Czech scientists uncover reason behind high incidence of abortions in mares

In this month’s edition of Science Journal: A team of Czech researchers may have found the answer to a question that has puzzled veterinarians, horse breeders and biologists for decades – why such a high percentage of pregnancies in mares end in natural, chemically triggered abortions. A recent study released by a Czech scientist suggests the answer may be that keeping pregnant mares close to stallions at their home stable makes them more likely to abort.

Dr. Luděk Bartoš feeds a mare at a stable on the outskirts of Prague. Female horses are also the subject of his latest study into the question of the species’ high abortion rates. Dr. Bartoš’ team from the Institute of Animal Science in the Czech Republic recently published their surprising findings in the journal Behavioral Ecology and Sociology.

“We found evidence that bringing a mare out of her home stable for mating and bringing her back when she is already pregnant, to an environment containing either stallions or geldings, increases the probability for abortion.”

It is common practice among horse breeders to send mares away from their home stable to be mated with stallions. But once pregnant, the new study suggests, they often engage in promiscuous sex with the males when they return to their home stable, says the researcher.

“The mare tries to mate with the local stallion to convince him that her future foal is actually his. When she fails to attract him, or get into contact with him, she aborts.”

This peculiar sexual behavior may have evolved as a response of certain species to infanticide – when a new dominant male in the group will kill the offspring of other males. However, the high abortion rate amongst horses has puzzled experts for a long time, says Dr. Bartoš.

“This has been a question for generations of veterinarians for about forty years. The average percentage of abortions is reaching 40 percent, which is unusually high in comparison to other domestic ungulates, such as cattle, pigs, goats, sheep, etc.”

Dr. Luděk BartošDr. Luděk Bartoš Chemically triggered abortions do not just occur in horses. Dr. Bartoš and his team of researchers were inspired by a discovery of zoologist Hilda M. Bruce, he explains.

“Originally, we were inspired by the so-called Bruce effect, which was discovered in mice, in the late 1950s. When a pregnant mouse comes into contact with either a dominant male, or even just the scent of his urine, she aborts.

“And it took forty years until they discovered this principle, and several teams globally had to contribute to this research, and very briefly, this is the mechanism: when the female sniffs the urine of the dominant male, she gets pheromones contained in the urine into her Jacobson organ, which is the organ that analyses pheromones, this triggers the whole cascade of processes leading to changes in prolactin profiles, resulting in abortion.

“Originally, we were expecting the same. But in our case, it cannot be like that, because those whose incidence of abortions was the highest were actually physically isolated from males, so that they couldn’t come in touch with urine.”

In the case of horses, mares may abort foals even if they are kept close to male horses that have been castrated, he says.

“Not too many breeders have a real stallion, and this is also a very important question, that the mare does not distinguish if the male who is present is an intact male, a stallion, or a castrated male, which is a gelding. So it is very common to have mares close to geldings, and we found out that the effect of the gelding is equal to that of the stallion.”

Dr. Bartoš’ study, based on a survey of 100 Czech horse breeders, suggests that the current breeding practices may be the reason for this high incidence of abortions. If pregnant mares are close to but physically separated from a stallion or gelding, and therefore unable to disguise the origin of their offspring, they will abort their foals. But breeders are hesitant to adjust their techniques to the findings, says Dr. Bartoš

“At this stage, we have already been approached by various breeders, but it is usually ‘What should we do?’”

And what is your answer to that question?

“The answer doesn’t make them happy, because it’s quite obvious; the best way is to keep the horse and the stallion at the same environment for the whole period of reproduction.”

However, for most breeders this is an inconceivable notion – horse pregnancies last eleven months and keeping the stallion with the mare for this period would be very expensive. Another possibility would then be to allow mares to mate with males at their home stable. Lenka Skoupá, who breeds her horses using this technique, says she expects other breeders will be slow to embrace Dr. Bartoš’ findings.

“I think that breeders will perceive this as very problematic, because very few would be ready to lend a stallion to another stable. So I can understand that stud farm owners do not like this method of breeding, do not have an understanding for it and do not want to support it.”

Dr. Bartoš says he has been overwhelmed by the great response to his study from both the scientific community and the international media.

“To my great surprise, this has gotten huge audience in the scientific world. This was based first of all on the press release of the publisher of the journal, which is Springer, which publishes quite a lot of scientific journals, including behavioral ecology and sociobiology, and through this release, we got to BBC, to New Scientist, to some of the American magazines, and some in Denmark, and when you search for the public release and subsequent reports, you will find some 100 results. So the response is really unbelievably high and huge.”

But while the scientific community was quick to embrace the study, Dr. Bartoš says that horse breeders were rather hesitant.

“We have very varied responses. First of all, we had the chance to present our data in Kladruby, which is famous for its Kladruby horse breed. And I would say that it depended more or less on the age of the breeders. Those of my age, which means very experienced breeders, were quite surprised and they did not take it very seriously. On the contrary, we got a very positive response from younger breeders, who were at least interested in what might be the principle behind the whole thing.”

But even with some of the younger breeders interested in the results of his study, he says it’s hard to say how much actual impact his ground-breaking research will have ob breeding techniques.

“Being an experienced man, I would say that my ambitions are not too high. I would be absolutely happy if breeders started thinking about the biology of their horses. It would be too nice if they accepted this and applied it in practice straight on.”

Dr. Bartoš and his team are planning to further expand their research into the phenomenon of abortions in mares, and shed light on additional factors that may influence their occurrence.

“We have a lot of ideas how to proceed with horses. For me, it’s some sort of… the bad part of this is that I have spent my entire life studying deer, and this result came from horses, but at least they are ungulates, so it’s not that far off. But I am still hoping to have an equally quality result on my deer research.”