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