No, mules are born male or female, just like their parents. The idea that they are all male probably persists because both sexes are almost always unable to breed, so sex seems to matter very little. But sterile does not mean the reproductive organs don’t work, and female mules in particular often still cycle like mares.
What Male and Female Mules Are Called
A mule is the offspring of a male donkey, called a jack, and a female horse, a mare. A male mule is called a john, a john mule, or formally a horse mule.1Wikipedia. Mule A female mule is called a molly, a molly mule, or formally a mare mule.2Wikipedia. Mule The formal names come from the mule’s sex, not from which parent it resembles.
That is where the confusion starts. “Horse mule” sounds like a horse, and “mare mule” sounds like a mare, but both describe mules. The formal terms turn up most in sale ads, registry papers, and show classes, while barn talk leans toward molly and john. A buyer who skims past the word “mule” in a “mare mule” listing can come away with the wrong picture. If a listing is unclear about sex, ask the seller directly, and ask for a photo of the animal standing square so you can check yourself.
Checking is simple, because a mule’s outward anatomy matches a horse’s. A male has a sheath under the belly, as a stallion or gelding does. A female looks like a mare underneath, with an udder area and a vulva under the tail. Most male mules are gelded young, just like colts, so the testicles may be missing. A gelded john is identified by the sheath and by his history rather than by testicles.
Among more than 1,400 recorded mule births, about 44 percent were male.3Journal of Animal Science. The Sex Ratio in the Mule That works out to roughly 80 male mules for every 100 females.4Journal of Animal Science. The Sex Ratio in the Mule Horses, by comparison, are born close to half male and half female.5Journal of Animal Science. The Sex Ratio in the Mule If anything, female mules are the more common sex.
Why Mules Usually Cannot Have Babies
A horse has 64 chromosomes and a donkey has 62.6Wikipedia. Mule A mule gets 32 from its horse mother and 31 from its donkey father, for 63 total.7The Tech Interactive. Why can’t mules breed? Each parent passes on half of its chromosomes, so the mule lands between the two parents’ counts, with an odd total.
Chromosomes normally come in matching pairs, so an odd count leaves one without a partner. On top of that, the horse and donkey chromosomes differ enough in structure that many don’t match even when the counts line up. As a result, the cell division that makes eggs or sperm stalls, and no complete egg or sperm results.
The block happens when eggs or sperm are being made, not at conception, so a mule is born healthy and grows up normally. Sperm development in male mules usually stops at an early stage, before mature sperm form.8PubMed Central. Testicular Characteristics and the Block to Spermatogenesis in Mature Hinny9PubMed Central. Testicular Characteristics and the Block to Spermatogenesis in Mature Hinny10PubMed. Comparative testis morphometry and seminiferous epithelium cycle length in donkeys and mules
Sterility is a very strong tendency rather than a perfect law, which is why rare exceptions in females show up in the record. And sterile does not mean sexless. A male mule still has testes and makes testosterone, so an ungelded john can act like a stallion.
Male Mule Hormones and Gelding
A male mule’s testes still respond to reproductive hormones by raising testosterone.11PubMed. Effect of kisspeptin-10, LH and hCG on serum testosterone concentrations in stallions, donkeys and mules An intact john can therefore show stallion-like behavior despite being sterile. Like colts, male mule foals do not produce testosterone until puberty.12PubMed Central. Postnatal Dynamics of Circulating Steroid Hormones in Mule and Equine Neonates A young john may seem calm for his first year or so, and studdy behavior usually shows up as he matures. An owner who sees a young male mule getting pushy or interested in mares should raise it with a vet, who can advise on timing.
Gelding is done for behavior and handling, since there are no foals to prevent. An intact john may fight other animals, chase mares, or be harder to control around people. Gelded johns are the usual working and riding mules. Surgical gelding is the standard way to stop stallion behavior. An experimental hormone vaccine has also lowered testosterone in young intact male mules.13Preprints. GnRH Vaccine could Suppress Serum Testosterone in Stallion Mules
Female Mules Often Cycle and Can Carry Foals
In one group of female mules, about 6 in 10 ovaries showed signs of cycling.14Journal of Equine Veterinary Science. Evidences of Regular Estrous Cycles in Mules and Successful Use of These Animals as Recipients for Donkey Embryos Many mollies ovulate and cycle regularly despite being infertile.15Equine Veterinary Journal. Reproduction in mules: Embryo transfer using sterile recipients A molly may show heat signs around horses or donkeys: she may become squealy, hold her tail aside, or flirt with geldings and stallions. Some mollies show little or no heat behavior, and a flirty one isn’t about to be bred.
