Do Mules Have Sex and Can They Reproduce?

Mules do mate, and they show many of the same sexual behaviors as horses and donkeys, but the overwhelming majority cannot produce offspring of their own. A mule inherits 32 chromosomes from its horse mother and 31 from its donkey father, leaving it with an odd total of 63. That mismatch makes it nearly impossible for the mule’s cells to divide properly during the specialized process that creates eggs or sperm. The word “nearly” matters, though, because a handful of documented cases over the past century show that rare female mules have beaten those odds and given birth to live foals.

Why Mules Exist but Struggle to Make More Mules

A mule is the offspring of a male donkey (jack) and a female horse (mare). The reverse cross, a male horse (stallion) bred to a female donkey (jenny), produces a hinny. Both hybrids end up with 63 chromosomes, and both face the same fundamental reproductive problem. When any animal makes eggs or sperm, its chromosomes need to line up in matching pairs so they can be divided evenly. Horses carry 64 chromosomes and donkeys carry 62, and the two species’ chromosomes differ not just in number but in shape: donkeys have a greater proportion of certain chromosome types that look and behave differently from their horse equivalents.¹ Those structural differences, combined with the odd number, mean the chromosomes cannot form proper pairs during the cell division that produces sex cells. Most of them stall out partway through and die.

This is why the standard description of mules as “sterile” has persisted for thousands of years. The majority of first-generation horse-donkey hybrids do conform to what geneticists call gametogenic breakdown, meaning the process of making functional eggs or sperm simply fails.² But “majority” is not “all,” and the exceptions have been fascinating researchers for decades.

Male Mules Are Sexually Active but Reproductively Dead-End

Male mules (called johns or horse mules) develop normal-looking external reproductive anatomy and produce testosterone, which means they will mount mares, show aggression toward other males, and display the full range of stallion-like sexual behavior. This is why most working male mules are gelded early: an intact male mule can be just as difficult to handle as an intact stallion, even though he cannot sire foals.

The problem is internal. In studies of hinny testes, researchers found that sperm-producing cells stall during mid-meiosis. Most spermatogonia and spermatocytes stop developing at the point where chromosomes attempt to pair up, which prevents the formation of mature sperm cells entirely.³ The same block applies to male mules. No verified case of a male mule siring offspring has ever been genetically confirmed. There have been occasional claims, but when parentage testing has been applied rigorously, the supposed sire has turned out to be a donkey or horse that was mistakenly identified as a mule, or the claim could not be substantiated. For practical purposes, male mule fertility does not exist.

Female Mules Are a Different Story

Female mules (called mollies) are biologically more interesting than their male counterparts when it comes to reproduction. While the meiotic block hits males almost totally, the situation in females is more nuanced. Researchers who examined the ovaries of fetal and newborn mules and hinnies found that germ cells migrated into the fetal ovary normally and divided normally at first, but most of them died when they entered meiosis in early neonatal life, apparently because the mismatched chromosomes could not pair up.⁴ “Most” died, but not all. A small number of egg cells appear to survive in some female mules.

This matters because many female mules show clear, regular estrous cycles. Ultrasound imaging and blood hormone measurements confirm that these mules develop follicles, ovulate, and have the same progesterone and estradiol fluctuations you would see in a cycling mare.⁵ A Brazilian research team documented consistent evidence of regular cyclicity in mules and even used cycling mules as recipients for donkey embryo transfers, resulting in two live donkey foals born to mule surrogate mothers.⁶ So female mules are not just behaviorally sexual; their reproductive tracts are functional enough to cycle, conceive a transferred embryo, carry a pregnancy to term, and deliver a healthy foal.

Documented Cases of Female Mules Producing Their Own Offspring

The rarest and most remarkable finding is that some female mules have conceived and given birth to their own genetic offspring, not embryo transfers, but foals sired by a stallion or jack donkey. These cases have been met with enormous skepticism historically, since mule fertility was considered impossible. Modern genetics has changed that. Researchers have used chromosome analysis and DNA microsatellite testing to verify that certain foals are genuinely the biological offspring of cytogenetically confirmed mules.

