Mules come in both sexes. A male mule is sometimes called a john or horse mule, while a female mule is called a molly or mare mule. The confusion around their sex likely stems from the more interesting half of the question: mules are almost always sterile, regardless of whether they are male or female. That near-total infertility is baked into their genetics as interspecies hybrids, but “almost always” leaves room for some genuinely surprising exceptions, particularly among females.
How Mules Get Their Chromosome Count
A mule is the offspring of a male donkey (called a jack) and a female horse (a mare). Horses carry 64 chromosomes and donkeys carry 62, so the mule ends up with 63, an odd number that creates serious problems when the animal’s body tries to produce eggs or sperm. For normal reproduction, chromosomes need to line up in neat pairs during the cell division that creates reproductive cells. With 63 chromosomes, there is always one left over with no partner, and the pairing process stalls or produces defective cells. This is the core reason mules are sterile, and it applies to both males and females.
A hinny is the reverse cross: a male horse (stallion) bred with a female donkey (jenny). Hinnies also have 63 chromosomes and face the same fertility barrier. They are less commonly produced because the size mismatch between a stallion and a smaller jenny makes breeding more difficult and conception rates lower. When people ask “can mules reproduce,” the answer covers hinnies too, since the underlying chromosome problem is identical.
Why Males Are Almost Completely Sterile
Male mules and hinnies are, for all practical purposes, completely infertile. Their testes typically develop to a smaller size than those of a horse or donkey, and the internal machinery for producing sperm breaks down at a specific stage. Research on male hinnies found that most sperm-producing cells stall during a mid-stage of cell division called meiosis, which prevents them from forming mature sperm.1PubMed Central. Testicular Characteristics and the Block to Spermatogenesis in Mature Hinny Cytogenetic studies of both mules and hinnies found the same pattern: abnormal chromosome pairing in nearly all cells at the stage where chromosomes need to match up, and spermatogenesis almost totally arrested as a result. A few mature sperm were recovered from one hinny, but such vanishingly small numbers have no practical reproductive significance.2PubMed. Cytogenetic studies of three equine hybrids
Male mules are routinely gelded (castrated) not because owners worry about unwanted pregnancies, but because intact males can still exhibit stallion-like behavior driven by testosterone. They may mount mares, become aggressive around other males, and generally be harder to handle. Gelding makes them calmer and easier to work with, the same rationale behind gelding horses. Despite the testosterone-driven behavior, intact male mules have never been confirmed to sire offspring under natural conditions. There is no verified case in the scientific literature of a male mule producing a live foal.
Why Females Are the Exception
Female mules are a different story, at least in rare cases. While the vast majority of molly mules are infertile, scattered reports over the centuries have documented female mules giving birth. These cases are uncommon enough to have been treated with heavy skepticism for a long time, but modern genetics has confirmed several of them beyond doubt.
One well-documented case involved a female mule whose chromosome count was verified as 63, confirming she was indeed a mule and not a misidentified horse. She gave birth to a male foal sired by a donkey jack, and the foal’s karyotype was also that of a mule, meaning he inherited the mule-typical chromosome count.3PubMed. Male mule foal qualifies as the offspring of a female mule and jack donkey Another confirmed case similarly documented a fertile female mule producing a colt whose chromosomal makeup and blood type both confirmed the parentage.4ScienceDirect. A fertile female mule
The reason female mules are occasionally fertile while males never are comes down to a broader pattern in mammalian hybrids. When two closely related species interbreed, the resulting hybrid’s fertility problems tend to hit the male sex harder than the female. This pattern has been observed across many mammalian species, not just equines. Research on hybrid mice found that the underlying cause involves differences in how male and female reproductive cells handle chromosome-pairing irregularities, combined with specific vulnerabilities related to the sex chromosomes. Female reproductive cells can sometimes tolerate the mismatched chromosomes well enough to produce a viable egg, while male cells are far less forgiving of the same irregularities.5PubMed. Meiotic abnormalities in hybrid mice of the C57BL/6J x Mus spretus cross suggest a cytogenetic basis for Haldane’s rule of hybrid sterility
Even so, fertile female mules are genuinely rare. Estimates in older literature sometimes cite figures like one in a million, though no rigorous census has ever been conducted to pin down the actual rate. It is safe to say that the overwhelming majority of molly mules will go through life without ever producing a foal, and the ones that do tend to make headlines in the agricultural and veterinary press.
