Mares' Social Bonds Might Enhance Reproductive Success

Mares that form close social bonds with other mares have improved pregnancy, foal birth, and foal survival rates, said a group of researchers studying wild herds in the North Island of New Zealand. The mares' enhanced reproductive success appears to be linked primarily to mutual protection from harassment by stallions, they said.

Unlike most other mammal groups, the mares were able to form these tight bonds among unrelated females. Benefits of social integration were independent of the quality of the habitat, the type of social group, dominance status, and the age of the mare, the study authors reported.

"The horse is clearly a social animal, and we decided it was worth investigating what kinds of benefits the horses were getting from their social natures," said Elissa Cameron, PhD, director of the Mammal Research Institute at the Department of Zoology and Entomology at the University of Pretoria (South Africa). "Above all, it seems that by forming bonds, these feral mares are getting protection from male harassment, which is then translated into reproductive success."

Harassment was defined as any kind of negative interaction such as biting, kicking, and chasing, said Cameron, who has previously published research on stallion harassment. Male aggression towards females is known to reduce female reproductive rates, and the social bonds among the females can be considered an adaptive strategy. Additional benefits could include reduced stress levels and better body condition.

"As handlers, we need to be aware of the importance of social bonds in such a social animal," she said. "Horses with strong social bonds may perform--and breed--better than horses kept alone.

"Even so, the fact that they bond with non-relatives means that they probably also bond well with humans. Although humans can’t replace the presence of other horses entirely, this might explain why they have been so successfully domesticated."

The study, during which 55 adult mares within stable wild herds were followed for a four-year period, is part of a larger research effort to understand the mating system of horses and how mares invest in their foals from conception to independence.

The study, "Social bonds between unrelated females increase reproductive success in feral horses," was published in the Aug. 10, 2009, edition of the Proceedings of the National Academy of Sciences of United States of America. The abstract is available on PubMed.

About the Author

Christa Lesté-Lasserre, MA

Christa Lesté-Lasserre is a freelance writer based in France. A native of Dallas, Texas, Lesté-Lasserre grew up riding Quarter Horses, Appaloosas, and Shetland Ponies. She holds a master’s degree in English, specializing in creative writing, from the University of Mississippi in Oxford and earned a bachelor's in journalism and creative writing with a minor in sciences from Baylor University in Waco, Texas. She currently keeps her two Trakehners at home near Paris. Follow Lesté-Lasserre on Twitter @christalestelas.

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