One well-documented case involved a female mule whose male foal was confirmed through karyotyping and parentage analysis to be the product of mating with a jack donkey. The mule’s own karyotype was verified as 63 chromosomes, ruling out the possibility that she was actually a horse or donkey misidentified as a mule.⁷ In another case, a female mule produced a colt whose karyotype also turned out to be that of a mule, with blood grouping analysis confirming the donkey sire.⁸

These cases raise an obvious question: how does a cell with 63 chromosomes manage to produce a viable egg? The honest answer is that nobody fully understands the mechanism. One possibility is that a rare egg cell undergoes a form of meiosis where the chromosomes happen to sort in a way that produces a roughly functional gamete, even if the chromosome count is not perfectly halved. Another is that some form of premeiotic endoreduplication (the cell doubling its chromosomes before dividing) allows pairing to work. Whatever the mechanism, it is vanishingly rare. Across the entire documented history of mule breeding worldwide, confirmed cases of fertile female mules number in the single digits to low dozens. It happens, but you would not want to plan a breeding program around it.

How Parentage Verification Works for Unlikely Foals

When someone claims a mule has produced a foal, the scientific bar is high. A mule looks distinct from a horse or donkey, but visual identification alone is not enough. Researchers verify the parentage through multiple independent methods. In one such case, investigators compared 12 microsatellite DNA sites between the foal and the supposed mule dam, analyzed mitochondrial DNA sequences to confirm the maternal line, and performed karyotyping on both the foal and its parents to confirm species identity.⁹ This level of rigor is necessary because the default assumption, rightly, is that a fertile “mule” was actually a horse or donkey all along. The handful of cases that have survived this scrutiny are all the more remarkable for it.

Mules as Surrogate Mothers

Even setting aside the rare fertile mule, there is practical interest in using female mules as embryo recipients. Because many mollies cycle regularly and have functional uteruses, they can carry embryos from other equids. Researchers successfully transferred horse embryos non-surgically into cycling mules. The embryos transferred to mules that were actively cycling resulted in live colt foals born at normal gestational length, and the foals developed normally to weaning. By contrast, embryos transferred to mules that were not cycling did not establish pregnancies.¹⁰ This finding means a cycling female mule’s reproductive tract is not just going through the motions hormonally. It is physiologically competent to support a full equine pregnancy.

This has practical value because mules are common in many parts of the world and are generally hardier and longer-lived than horses. In settings where mule populations are large and horse broodmares are scarce or expensive, using cycling mules as recipients could expand the breeding options for both horse and donkey programs.

The Cloning Angle

In 2003, researchers achieved another milestone by cloning a mule using nuclear transfer from fetal cells. The resulting animal, a mule named Idaho Gem, was the first equine of any kind to be cloned.¹¹ Cloning sidesteps the meiotic problem entirely because it does not involve eggs or sperm in the traditional sense. Instead, the nucleus from a body cell is placed into an emptied egg cell and stimulated to develop. The resulting clone has the same 63 chromosomes as the original mule, is genetically identical to it, and is just as sterile by natural reproduction. Still, the achievement showed that mule cells are perfectly capable of directing normal embryonic development when the meiotic hurdle is removed from the equation.

Why Chromosome Number Does Not Always Determine Fertility

If mismatched chromosome numbers cause sterility, you might expect that any cross between species with different chromosome counts would produce sterile offspring. That is not always the case, and the comparison is instructive. Przewalski’s horse, the last truly wild horse species, carries 66 chromosomes compared to the domestic horse’s 64. Yet hybrids between the two are fully viable and fertile.¹² The reason is that the difference between Przewalski’s and domestic horse chromosomes involves a simple fusion event: two separate Przewalski’s chromosomes correspond to one large domestic horse chromosome, so the genetic content is essentially the same and pairing during meiosis can still work.

