A mule is produced by breeding a male donkey (called a jack) with a female horse (called a mare). This specific combination matters: reversing the parents, a male horse with a female donkey, produces a different hybrid called a hinny. The two hybrids look similar at a glance but differ in size, temperament, and physical features.
The Parent Combination
The father is always a donkey and the mother is always a horse. This isn’t just a naming convention. Which species carries the pregnancy and which contributes the sperm changes the outcome in measurable ways. Mules tend to have more donkey-like features: longer ears, a heavier head, and tough, boxy hooves inherited from their donkey father. Hinnies, the reverse cross, lean more horse-like with shorter ears and a lighter head.
Getting the cross to succeed is harder than breeding within a single species. In one study using artificial insemination, the pregnancy rate for the mule combination (donkey male, horse female) was about 43%, compared to 87.5% for horse-to-horse breeding. The reverse cross, producing a hinny, had an even lower success rate of just 16.3%. Overall, interspecies crosses succeeded about 22% of the time versus nearly 68% for same-species pairings. So while mules are common working animals worldwide, each one represents a pregnancy that was genuinely difficult to achieve.
Interestingly, mule pregnancies are slightly shorter than normal horse pregnancies. A mare carrying a mule foal averages about 341 days of gestation, roughly a week less than the 347-day average for a standard horse foal.
Why Mules Are Almost Always Sterile
Horses carry 64 chromosomes. Donkeys carry 62. When they 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 their chromosomes into matching pairs. With 63 chromosomes, there’s always one left over with no partner, and the chromosome structures inherited from each parent are so different in shape that they can’t line up properly. This makes normal cell division during reproduction essentially impossible.
The result is that nearly all mules, both male and female, are infertile. Male mules are universally sterile. Female mules, on rare occasions, have produced offspring. One scientifically documented case confirmed through chromosome analysis that a female mule gave birth to a colt, and the colt’s chromosome pattern confirmed it was also a mule (the sire was a donkey). These cases are extraordinary exceptions, not something breeders can plan for. Every working mule you see was produced from scratch by crossing a donkey with a horse.
What Mules Inherit From Each Parent
Mules combine traits from both species in ways that often outperform either parent, a phenomenon sometimes called hybrid vigor. From their donkey side, they inherit remarkable endurance, sure-footedness, and dense, upright hooves with thick soles and tough walls. Their hoof shape is distinctly donkey-like: small, narrow, and oval rather than round like a horse’s. From their horse side, they get a larger body, greater strength, and more speed.
The practical advantages stack up. Mules are stronger than donkeys but require less food than horses of comparable size. Their skin is less sensitive than either parent species, making them better suited to harsh weather and strong sun. Their tough hooves handle rocky, uneven terrain that would wear down a horse’s feet. These traits made mules the preferred pack animal for centuries in mountainous regions and demanding work environments.
Behaviorally, mules are known for intelligence and a strong instinct for self-preservation. What people sometimes call stubbornness is often the mule refusing to do something it perceives as dangerous, a trait that makes them remarkably safe on narrow mountain trails or around hazards. They tend to be bolder and more willing to take on complex tasks than hinnies, which are generally more cautious and reserved.
How Mules Differ From Hinnies
Since both are donkey-horse hybrids with 63 chromosomes, people sometimes assume mules and hinnies are interchangeable. They’re not. Hinnies are generally smaller than mules and less suited to carrying heavy loads. Their movement patterns differ too: mules combine the strides of both parents in a way that makes them versatile workers, while hinnies show more influence from their donkey mothers in how they move.
Temperament is the biggest practical difference. Mules display more boldness and are suited for demanding, complex tasks. Hinnies tend to be less reactive, showing steadier behavior in quiet, controlled settings, which makes them better companions for lighter work. Hinnies are also far less common than mules, partly because the pregnancy rate for the hinny cross is so much lower, and partly because mules are simply more useful for most purposes.
The reason which parent is which species matters so much comes down to more than just chromosomes. The uterine environment, maternal nutrition, and even which parent’s genes are more active during development all differ depending on whether the mother is a horse or a donkey. A horse mare provides a larger womb and more resources during pregnancy, which is one reason mules end up bigger and stronger than hinnies.