Mules are equines, but they occupy a unique space within that family. An equine is any member of the biological family Equidae, which includes horses, donkeys, and zebras. The mule is a hybrid animal, resulting from the crossbreeding of two distinct species within that family.
What Defines the Equidae Family
The term “equine” refers to any animal belonging to the family Equidae, a group of mammals known as odd-toed ungulates. This family currently contains only one surviving genus, Equus, which encompasses all living horses, asses, and zebras. Members of the Equidae family share a common body plan, characterized by long, slender limbs and a single functional toe encased in a hoof.
These animals are all herbivores, relying on a fibrous diet of grass and other plant matter. Their teeth are specialized with high crowns to continuously grind down tough forage. Because the mule’s parents are both distinct species within the Equus genus, the mule is automatically classified under the umbrella of the Equidae family.
The Specific Parentage of Mules
A mule is the product of a specific interspecies cross: a male donkey, known as a jack, bred with a female horse, or mare. This pairing is the most common method for creating the hybrid. Mules often exhibit hybrid vigor, inheriting desirable traits from both parents, such as the horse’s size and the donkey’s endurance and intelligence.
The reciprocal cross involves a male horse, a stallion, and a female donkey, or jenny, and produces a different hybrid called a hinny. While both mules and hinnies are first-generation hybrids of the same two species, they typically differ in appearance and stature. Mules are larger and stronger than hinnies, which is why they have historically been the sought-after working animal.
Why Mules Cannot Reproduce
The mule’s hybrid status comes with a genetic limitation that prevents it from reproducing: sterility. This condition is caused by the mismatch in chromosome numbers between its parent species. Horses possess 64 chromosomes, while donkeys have 62.
The resulting mule inherits 32 chromosomes from its horse mother and 31 from its donkey father, yielding a total of 63 chromosomes. The problem arises during meiosis, the specialized cell division process required to produce sperm or eggs. During meiosis, chromosomes must align in homologous pairs to divide successfully.
The mule’s odd number of 63 chromosomes means there is one chromosome without a partner, and the chromosomes inherited from the different species are often too dissimilar to pair up correctly. This prevents the proper formation of viable gametes (sperm or eggs), which is why mules are almost universally sterile. Though extremely rare cases of female mules giving birth have been documented, these anomalies do not change the fundamental biological rule of sterility.