A risk factor, responsible for nearly a third higher rate of abortion among mares, has been recently identified by Luděk Bartoš and his colleagues at the Institute of Animal Science in the Czech Republic. In the same paper, the researchers also discuss another newly-discovered phenomenon of mare mating behaviour. These findings are likely to affect the structure of horse breeding.

Bartoš et al. used questionnaires from horse breeders to determine if there is some link between breeding away and foetal loss. They discovered that breeding a mare away from its home environment, and then returning it around home males, increases the rate of abortion by a striking 31%. This is the case for both pregnancies by mating and artificial insemination. Mares bred within the home stable did not show this effect, clearly showing that the increased abortion rate is a result of away-breeding. Furthermore, it was noted that away-bred mares, when placed in enclosures adjacent to other home males, tend to have higher rates of pregnancy disruption.

The researchers also observed that the females bred away engaged in promiscuous sexual behavior upon return, either with a stallion or dominant gelding. If the pregnant female is separated from another male by a fence, she will even attempt to solicit him over that fence. This behavior, which has not been scientifically documented before, is believed to be an attempt to confuse other males as to the paternity of the foal. It is thought to be an evolutionary response to the practice of infanticide by competitor males, which is common among wild horses.

The increased rate of abortion amongst away-bred mares returned home is also believed to be an adaptive behavior. Invading male horses in the wild are known to cause abortions through forced copulations, followed by impregnation. The 31% increase in foetal loss may reflect an instinctual mechanism in mares to cut losses. She may on a primal level recognise that she is with another herd than the one she conceived with. In aborting, she does not further invest resources in the foetus; they would be lost by forced copulations in a natural setting. The mare’s instinct is to save nutrients. The phenomenon of higher abortion rates among away-bred mares is thought to be a major cause of the high percentage of pregnancy disruption observed in domesticated horses.