What Happens When a Fertile Mule Breeds
One of the more intriguing aspects of fertile female mules is what kind of offspring they produce. You might expect the foal to be some complicated mix of horse and donkey genetics, but the confirmed cases tell a simpler story. In both documented cases described above, the female mule was bred to a donkey jack, and the resulting foal had a mule karyotype.3PubMed. Male mule foal qualifies as the offspring of a female mule and jack donkey This suggests the fertile mule’s egg carried a horse-like chromosome set rather than a mixed one. In other words, when a female mule does manage to produce a viable egg, she appears to pass along her horse chromosomes preferentially rather than a scrambled combination.
This selective transmission makes sense in a rough way. If the egg contained a random assortment of horse and donkey chromosomes, the resulting embryo would likely be inviable. The eggs that do survive are probably the ones where the chromosomes sorted into a workable set, and a set resembling one parent species has a better chance of being workable than a random mix.
Genomic Imprinting and Why Direction of the Cross Matters
One reason mules and hinnies differ from each other, despite having the same species mix, is a phenomenon called genomic imprinting. Certain genes behave differently depending on whether they came from the mother or the father. In interspecies hybrids, this parent-of-origin effect can become exaggerated, contributing to the abnormal features seen in hybrid offspring. Research has linked loss of normal imprinting patterns to hybrid incompatibility in several mammalian groups, and the effects include disrupted growth and developmental genes.6PubMed Central. Imprinted gene expression in hybrids: perturbed mechanisms and evolutionary implications
In practical terms, this is why mules and hinnies look and behave somewhat differently even though they share the same two parent species. Mules tend to be larger because the horse mother provides the uterine environment and certain maternally expressed growth genes. Hinnies, carried in the smaller donkey uterus with a different set of imprinting biases, tend to be smaller and are often described as more donkey-like in build and temperament. The genetic cards are the same, but which parent dealt them changes the outcome.
Mules as Surrogate Mothers
Even though female mules cannot usually produce their own offspring, they can carry someone else’s. Embryo transfer technology has successfully used mules as surrogate mothers for both horse and donkey embryos, opening up a practical role for molly mules in breeding programs.
In one study, horse embryos were transferred nonsurgically into cycling female mules. Both embryos resulted in live, healthy colt foals that developed normally to weaning. The mules’ hormonal profiles during pregnancy closely resembled those of normal pregnant mares, with the expected patterns of hormone secretion and the development of the structures that support pregnancy.7PubMed. Successful non-surgical transfer of horse embryos to mule recipients Parturition occurred spontaneously at about 348 days, and the foals were completely normal horses despite being born from a hybrid mother.
More recently, researchers demonstrated that even non-cycling mules, those without active reproductive cycles, could serve as surrogates if given the right hormonal support. A donkey embryo was transferred into a non-cycling mule that had been treated with estradiol and long-acting progesterone. The mule carried the pregnancy to term and delivered a live donkey foal after 375 days.8Spanish Journal of Agricultural Research. Short communication: Establishment and maintenance of donkey-in-mule pregnancy after embryo transfer in a non-cycling mule treated with oestradiol benzoate and long-acting progesterone This finding is relevant to conservation. Several donkey breeds around the world are endangered, and using mules as surrogates could expand the pool of available recipient animals for embryo transfer programs without needing additional breeding jennies.
Cloning Gets Around the Problem Entirely
Because mules cannot breed to produce more mules, every mule has always been a first-generation cross. If you have an especially talented working mule or racing mule, there has historically been no way to reproduce that animal. Cloning changed that, at least in principle. In 2003, researchers produced the first cloned mule, a male named Idaho Gem, born after a normal gestation period in the womb of a horse mare. He was the first member of the horse family to be cloned and the first sterile animal of any species to be produced through cloning.9Science. First Cloned Mule Races to Finish Line
Idaho Gem went on to compete in mule racing, which demonstrated that the clone was physically functional and healthy. Cloning remains expensive and technically demanding, so it is not a routine way to produce mules. But the proof of concept matters: a particularly valuable mule can, in theory, be replicated without needing to cross-breed a horse and donkey again and hope for the same combination of traits.
