. In addition, a single stallion lived in the same stable; he had the ability to move freely between his stall and the stable aisle 17 hours a day. The researchers took blood samples to determine the mares’ and stallions’ MHC serotypes.

The researchers artificially inseminated the mares at their time of ovulation with semen from a stallion selected by each mare’s owner; the researchers did not use any of the teaser stallions’ or the resident stallion’s semen. Veterinarians checked the mares for pregnancy via transrectal ultrasound at 14 to 17 days post-insemination.

Study Results

The team found that no teaser stallion had more effect on the mares’ positive pregnancy results as a whole, Burger said. However, on an individual level, the teaser stallion made a huge difference. Mares were much more likely to be pregnant at the two-week check if they’d been exposed to a teaser stallion with dissimilar MHC.

“The mares had no social contact whatsoever with the actual sire of their embryos,” Burger said. “So the mare’s organism would have ‘assumed’ that the teaser stallion was the sire. The odors stemming from the MHC of the teaser stallion had a clear effect on the mares’ fertility.”

This is likely because the mare might spontaneously abort an early embryo if the supposed sire had similar MHC, he said. That might be for evolutionary purposes—preserving genetic diversity by avoiding similarities. But the definite reasons and mechanisms have yet to be clarified, Burger said.

These results could have bearing on mammal reproduction in general, Burger added, noting a recent citation of this equine study in the highly renowned journal Science.

The team is currently working on very practical applications of their research so as to provide breeders with concrete recommendations, Burger said.

The study, “Major histocompatibility complex-linked social signalling affects female fertility,” was published in Proceedings: Biological Sciences.