Feral horse populations are alive and well on several continents including North America, Europe, Australia and India.  Feral horses – referring to specimens that are free roaming but with a domesticated ancestry – live in herds (harems) that are comprised of a dominant stallion, sub-dominant males, and several females.  The majority of the foals in a harem are sired by the dominant male – those that are not have a lower survival rate and may even be subjected to infanticide by the stallion.  As with many mammalian species, female equids often employ a promiscuous sexual strategy in order to confuse paternity and prevent infanticide by dominant males.

Complex social behaviors are much more limited in populations of domesticated horses – where humans carefully monitor most aspects of social interaction. Domestic horses exist in groups that are comprised of a wide range of sexes and ages – not necessarily resembling anything like a harem….and reproduction is not something that is not generally left to the ‘birds and bees’.  It is a common practice among domestic horse breeders to send females away from their home group for either fertilization by an unfamiliar stallion or by artificial insemination.  However, the rate of abortion after a successful implantation in such cases is astonishingly high.  Researchers recently set out to investigate this phenomenon by gathering data on reproductive practices from a broad spectrum of horse-breeders across Eastern Europe.

The results were little short of astonishing.

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The data show very clearly that returning an ‘outsider-inseminated’ female to her home group (containing males that were not the father) resulted in a dramatic increase in abortion (31%) compared to females achieving pregnancy ‘the old fashioned way’ in the home stable with home-based stallions (0%).  Of the outside inseminated females – two factors were found to be statistically correlated with abortion frequency:  the number of foals she had already birthed, and whether she was in direct contact with the other males of her group.  The latter of these represents the most interesting result…

As I mentioned above, equid females are among the many mammalian ladies that employ promiscuous sex as a means to confuse paternity and to decrease the potential for infanticide by competing ‘fathers’.  When outside inseminated females were returned to their groups and allowed to associate with their male counterparts, they frequently engaged in sexual activities.  When females were returned home but segregated into private enclosures they were SEVEN TIMES MORE LIKELY to have an abortion.  Females in segregated pens were also observed to ‘solicit’ attention from males across the fence by standing closeby and urinating frequently (perhaps ‘in vain’ attempts to confuse paternity from afar?).

These results provide an excellent jumping off point for further studies on reproductive behavior and the physiological mechanisms of abortion in domestic horses.  These numbers are far too compelling to conclude that females have no control over whether to abort.  The researchers here suggest that females in segregated pens induce abortion due to their lack of opportunity to confuse paternity.  Why waste energy rearing an offspring that is likely to be subjected to infanticide anyway?

I look forward to reading more work in this area – not only because I find the biology and behavior to be extremely interesting, but because it could potentially provide a happier future for mares that are ‘prostituted out’ to other farms.  If reproductive success is so much higher at home, it may be best to allow nature to run its course on this one

Bartoš, L., Bartošová, J., Pluháček, J., & Šindelářová, J. (2011). Promiscuous behaviour disrupts pregnancy block in domestic horse mares Behavioral Ecology and Sociobiology DOI: 10.1007/s00265-011-1166-6