Donkeys are not all male. Like virtually every other mammal, donkeys come in both male and female forms, and healthy populations maintain a roughly even split between the sexes. The confusion is surprisingly common and stems from a tangle of folk terminology, the widespread visibility of working male donkeys, and mix-ups between donkeys and their hybrid offspring, mules. Once you untangle the vocabulary, the biology is straightforward: female donkeys exist in enormous numbers worldwide and play critical roles in breeding, dairy farming, and conservation of heritage breeds.
Why So Many People Think Donkeys Are Male
The misconception has a few distinct roots, and they tend to reinforce each other. First, there is the naming problem. A male donkey is called a “jack” (or “jackass”), and a female donkey is called a “jenny” (or “jennet”). Many people have heard the word “jackass” used as a generic insult and assume it refers to all donkeys rather than specifically to males. Meanwhile, “jenny” is far less common in everyday speech, so the female term fades from public awareness.
Second, mules muddy the water. A mule is the hybrid offspring of a male donkey and a female horse. Because the sire is always a donkey and the dam is always a horse, people sometimes conclude that donkeys only come as males and horses only come as females, which is wrong on both counts. The reverse cross, a male horse bred to a female donkey, produces a different hybrid called a hinny. Hinnies are less common mainly because jennies have a harder time carrying horse-sired pregnancies to term, not because female donkeys are rare.
Third, in many working cultures, male donkeys are more visible. Jacks and geldings (castrated males) are often preferred as pack and draft animals because of their larger size and greater physical strength. Jennies, meanwhile, may be kept closer to the farm for breeding or milking. This selection bias means a traveler encountering donkeys on a road or work site is disproportionately likely to see males, which feeds the impression that all donkeys are male.
How Male and Female Donkeys Differ Physically
At first glance, jacks and jennies look quite similar. Unlike some species where males are dramatically larger or more colorfully marked, donkeys show only modest differences between the sexes. A study measuring nearly 200 donkeys across Bulgaria found that males averaged about 120.4 cm at the withers while females averaged about 119.8 cm, a gap of less than a centimeter. Body length was similarly close, with males at roughly 124.5 cm and females at roughly 123.7 cm. Males did tend to have slightly larger chest and cannon bone circumferences, but the overlap between sexes was considerable.1Journal of Central European Agriculture. CHARACTERISTICS OF THE MAIN BODY MEASUREMANTS OF MALE AND FEMALE DONKEYS IN BULGARIA
The most reliable way to tell jacks from jennies is the same as with horses: check the anatomy directly. Intact males have visible external genitalia and, once mature, tend to develop a thicker neck and a more muscular jaw. Jacks are also generally more vocal and territorial, with the classic donkey bray being louder and more frequent in males during breeding season. Geldings lose some of these behavioral markers after castration but retain the slightly heavier build. Jennies have a somewhat finer bone structure and tend to be calmer in temperament, though individual personality varies enormously within both sexes.
The Reproductive Life of a Jenny
Female donkeys have a well-studied reproductive cycle that differs in several ways from a horse’s. Research tracking nine jennies through multiple ovarian cycles found that the average interval between ovulations was roughly 21 to 26 days, with most of the variation coming from differences in the follicular phase rather than the luteal phase. In other words, how long it takes the next egg to mature and ovulate varies from cycle to cycle, but the period of progesterone dominance stays relatively constant.2PubMed. Ovarian dynamics and estrous cycle length in the donkey (Equus asinus)
Jennies also experience what is called “foal heat,” the first estrous cycle after giving birth. This typically begins about seven days after foaling, and ovulation follows roughly 13 days after parturition. Research comparing foal-heat cycles to regular cycles found that the duration of estrus behavior and the pattern of follicle growth were similar in both situations, meaning jennies can conceive again relatively soon after delivering a foal.3Animal. Reproductive characteristics of foal heat in female donkeys Double ovulations, where two eggs are released in a single cycle, occurred in about one in five jennies, which can lead to twin pregnancies.
