Can Mules Have Babies? Why Mules Are Almost Always Sterile

What is a mule?

A mule is the offspring of a jack (male donkey) and a mare (female horse). The reverse cross, a stallion with a jenny (female donkey), produces a hinny. Both are first-generation hybrids between two different species, and both are bred deliberately by people for their strength, endurance and steady temperament.

Mules have been bred for at least 3,000 years in western Asia and the Mediterranean, and they were vital to farming, mining and armies until the age of the tractor and the truck. Because mules cannot normally breed, every new mule has to come from a donkey and a horse. The differences between a mule and its donkey parent are covered in donkey vs mule.

Why are mules sterile? The chromosome problem

Horses and donkeys split from a common ancestor several million years ago. Over that time their chromosomes, the packages that carry DNA, were rearranged in different ways.

A mule’s body cells work perfectly well with this mixed set, which is why mules are healthy and strong. The problem comes when it tries to make eggs or sperm. That process, called meiosis, needs every chromosome to find and pair up with its matching partner before the set is split in half.

In a mule, the horse and donkey chromosomes do not match up neatly. Some are shaped differently, some carry genes in a different order, and one chromosome is left without any partner at all. Pairing fails, cell division breaks down, and the result is that almost no viable eggs or sperm are made.

Male mules vs female mules

Male mules (johns) produce no working sperm. Not a single fertile male mule has been confirmed, so a john cannot father a foal. Most are castrated when young to make them easier to manage.

Female mules (mollies or mare mules) are a slightly different story. Many have heat cycles, and their ovaries occasionally release an egg that, by chance, carries a usable set of chromosomes. This lopsided pattern fits a wider rule in biology, known as Haldane’s rule: in hybrid mammals, the male is more often sterile than the female.

The rare mules that gave birth

Mule births are so unusual that in ancient times they were treated as omens. The Greek historian Herodotus described a mule giving birth as a portent before Xerxes invaded Greece in 480 BCE, and the Romans had a saying that compared impossible events to “when a mule foals”.

Modern records show that it does happen, very rarely. Roughly 60 births to female mules were reported between 1527 and 2002, although some of these older reports are doubtful. Well-known cases include:

• “Old Bec” in Texas: in the late 1920s, a mare mule owned by Texas A&M had two foals. One, sired by a jack, was a sterile female. The other, sired by a Saddlebred stallion, looked entirely like a horse and later fathered normal horse foals.
• Brazil: in 1995 researchers described a female mule that had been pregnant seven times. Foals sired by a stallion had 64 horse-like chromosomes and the older ones proved fertile.
• Morocco and Colorado: a mare mule in Morocco gave birth in 2002, and another in Colorado in 2007. In the Colorado case, DNA tests on blood and hair confirmed that the mother was a mule and the foal was hers.

In several of these cases, the mule seems to have passed on an egg containing only, or mostly, the chromosomes she inherited from her horse mother. Her foal by a stallion was then effectively a horse.

What about hinnies?

A hinny has the same 63 chromosomes as a mule, just inherited the other way around, and it is sterile for the same reasons. Hinnies are rarer than mules partly because stallions are often less willing to mate with jennies and such pregnancies are more often lost early, and partly because a small donkey mother produces a smaller foal.

The one widely cited case of a fertile hinny is a foal born in Henan, China, in 1981, but reports disagree on whether the mother was a hinny or a mule.

Other equid hybrids

All members of the horse family can interbreed, which produces a whole group of hybrids, mostly sterile:

• Zorse: zebra stallion and horse mare. Zebras have between 32 and 46 chromosomes depending on the species, so the chromosome mismatch is even larger than in a mule.
• Zonkey or zedonk: zebra and donkey. Both mules and zebra hybrids often show stripes on the legs. See the plains zebra for the zebra side of the family.
• Kunga: an ancient hybrid of a domestic donkey and the Syrian wild ass, bred in Mesopotamia about 4,500 years ago and used to pull royal war wagons.

An interesting exception is Przewalski’s horse, which has 66 chromosomes. When it is crossed with a domestic horse (64), the hybrid has 65, yet it is fertile. The two horses are close relatives and their chromosomes differ by a simple split, so pairing still works. The same principle explains why some hybrids outside the horse family, such as those described in polar bear vs grizzly bear, can be fertile.

Can a mule be a mother at all?

Even though a mare mule can rarely conceive herself, she can still raise a foal. Using embryo transfer, vets can place an embryo from a horse or another mule into a female mule, which carries the pregnancy and nurses the foal as a surrogate. Female mules are also known to be protective of foals, and Italian horse breeders once kept them with herds partly for this reason.