Males make pregnant horses abort

By Victoria Gill
Science and nature reporter, BBC News

Do pregnant mares disguise the paternity of their foals?

Pregnant mares are likely to lose their foals if they are kept close to stallions, researchers have discovered.

Horse breeders, including thoroughbred breeders in the UK, often send mares to stables to be mated with stallions.

But a study reveals that, when they return, the pregnant mares engage in "promiscuous sex" with males in their home stables, in an attempt to disguise the paternity of the foal.

When this is not possible, the mares often abort the pregnancy.

Mares were more likely to have disrupted pregnancies when home males were in adjacent enclosures Dr Ludek Bartos

The strange sexual behaviour may have evolved because of the risk of infanticide - seen in many species that live in male-dominated social groups, where a male will kill offspring of other males in a struggle for dominance.

The team says the findings could explain the high rate of pregnancy disruption in domestic horses.

Ludek Bartos from the Institute of Animal Science in the Czech Republic led the study, published in the journal Behavioural Ecology and Sociobiology.

He and his colleagues distributed a questionnaire to horse breeders via an online magazine. Their responses revealed that mares mated with "foreign stallions" aborted in almost a third of cases.

"None of the mares mated within the home stable aborted," Dr Bartos told BBC News.

"And mares were more likely to have disrupted pregnancies when home males were in adjacent enclosures."

Survival increases after foals reach one month

Mares that were separated from males, and therefore physically unable to disguise the paternity of their foal, were more likely to abort. This, the researchers say, prevented "the waste of energy in producing offspring likely to be lost" to infanticide.

Paternity test

The idea that domestic horses might adopt such a strategy came from the study of infanticide in horses' wild cousins, the zebras.

To avoid inbreeding in captive zebras, zoos occasionally introduce a new male into their herd. But Dr Bartos found that this practice increased the likelihood of foals being lost.

A previous study by him and his colleagues revealed that most zebra foals, rather than being killed by the new male, were lost during pregnancy.

Pairing mares with foreign stallions may not be the best strategy

"If a new male was brought into the herd just after a female became pregnant, the chance of a foal surviving was less than 5%," Dr Bartos explained.

"Survival increased to more than 60% after the foal reached one month of age."

He believes that zebras and domestic horses have both evolved similar "abortion strategies". He thinks it is possible that horses "make the decision" to terminate their own pregnancy.

Natural chemically triggered abortion is a well-known phenomenon in biology. It is known as the Bruce effect, and has been observed mainly in rodents, where the scent of male urine causes a pregnant female to abort.

Although the mechanism behind this high rate of abortion in horses is not yet known, Dr Bartos says the research has a very practical message for horse breeders.

SOURCES Learn more about how horses lose their foals at Behavioral Ecology and Sociobiology

"The practice of transporting the mare for mating or artificial insemination with a foreign stallion and then bringing her back to an environment with home males is wrong," he said.

"It is very likely it is one of the main causes of such high percentages of pregnancy disruption in domestic horses."