Gestation in donkeys is longer than in horses. A large study of Spanish donkey breeds found an average gestation length of about 362 days, just under a full year. The timing of breeding within the season influenced gestation length, with jennies bred earlier in the year carrying slightly longer. Interestingly, breed, the jenny’s age, the sex of the foal, and other factors did not significantly change how long pregnancy lasted.4PubMed. Factors affecting gestation length and estrus cycle characteristics in Spanish donkey breeds reared in southern Spain
Donkey Milk and the Agricultural Value of Jennies
One of the clearest demonstrations that female donkeys are both real and economically important is the growing market for donkey milk. Donkey milk has attracted attention as a specialty product, particularly in parts of Europe, because its composition is closer to human breast milk than cow’s milk is. It is lower in fat and protein than cow’s milk but higher in lactose, and it contains bioactive compounds that researchers are still characterizing.
A study of grazing jennies in a free-range system found that individual milk production averaged about 0.61 kg per animal per day, which is modest compared to dairy cattle but meaningful for niche markets. The composition of the milk changed as lactation progressed: total solids, protein, and casein decreased over time, while lactose and fat stayed relatively stable. Older jennies produced milk with slightly higher protein and fat content, a difference that accumulated gradually with each year of age.5Journal of Food Composition and Analysis. Donkey milk composition is altered by lactation stage and jennies age
Donkey milk has traditionally been used in parts of southern Europe and the Middle East, and demand has been rising in the cosmetics industry as well, where it appears in soaps, creams, and face masks. Several European countries now have small-scale donkey dairies that rely entirely on herds of jennies, making the notion that all donkeys are male particularly ironic in these communities.
Jacks, Jennies, and Hybrids
The existence of mules is probably the single biggest driver of the “all donkeys are male” myth, so it is worth laying out exactly how the crosses work. A mule is produced by mating a jack (male donkey) with a mare (female horse). A hinny is produced by mating a stallion (male horse) with a jenny (female donkey). Both hybrids are possible because horses and donkeys are closely related, but they carry different chromosome numbers: donkeys have 62 chromosomes and horses have 64, giving hybrids 63, an odd number that makes normal cell division during egg and sperm formation extremely difficult.
The result is that mules and hinnies are almost always sterile. Both male and female mules exist, and both male and female hinnies exist, but they cannot typically reproduce. That said, “almost always” is doing real work in that sentence. Reports of fertile female mules have persisted for a long time, and at least one case has been verified through chromosome analysis: a female mule produced a male foal sired by a jack donkey.6PubMed. Male mule foal qualifies as the offspring of a female mule and jack donkey These cases are exceedingly rare, but they demonstrate that the sterility barrier is not absolute. Fertile male mules, by contrast, have never been reliably documented, which makes the female mule fertility cases all the more unusual from a genetic standpoint.
Hinnies are less commonly produced than mules for practical reasons. Jennies have a smaller body frame than mares, so carrying a horse-sired foal with its potentially larger size is harder on them. Conception rates for the hinny cross are also lower. None of this means jennies cannot breed; it just means the interspecies cross is more challenging when the jenny is the mother. Within-species breeding, a jack with a jenny, produces a normal donkey foal with no fertility issues at all.
Donkey Chromosomes and Sex Determination
Like other mammals, donkeys determine sex through X and Y chromosomes. A jenny carries two X chromosomes (XX), while a jack carries one X and one Y (XY). The donkey karyotype consists of 62 chromosomes total, and research into their chromosome structure has revealed some interesting features. The chromosomes of Equus asinus show distinctive patterns of constitutive heterochromatin, with bands appearing in positions that differ from those seen in horses.7PubMed. Polymorphic organization of constitutive heterochromatin in Equus asinus (2n = 62) chromosome 1 These chromosomal differences between donkeys and horses are part of why hybrids end up with mismatched chromosome sets and reduced fertility.
The sex ratio at birth in donkeys is close to 50:50, as it is in most mammals. Human management sometimes skews the visible ratio. In some breeding programs, a single jack is kept to service many jennies, so a herd might have 20 females and one or two males. That arrangement reflects a management choice, not a biological reality about what sex donkeys can be. In feral donkey populations, the ratio is closer to even, though dominant jacks may control access to groups of jennies, making the social structure look superficially male-dominated even when the numbers are balanced.
