A mule is the infertile offspring of a horse and a donkey. Mules have historically been used as mounts and beasts of burden, especially in rough terrain such as mountains or the Grand Canyon. Mules are generally stronger than either horses or donkeys, can tolerate more extreme temperatures, have greater endurance, are longer-lived, and may require less food and water.

There are places in America and other parts of the world where herds of wild horses live, well-adapted to the environment. If mules are so much better adapted to harsh environments than either horses or donkeys, why have there never been populations of wild mules?