Mules are not all one sex. Like horses and donkeys, they are born male or female. What nearly all mules share is not a sex but an inability to have offspring. Even that rule is not quite absolute, and the rare exceptions are female.
Where Mules Come From and Why Both Sexes Exist
A mule is the offspring of a male donkey (a jack) bred to a female horse (a mare). The reverse pairing, a male horse with a female donkey (a jenny), produces a hinny. Mules and hinnies can differ in looks and behavior depending on which parent was the donkey, because some genes act differently depending on which parent they came from.1PubMed Central. Comparing and Contrasting Knowledge on Mules and Hinnies as a Tool to Comprehend Their Behavior and Improve Their Welfare
Sex is settled at conception the same way it is in horses and donkeys. Every mule gets one sex chromosome from each parent. The mare always gives an X, so the jack’s contribution decides the matter: if the sperm that fertilizes the egg carries an X, the foal is a female, and if it carries a Y, the foal is a male. The two parent species have different chromosome counts, but this step works the same in either, so nothing about the cross favors one sex. A mule foal is about equally likely to be a colt or a filly.
The all-male or all-female myth usually grows from confusing “sterile” with “one sex” or “no sex.” Because mules can’t breed, some people assume they lack normal reproductive organs. They have them: males have testes, and females have ovaries and a uterus.
Telling a mule from a hinny by eye is unreliable. Mules often have a more horse-like body and larger size, since the mother is a horse, while hinnies are often smaller with more donkey-like features. The looks overlap, though, so only knowing the parents confirms which one you have.
What Male and Female Mules Are Called
- Male mule: formally a horse mule, informally a john or john mule.2Wikipedia. Mule
- Female mule: formally a mare mule, informally a molly or molly mule.3Wikipedia. Mule
- Young mules: a mule colt (male) or a mule filly (female).4Wikipedia. Mule
- Which to use: among owners, horse mule and mare mule are the proper terms, while john and molly are casual but widely used.5Pea Ridge Times. Mule talk: Learning the language of mule owners
“Horse mule” trips up newcomers. It means a male mule, not a horse and not a horse-mule cross. “Mare mule” likewise means a female mule, not a mare. Sale ads usually use these formal terms, so knowing them keeps you from misreading a listing or a vet’s notes.
If an ad says only “mule,” ask which sex it is, since price, handling, and care questions differ between the sexes. Mixed use is normal: a seller may say “molly” while a vet record says “mare mule,” and both mean the same animal.
A castrated male mule is usually called a gelding or a gelded john. Most male mules you meet will already be gelded, and sellers rarely offer intact males for riding or work.
Why Mules Can’t Have Babies
Chromosomes are the packages of genes in every cell. Horses have 64 and donkeys have 62.6Proceedings of the Japan Academy. A Revised Study of the Chromosomes in the Horse, the Ass and the Mule A mule gets 32 from its horse mother and 31 from its donkey father, for 63.7The Tech Interactive. Why can’t mules breed?
Making eggs or sperm requires each chromosome to pair with a matching partner. Horse and donkey chromosomes differ in arrangement, so many can’t find partners.8The Tech Interactive. Why can’t mules breed? The process that makes sperm and eggs then breaks down.9The Tech Interactive. Why can’t mules breed?
The odd count adds to the problem: with 63 chromosomes, at least one is always left without a partner. But the odd number is not the only cause, because the mismatch in arrangement matters as well.
That mismatch exists because horse and donkey chromosomes carry mostly the same kinds of genes, packaged and arranged differently. A hybrid animal can still be healthy, because the body can run on mixed gene sets, yet fail at the one step that demands exact matching.
In male mules, sperm production stops partway and the developing cells break down.10Heredity. The pachytene checkpoint and its relationship to evolutionary patterns of polyploidization and hybrid sterility No mature sperm are found in mule testes.11Indian Journal of Veterinary Anatomy. Morphometric and Histological Observations on the Testis of Mule and Donkey of Himalayan Region In one study of a hinny, the testes held no developing sperm at all, while a stallion and a jack had plenty.12PubMed Central. Testicular Characteristics and the Block to Spermatogenesis in Mature Hinny For practical purposes, male mules and hinnies are always sterile.
Can a Female Mule Ever Get Pregnant
Female mules usually go through heat cycles but are rarely fertile.13PubMed. Key Aspects of Donkey and Mule Reproduction In heat they act much like female donkeys, with chewing movements, frequent urination near males, and pushing for male attention.14Journal of Equine Veterinary Science. Evidences of Regular Estrous Cycles in Mules and Successful Use of These Animals as Recipients for Donkey Embryos
Heat comes from hormones working normally, which is separate from whether a viable egg is released. A mule can cycle regularly for years with essentially no chance of pregnancy, since her eggs are almost always unusable.
Still, real exceptions exist. In China, a female mule and a female hinny were confirmed by chromosome testing to be true hybrids, and each foaled after mating with a donkey.15PubMed. A fertile mule and hinny in China Another confirmed female mule produced a male foal fathered by a donkey.16PubMed. Male mule foal qualifies as the offspring of a female mule and jack donkey The likely reason is that some horse and donkey chromosomes are similar enough that an egg occasionally forms viable.17PubMed. Refined genome-wide comparative map of the domestic horse, donkey and human based on cross-species chromosome painting: insight into the occasional fertility of mules
Male mules never get the same exceptions. Sperm development fails at a stage every male mule reaches, while eggs form through a different path that occasionally survives. Claims of fertile mules are therefore mostly about females, and they need chromosome testing to confirm.
Many “my mule had a foal” stories turn out to be a hinny, a donkey, or a misidentified animal. A mule might also have simply mothered another mare’s foal. Only a chromosome test proves the mother was a mule, and verified cases are rare enough to be published one at a time.
Female mules can also carry pregnancies that aren’t their own. A cycling mule can carry a horse embryo placed in her uterus and give birth to a normal foal.18PubMed. Successful non-surgical transfer of horse embryos to mule recipients In that work, a mule that wasn’t cycling did not get pregnant when given embryos.19PubMed. Successful non-surgical transfer of horse embryos to mule recipients Cycling mules have also been used to carry donkey embryos.20Journal of Equine Veterinary Science. Evidences of Regular Estrous Cycles in Mules and Successful Use of These Animals as Recipients for Donkey Embryos
Living With a Male or Female Mule
Intact male mules are very sexually active even though they are sterile.21Pea Ridge Times. Mule talk: Learning the language of mule owners An intact john acts like a stallion, which can mean mounting, aggression toward other animals, and trouble focusing on training. That is why males are usually castrated before work training,22Preprints. GnRH Vaccine could Suppress Serum Testosterone in Stallion Mules and owners generally advise gelding all male mules.23Pea Ridge Times. Mule talk: Learning the language of mule owners
Castration (gelding) carries some risk, though: donkeys, mules, and mature horses face the highest risk of complications.24CVMA. Surgical Castration of Horses, Donkeys, and Mules So have it done by a vet experienced with mules, ideally while the animal is young. Ask about timing, and about aftercare such as stall rest, watching for swelling or bleeding, and who to call if something looks wrong.
Mare mules have a reputation among longtime mule users as slightly easier to handle.25RiverEarth.com. Mare Mule vs Horse Mule: Which Is Better for You? That is an owner opinion, not a measured difference. Because mare mules cycle, expect heat behavior around males.
For a first-time owner, temperament and training matter more than sex, and a gelded john can be as calm as a molly. Ask sellers about heat behavior in mares and about when a john was gelded. Watch the mule around other animals before you buy.