Behavioral Differences Between the Sexes
Jacks are more aggressive than jennies, full stop. Intact male donkeys are territorial and can be dangerous around other livestock, other male donkeys, and sometimes people. This is one reason many working donkeys are gelded: castration reduces the hormonal drive behind territorial behavior, biting, and kicking. A gelding retains the size and strength of a male donkey with a temperament much closer to a jenny’s.
Jennies are generally described as more docile and cooperative, though “docile” in donkey terms is still more independent-minded than most horses. Donkeys of both sexes are cautious, deliberate animals that resist being rushed, a trait sometimes mistaken for stubbornness. Jennies with foals at their side can be fiercely protective and will kick or bite perceived threats, which is why donkeys of both sexes are sometimes used as livestock guardians for sheep and goat herds. A single jenny placed with a flock of sheep will bond with them and actively chase off coyotes and dogs, a practical role that again depends on female donkeys being real, available, and common.
In feral and semi-feral populations, jennies form stable social groups with their offspring, while jacks tend to be more solitary or loosely associated with groups of females. A dominant jack will defend access to a group of jennies against rival males, using loud braying, posturing, and physical combat when necessary. These social dynamics mirror what is seen in wild equid species more broadly, where females form the core of social groups and males either defend a harem or live as bachelors.
Sex-Specific Health Considerations
Veterinary care for donkeys requires awareness of sex-specific issues, just as it does for horses. Jacks are prone to conditions affecting the reproductive tract, including inguinal hernias and sheath infections that require regular cleaning. Geldings largely avoid these issues but still need monitoring for complications at the castration site, especially if gelded later in life.
Jennies face their own set of concerns. Reproductive tract infections, difficult births (dystocia), and retained placentas are all risks that increase with age and the number of previous pregnancies. Mammary health matters for jennies used in milk production, where somatic cell counts in the milk serve as an indicator of udder health. Jennies are also susceptible to ovarian tumors, though these are less common than in horses. One quirk of donkey medicine is that drug dosing and anesthetic protocols often need to be adjusted from horse guidelines, and in some cases the adjustments differ by sex due to differences in body composition and fat distribution between jacks and jennies.
Perhaps the most important sex-specific health concern is unwanted breeding. Jacks reach sexual maturity by about one year of age and can be persistent breeders. In mixed herds or on farms where donkeys are kept alongside horses, an intact jack can impregnate mares to produce mule foals, which may or may not be desired. Managing this risk is a routine part of donkey husbandry and underscores why understanding that donkeys come in both sexes matters for anyone keeping them.
Heritage Breeds and the Conservation of Jenny Bloodlines
Many traditional donkey breeds around the world are now endangered, and conservation efforts depend heavily on jennies. Breeds like the Poitou donkey of France, the Asinara donkey of Sardinia, and the American Mammoth Jackstock all have limited breeding populations where every jenny’s reproductive contribution counts. In these programs, jennies are the bottleneck: a single jack can sire dozens of foals per year through natural service or artificial insemination, but each jenny can produce only one foal per year given the roughly 362-day gestation.4PubMed. Factors affecting gestation length and estrus cycle characteristics in Spanish donkey breeds reared in southern Spain
This asymmetry means that the effective population size of a donkey breed is driven more by the number of breeding jennies than by the number of jacks. Losing even a few jennies from a critically endangered breed can set conservation back by years. Breed registries track jenny lineages meticulously, and some conservation programs use assisted reproduction techniques, including embryo transfer, to maximize the genetic contribution of high-value jennies. The idea that donkeys are all male is not just wrong; in the context of breed conservation, it obscures the very animals on which the survival of these breeds depends.
Donkey populations globally have been declining in many regions as mechanized transport replaces animal labor, while increasing in others where donkeys remain essential for transport, agriculture, and milk production. In both cases, reproductive management of jennies is central to maintaining healthy, sustainable numbers. Whether the goal is preserving a rare French breed or supporting a working herd in rural Ethiopia, the female donkey is the linchpin.