Female horses (mares) exhibit a very strange behaviour after they mate with a male. A mare will mate with every other male in the stable, almost immediately after mating with the first one. This strange post-coital promiscuity does not appear to make any sense from an evolutionary perspective, as one would think that pregnancy would lead to a decrease in libido so the mother can focus on caring for her embryo. However, this promiscuity is part of a strategy that helps protect the baby horses.
It is common behaviour in the animal kingdom for a male to kill a female’s young so that he can mate with her and produce babies with his own genetic material. This also happens in horses, where stallions engage in infanticide, kicking a foal to death if it is not his. For a mother horse, this is not only heartbreaking, but also a tremendous waste of the energy she put into pregnancy. So to protect her offspring and conserve energy, she ensures that no male knows who the father of her baby is. It has been shown that males who mated with a female who gave birth will not attack the foal, as there is a chance that it is his. Thus, a mother horse guarantees the protection of her child by prostituting herself to all of the males around.
Although the story of a mother willing to sacrifice everything, even her dignity, for the safety of her offspring may be inspiring, it has also been observed that if a mare cannot mate with all of the males in the stable, she will instead abort the pregnancy. For example, if a new horse is brought into the herd after the mare becomes pregnant, the mare senses the danger to her eventual foal and proceeds to abort it. In a study involving zebras, it was found that bringing in a new male made the foal’s chances of survival fall to less than 5 percent.