Because they are hybrids with an odd number of chromosomes, mules, which are produced by breeding a horse and a donkey, are not generally able to reproduce. Mules have 63 chromosomes, while horses have 64 and donkeys have 62. In very rare cases, female mules have become pregnant after being bred to purebred horses.
Even though mules are incapable of reproducing in nearly all cases, they do still produce reproductive hormones and have urges to mate. For this reason, most male mules are gelded to make them easier to handle and more social. Gelded male mules are sometimes referred to as "John mules," while intact male mules are often called "stallion mules." Female mules are often called "Molly mules" or "mare mules."
Although often used to refer to any offspring of a horse and donkey pairing, the term "mule" technically refers specifically to the offspring of a female horse and a male donkey. When a female donkey and a male horse are bred, the resulting offspring is called a "hinny." Like mules, hinnies are sterile and have 63 chromosomes. Hinnies and mules have similar appearances, though hinnies tend to have rounder, more donkey-like hooves than mules. Hinnies also tend to be smaller than mules, and their heads tend to be more horse-like, featuring shorter ears and more refined features.