A mule is the offspring of a male donkey (called a jack) and a female horse (called a mare). That specific pairing is the only way to get a mule. If you reverse the cross and breed a male horse (stallion) to a female donkey (jenny), the result is a different hybrid called a hinny. While mules and hinnies share some traits, they differ in size, body proportions, and temperament.
Why It Has to Be a Jack and a Mare
The direction of the cross matters because each parent contributes differently to the offspring’s size, build, and behavior. The mare carries the foal, so her body size sets the upper limit on how large the baby can grow in the womb. A draft horse mare bred to a jack can produce a large, powerful mule, while a pony mare produces a much smaller one. The jack contributes the mule’s most recognizable features: long ears, a larger skull, and narrower, more upright hooves.
Flip the cross, and you get a noticeably different animal. Historical descriptions of hinnies note they tend to be no larger than their jenny mother, often smaller, with narrower heads, shorter ears, and thinner necks. Their body proportions resemble a donkey’s more than a horse’s, and they neigh like a horse rather than braying like a donkey or mule. Most breeders prefer the mule over the hinny because mules are generally larger and stronger.
The Breeding Process Isn’t Simple
Producing mules comes with practical challenges that don’t exist when breeding horse to horse. Jacks are slower breeders than stallions, and mares often don’t cooperate. Studies on pasture breeding found that fewer than 40% of mares in estrus willingly accepted the jack’s approach. When a mare showed hostility during the jack’s initial approach, even while in heat, the jack would immediately lose interest.
Size differences between jacks and mares create additional complications. Standard-sized donkeys can have difficulty mounting taller mares, and the mismatch in sexual behavior between the two species adds another layer of difficulty. For these reasons, many breeders use breeding stocks, which are sturdy enclosures that restrain the mare during mating. This protects both the animals and the handlers. Artificial insemination is also common in professional mule-breeding operations.
Once the mare is successfully bred, gestation is slightly shorter than a typical horse pregnancy. Mares carrying mule foals average about 341 days, compared to roughly 347 days for a normal horse pregnancy.
Why Mules Can’t Reproduce
Horses have 64 chromosomes. Donkeys have 62. When the two species cross, the mule ends up with 63, an odd number that creates a fundamental problem during reproduction. To produce eggs or sperm, cells need to divide in a way that pairs up matching chromosomes. With 63 chromosomes, there’s no clean way to do this. The mismatched chromosomes can begin the pairing process, but they can’t finish it. Cell development stalls partway through, and functional eggs or sperm are never produced.
This is true for both male and female mules, though the block works slightly differently in each. In males, sperm-producing cells get stuck at an early stage and never mature. The result is that mules are almost always completely sterile. A handful of fertile female mules have been documented over the centuries, but they’re extraordinarily rare.
What Mules Inherit From Each Parent
A mule’s appearance is a genuine blend of horse and donkey, though certain traits lean heavily toward one parent. From the donkey sire, mules get their long ears, slightly larger skull, and distinctive hooves. Mule hooves are smaller, narrower, and more upright than horse hooves, with a thicker sole and tougher wall inherited from the donkey side. This hoof structure is actually an advantage on rocky or uneven terrain, though it means mules need different trimming techniques than horses.
From the horse dam, mules inherit their overall body size and much of their build. A mule’s body proportions are closer to a horse’s than a donkey’s, with a longer underline relative to the topline (back and loin) that approaches the 2:1 ratio seen in well-balanced horses. Color and size vary enormously depending on the mare. Mules can range from pony-sized to draft-sized, and they come in every coat color horses do.
Physiologically, mules track closer to their horse mothers in some surprising ways. Their resting body temperature averages about 37.24°C, nearly identical to a horse and slightly warmer than a donkey or hinny. Their blood values, including red blood cell counts and hemoglobin levels, also align more closely with horses.
Why People Breed Mules at All
Given the difficulty of producing them and the fact that they can’t breed on their own, it’s fair to wonder why mules exist. The answer is hybrid vigor. Mules combine the best working qualities of both parents in ways that make them exceptionally useful animals. They have the size and speed of the horse with the sure-footedness and endurance of the donkey. Their donkey-inherited hooves handle rough terrain better than horse hooves, and their tougher constitution means they’re generally hardier and less prone to leg injuries.
Mules are also famous for their self-preservation instinct. Unlike horses, which can be pushed past their physical limits, mules tend to refuse work that puts them at risk. This trait, sometimes mistaken for stubbornness, makes them safer and longer-lived working animals. They’ve been used for centuries in agriculture, military transport, and mountain packing precisely because they outlast horses in harsh conditions while carrying comparable loads.