The horse-donkey gap is different in kind, not just in number. Horses and donkeys diverged millions of years ago, and their chromosomes have undergone rearrangements, inversions, and structural changes that go far beyond a single fusion.¹ It is the depth of these structural differences, not simply the arithmetic of odd versus even, that makes meiosis fail in mules. Two species can differ by a couple of chromosomes and still produce fertile hybrids if the changes are minor. But when the chromosomes have been reshuffled extensively, even a small numerical gap becomes an insurmountable pairing problem.

The Role of Genomic Imprinting in Mules Versus Hinnies

Mules and hinnies have the same chromosome count and the same two parent species, but they are not identical animals. A mule has a horse mother and donkey father; a hinny has a donkey mother and horse father. The direction of the cross matters because of a phenomenon called genomic imprinting, where certain genes behave differently depending on which parent they came from. In interspecies hybrids, imprinting can be disrupted, and the direction of disruption depends on which species contributed which set of chromosomes.¹³

In practice, mules and hinnies differ in size, body shape, temperament, and even ear length, with hinnies tending to be slightly smaller and more donkey-like in conformation. These differences are not just random variation. They reflect systematic differences in which parent’s genes are more heavily expressed. Imprinting likely also contributes to the slight differences in reproductive biology between the two hybrids, though both are overwhelmingly sterile. Hinnies are also far less common than mules, partly because the cross is harder to achieve (donkey jennies are less receptive to horse stallions) and partly because they are less commercially useful.

Sexual Behavior Without Reproduction

One of the most common misunderstandings about mules is the assumption that sterile means asexual. Intact male mules will court, mount, and attempt to breed mares or jennies with the same enthusiasm as any stallion or jack. Female mules in estrus will display classic signs of heat, including “winking” of the vulva, increased urination, and receptivity to males. Their bodies are producing the same sex hormones as their parent species, and those hormones drive behavior regardless of whether functional eggs or sperm result.

This is why gelding male mules is standard practice. A john mule with testosterone coursing through his system can be aggressive, territorial, and difficult to manage in a working string. He will fight with other males and relentlessly pursue females, all to no reproductive end. Some mule owners keep intact males intentionally as “teaser” animals to detect estrus in mares, taking advantage of the mule’s sex drive without the risk of an unwanted pregnancy.

Female mules’ cycling behavior can occasionally be a management nuisance too, particularly in pack or riding strings where behavioral changes during estrus are undesirable. Some owners administer hormonal treatments to suppress cycling, similar to what might be done with mares. But for the most part, a molly mule’s estrous cycle is simply an uneventful biological rhythm with no practical consequence for her work life.

Why Mule Fertility Research Keeps Going

Given that confirmed fertile mules are extraordinarily rare, you might wonder why researchers keep studying the topic. Part of the answer is basic science: understanding why hybrid sterility is almost but not quite absolute in equines tells us something about how species boundaries form and hold. The mule sits in an interesting evolutionary gray zone. Its parent species diverged long enough ago that their chromosomes are quite different, but recently enough that the cross still produces a healthy, vigorous animal. Studying where exactly the reproductive machinery breaks down, and how it occasionally does not, illuminates the genetics of speciation more broadly.

There are practical motivations too. Mules are among the most widely used working animals on Earth, and in regions where they are economically important, any expansion of breeding flexibility has value. The demonstration that cycling mules can serve as embryo transfer recipients has already opened new possibilities for donkey and horse breeding programs.⁶ And cloning technology, while not widely applied to mules today, proved that the mule genome itself is not defective. The cells work fine; it is only the specific act of shuffling chromosomes into sex cells that breaks down.¹¹

There is also a conservation parallel. Hybridization between related species happens naturally in many animal groups, and understanding the mechanisms of hybrid sterility versus hybrid fertility helps predict what happens when populations overlap due to habitat loss or climate change. The mule is a conveniently well-studied model for those questions, since humans have been producing mules deliberately for at least three thousand years and recording the results, however anecdotally, for nearly as long.