How Mules Compare to Other Equine Hybrids
The horse family is notable for the number of hybrid crosses it can produce. Beyond mules and hinnies, there are zebroids (zebra crossed with a horse or donkey) and various other combinations. Most of these hybrids face the same fertility barrier as mules, since different equid species carry different chromosome numbers. Zebras, depending on the species, have anywhere from 32 to 46 chromosomes, making hybrid chromosome counts even more uneven than the mule’s 63.
An interesting contrast is the Przewalski’s horse, a wild horse species with 66 chromosomes to the domestic horse’s 64. Hybrids between the two are fertile and show normal sperm production, despite the chromosome difference.2PubMed. Cytogenetic studies of three equine hybrids The key difference is that Przewalski’s horses and domestic horses are much more closely related than horses and donkeys. Their chromosomes, even though they differ in number, are similar enough in structure that they can still pair up adequately during cell division. This tells us that chromosome count alone does not determine whether a hybrid will be fertile. What matters more is how structurally different the chromosomes are, and how well they can find matching partners during the cell division that produces eggs and sperm.
The Mule’s Place in the History of Genetics
Mules have been deliberately bred for at least 3,000 years, making them one of humanity’s oldest engineered animals. Long before anyone understood chromosomes or DNA, farmers in the ancient Near East figured out that crossing a jack donkey with a mare produced an animal with desirable qualities: the strength and endurance of a donkey combined with the size and speed of a horse, plus a temperament often described as more patient and surefooted than either parent.
The mule also played a surprisingly important role in the development of genetic thinking. The fact that a mule visibly combines traits from both parents was one of the earliest pieces of evidence that both the mother and the father contribute to the makeup of their offspring. Before formal genetics, the prevailing view in many cultures was that the male provided the essential “seed” while the female merely served as a vessel. The mule’s obvious blend of horse and donkey characteristics challenged that idea and helped establish the concept of dual parental contribution to heredity.10PubMed. The contribution of the mule to scientific thought
Behavioral Differences Between Male and Female Mules
While the fertility question dominates discussions of mule sex, the practical differences between johns and mollys matter more to people who actually work with these animals. Male mules, if left intact, produce enough testosterone to behave like stallions. They can be territorial, aggressive toward other males, and persistently interested in mares despite having no ability to impregnate them. Most working male mules are gelded young to avoid these problems, and a gelded john mule is generally calm, cooperative, and easy to train.
Female mules do cycle hormonally, at least many of them. Some show regular estrus behavior, while others cycle irregularly or not at all. Molly mules in heat can become distracted or difficult, just like mares, though the intensity varies from animal to animal. Some owners prefer gelded johns for steady work precisely because they avoid the hormonal ups and downs that come with a cycling female. Others prefer mollys for their generally gentler disposition when they are not in season. In practice, individual temperament varies enough that sweeping generalizations about one sex being “better” are not particularly useful.
The hormonal cycling in female mules is also what makes them candidates for embryo transfer surrogacy. A cycling molly mule’s reproductive tract can prepare for and sustain pregnancy much like a mare’s, even though her own eggs are almost never viable. This is why researchers have been able to use them as surrogates for horse and donkey embryos, with pregnancies that proceed largely like normal equine pregnancies and produce healthy foals at term.
Why the “One in a Million” Claim Is Hard to Verify
The notion that fertile mules occur at a rate of roughly one in a million gets repeated constantly, but it deserves some skepticism. No controlled study has ever tracked a large enough population of female mules to calculate a real fertility rate. The documented cases number in the low dozens over several centuries, and the global mule population at any given time is in the millions, which makes the one-in-a-million figure feel plausible as a rough order of magnitude. But there are confounding factors. Many mule owners never breed their animals and would never discover fertility even if it existed. Female mules in areas without access to genetic testing could produce foals that are simply assumed to be misidentified horses or donkeys. And historically, reports of fertile mules were often dismissed as fraudulent or mistaken before genetic confirmation tools existed.
The evidence is clear that fertile female mules exist and that their fertility has been genetically confirmed in multiple independent cases. What remains genuinely unknown is how common the phenomenon is. It could be rarer than one in a million or considerably more common. Until someone tracks a large cohort of female mules with systematic breeding attempts and genetic testing of any offspring, the true rate stays in the realm of educated guessing.