Male Horses: A Role in Early-Term Abortion?

Early-term abortion in mares is a frustrating problem for breeders, as often the cause is nearly impossible to determine. But a team of researchers from the Czech Republic might have uncovered one possible reason why mares abort in certain management situations. According to lead researcher Ludek Bartos, MSc, PhD, ScD, of the Institute of Animal Science in Prague, mares are more likely to have an early-term abortion if they're pastured adjacent to geldings or beside stallions that are not the sires of their fetuses.

"Fetal loss is a common phenomenon in domestic horses, being usually substantially higher than that in other domestic ungulates (hooved animals) and it has been a puzzle for generations of vets," noted Bartos, who is professor and head of the institute's Department of Ethology.

Their retrospective study idea grew out of an examination of infanticide--killing or attempted killing of young--in another equid species. "By studying infanticide in captive plains zebra, we found out that the probability of fetal death was greatest when the new male joined the herd just after conception and decreased with increasing time between conception and date of the new male introduction," Bartos explained. "The chance of a foal surviving was less than 5% when the male joined the group just after conception and more than 50% when he joined at the time of delivery. Based on this we predicted that something similar could happen in domestic horses."

The researchers surveyed breeders from across the Czech Republic and obtained 81 breeding records from mares of 21 different breeds. The mares were 4 to 24 years old and ranged from maiden mares to experienced broodmares. Artificial insemination was used in 20% of the breedings and the rest were bred via live cover. The team took into consideration normal foaling rates among this population of mares when examining the data.

In 45 of the 81 cases, the mare was transported to a different farm to be bred (Bartos termed this being bred to a "foreign" stallion); the remaining 36 mares were bred to a stallion on their home farm.

Bartos noted that upon return from a successful breeding to a foreign stallion, 13 mares were kept in pastures or enclosures adjacent to geldings or stallions and 32 mares shared pastures with geldings or stallions. Of these mares, 31% suffered early-term abortions: 54% of the mares kept adjacent to geldings/stallions and 22% of the mares sharing pastures with geldings/stallions aborted.

Conversely, none of the mares bred to a stallion at their home farm suffered fetal loss. He noted that more than half of them were trailered frequently while pregnant, so it is unlikely that transportation played a role in the abortions of mares bred to foreign stallions. Bartos noted that these mares bred at home were kept in the same close proximity to familiar stallions and geldings that the mares bred off premises were.

In looking at the data, Bartos noted that that statistically the probability of early-term abortion was seven times more likely when the mare had no male company in her enclosure and one or more home stallions or geldings in an adjacent enclosure.

He and his colleagues believe the basis for this phenomenon could be related to stallion infanticide: In bands of feral horses, dominant stallions have been reported to kill (or try to kill) foals that they didn't sire.

According to the research team, to combat infanticide mares have been reported to partake in "promiscuous matings," or sexual encounters with dominant stallions when already in foal (to other stallions). Such an encounter might "manipulate" the male's assessment of whether or not he sired the foal.

Bartos et al. also theorized that if mares aren't able to partake in promiscuous mating, they will abort their foals if the likelihood is high that the dominant stallion will kill the foal upon birth. This essentially would save the mare the energy of carrying the foal to term. The method by which a mare's body terminates pregnancy in this scenario is unknown, he noted, adding that this phenomenon has already been researched and confirmed in house mice.

He said the inability to partake in promiscuous mating could be key to why more of the mares pastured next to males--as opposed to with them--aborted: "Some respondents to our questionnaire reported an increased, repeated sexual activity either by a home stallion or dominant gelding shortly after the pregnant mare returned from mating with a foreign stallion if released into the enclosure with them. We interpret our results as suggesting that where possible, a mare manipulates the male's paternity assessment by promiscuous mating. If she has no chance to do that she may abort the current fetus.

"Several thousand years of domestication has not been enough to eliminate principles formed by the evolution of the species," Bartos added. "In this respect, the domestic horse does not differ from various other species."

The study, "Promiscuous behaviour disrupts pregnancy block in domestic horse mares," is scheduled to be published in an upcoming issue of Behavioral Ecology and Sociobiology. The abstract is available online.

About the Author

Erica Larson, News Editor

Erica Larson, news editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in eventing with her OTTB, Dorado.

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