In embryo transfer, a fertilized embryo from another mare is placed in a female’s uterus. Cycling mollies have carried horse embryos to normal, full-term foals.16PubMed. Successful non-surgical transfer of horse embryos to mule recipients A mule that was not cycling did not get pregnant from transferred embryos.17PubMed. Successful non-surgical transfer of horse embryos to mule recipients Mollies can carry either horse or donkey foals to term,18Equine Veterinary Journal. Reproduction in mules: Embryo transfer using sterile recipients and researchers describe them as excellent surrogate mothers.19The Horse. Study: Mules Make Excellent Moms for Equine Embryo Transfer
This is work for equine reproduction vets, not natural breeding. The foal is genetically the donor mare’s and the sire’s, not the molly’s. Her role is to carry the foal and supply the womb environment. Since she must be cycling for the transfer to work, vets track her cycle and time the transfer to it.
The Rare Fertile Molly
A female mule has been documented giving birth to a foal sired by a donkey, confirmed by chromosome and blood tests.20Journal of Equine Veterinary Science. A fertile female mule Another female mule produced several foals with different sires.21Biology of Reproduction. Mating Pattern and Chromosome Analysis of a Mule and Her Offspring Her foals by stallions had 64 chromosomes and looked like horses, while foals by donkeys were mules.22Biology of Reproduction. Mating Pattern and Chromosome Analysis of a Mule and Her Offspring Her eggs appear to have kept mostly the horse set of chromosomes.23Biology of Reproduction. Mating Pattern and Chromosome Analysis of a Mule and Her Offspring
Documented births are a handful of cases, not a trend a buyer should count on. No comparable confirmed foal from a male mule exists, since sperm production stalls.
If you hear of a mule foaling, it is a molly, and it should be verified by genetic testing. In pastures holding mares and mollies together, a foal at a molly’s side can be a mix-up, such as a mare’s foal that has attached to a molly. Only chromosome or DNA testing can confirm that a mule is the mother, and a claim made without testing should be treated as unverified.
Mules Versus Hinnies
A hinny comes from a stallion and a female donkey, the reverse of a mule’s parentage. A common memory aid is that the name follows the horse in the pairing: the mule’s horse is the mother, the hinny’s horse is the father.
Hinnies have the same 63 chromosomes and are nearly always sterile, though a few mature sperm have been found in male hinnies.24Cytogenetic and Genome Research. Meiosis in interspecific equine hybrids: I. The male mule (Equus asinus × E. caballus) and hinny (E. caballus × E. asinus) Which parent is which may also change a hybrid’s build and behavior.25PubMed Central. Comparing and Contrasting Knowledge on Mules and Hinnies as a Tool to Comprehend Their Behavior and Improve Their Welfare
Hinnies tend to be smaller and often have more donkey-like bodies, though individuals vary and looks alone are not proof of parentage. Mules often have a more horse-like body with donkey ears, but that too is a tendency, not a dependable test. Hinnies are rarer because the cross succeeds less often, so most people will meet mules far more often.
Hinnies come as males and females just as mules do. Many people use the same male and female terms for hinnies as for mules, though usage varies by place.
Choosing Between a Molly and a John
Mare mules have a reputation among experienced users for being slightly easier to handle.26RiverEarth.com. Mare Mule vs Horse Mule: Which Is Better for You? That is handler opinion, and individual training matters more. Mules of either sex are prized for endurance greater than either parent’s,27PubMed Central. Comparative Transcriptome Profiling Analysis Uncovers Novel Heterosis-Related Candidate Genes Associated with Muscular Endurance in Mules possibly in part because they inherit horse-type energy-producing cell parts from their mothers.28PubMed Central. Preliminary Evaluation of Muscle Fiber Composition in the Middle Gluteal Muscle in Race Mules and Mammoth Donkeys
When buying, ask whether a john is gelded and at what age, since this affects his behavior and whether he can live with other stock. Expect that a molly may come into heat and be distracted around other animals, especially geldings and stallions. Judge temperament, training, and soundness over sex.
Pasture mixing follows the same logic. An intact john should not run with mares or mollies, since he may chase and harass them. A molly in heat may unsettle geldings in a shared pasture. Gelded johns and mollies usually mix well with horses.