The high rate of failed horse pregnancies may have a simple explanation: pragmatism. Mares may simply abort because nearby stallions other than the sire could be expected to kill the foal anyway.

The rate of failed pregnancies among domestic horses – 37 per cent – is far higher than that in domestic cattle – 3.6 per cent – and sheep – 7.5 per cent. Luděk Bartoš and colleagues at the Institute of Animal Science in Prague, Czech Republic, decided to investigate why.

The team sent out a questionnaire to horse owners throughout the Czech Republic asking for information on mares that had recently either given birth or aborted a pregnancy. It asked for details of the mare’s age, the types of horses she was housed with, the number of foals she had given birth to in the past and the way she had been inseminated.

An intriguing pattern emerged: mares that were housed with one stallion after copulating with another were seven times more likely to abort their pregnancies than mares that were kept alone during pregnancy. No mare aborted after sharing housing with the stallion that had inseminated her.


Jealous stepfather

Bartoš argues that the results make sense in light of horses’ natural behaviour. Wild horses live in “harems” consisting of one or more mares and a stallion. A stallion can gain new mares by challenging and defeating another. If a mare becomes pregnant by one stallion but then becomes a member of another harem under another stallion, she will tend to lose the developing foal and breed with the second stallion instead, he says, because stallions often kill offspring that are not their own. “The mares are just trying to avoid wasting resources on a foal that will be killed when born,” he says.

Horse breeders could learn from the study, he thinks. They often send a high-quality mare away to mate with the best available stallion – but then house the pregnant mare with local stallions on her return, increasing the likelihood that the pregnancy will fail.

But whether horse breeders will put this finding to use is another matter. “It costs a lot to put a mare in a enclosure on her own and there is a lot of tradition in this field, so I think many people will not change their ways,” says Jose Ignacio Gonzalez, an independent horse breeder in Spain.

Journal reference: Behavioural Ecology and Sociobiology, DOI: 10.1007/s00265-011-1166-6