A female donkey is called a jenny, sometimes spelled “jennet.” The term applies to any female donkey regardless of age or breed, and it parallels the word “jack” used for an intact male donkey. These names show up constantly in veterinary literature, breeding registries, and livestock management guides, yet many people outside the equine world have never encountered them. The terminology is just the starting point, though, because jennies have a surprisingly rich story involving reproduction, milk production, livestock protection, and even endangered-breed conservation.
Donkey Terminology Beyond Jenny and Jack
Donkey naming conventions are straightforward once you learn the handful of terms. A jenny is any female donkey. A jack is an intact (uncastrated) male. A gelding is a castrated male. Young donkeys of either sex are foals until weaned, at which point they become fillies (young females) or colts (young males) until maturity. The species name is Equus asinus, placing donkeys in the same genus as horses (Equus caballus) but in a separate species.
You’ll also hear the word “burro,” which is simply the Spanish word for donkey and is commonly used in the American Southwest to refer to small feral or wild-roaming donkeys. “Ass” is the older English term for the species and still appears in scientific writing, though it has fallen out of casual use. None of these words changes based on sex; “jenny” and “jack” are the only sex-specific terms for the species.
How a Jenny’s Reproductive Cycle Works
Jennies are seasonally polyestrous in many climates, meaning they cycle repeatedly during warmer, longer-light months, though in tropical regions they can cycle year-round. The estrous cycle of a jenny averages roughly 23 to 24 days from one ovulation to the next. Research on Caribbean donkeys found a mean cycle length of about 23 days, with nearly all cycles showing a two-wave pattern of follicle development on the ovaries.1PubMed Central. Structural and Functional Dynamics of the Ovary and Uterus during the Estrous Cycle in Donkeys in the Eastern Caribbean A separate study categorized jenny cycles into short (about 21 days), medium (about 24 days), and long (about 26 days) groups, and found that variation in cycle length came mostly from differences in how long the follicular phase lasted rather than from the luteal phase, which stayed relatively constant.2Theriogenology. Ovarian dynamics and estrous cycle length in the donkey (Equus asinus)
Estrus, the portion of the cycle when a jenny is receptive to a jack, typically lasts about six to seven days. Work on the endangered Asinina de Miranda breed in Portugal recorded an average estrus of roughly 6.6 days, with body condition and age both influencing cycle timing.3Theriogenology. Characterization of the estrous cycle of Asinina de Miranda jennies (Equus asinus) Compared with mares, jennies tend to show longer and sometimes less predictable heat periods, which can make breeding management trickier. Behavioral signs of estrus in jennies are also subtler than in horses; a jenny in heat may show jaw-clapping, ear-flattening, and increased proximity to a jack, but she rarely “winks” the vulva as obviously as mares do.
Pregnancy and Foaling in Jennies
Donkey pregnancies are famously long. A jenny carries her foal for an average of about 371 days, which is roughly two weeks longer than a typical horse pregnancy. Research on Martina Franca jennies in Italy found that the sex of the foal was the only factor that significantly influenced pregnancy length: jennies carrying male foals tended to gestate longer than those carrying females.4Theriogenology. Factors affecting pregnancy length and phases of parturition in Martina Franca jennies Other variables like the jenny’s age, the month she ovulated, and the foal’s birth weight did not meaningfully shift the timeline.
Foaling itself tends to progress through three stages. In that same study, the first stage of labor, when the jenny becomes restless and her uterus begins contracting, averaged about 65 minutes. The second stage, actual delivery, averaged about 19 minutes. The third stage, passing the placenta, took around 58 minutes. These windows are broadly similar to horses, though individual jennies vary.
One area where jennies differ from mares is placental structure. A comparative study of donkey, pony, and Thoroughbred placentas showed strong similarities across species, but the donkey placenta had a more complex branching pattern in its microcotyledons, the tiny structures through which nutrients and waste are exchanged. The donkey placenta had a significantly higher surface density, meaning more exchange surface packed into a given volume of tissue.5Theriogenology. A comparative stereological study of the term placenta in the donkey, pony and Thoroughbred This denser architecture may help sustain the foal through the jenny’s longer gestation.
Artificial Insemination and Breeding Management
Breeding jennies is not always as simple as putting a jack in a pasture with them. Many conservation programs and commercial donkey operations now rely on artificial insemination to manage genetics carefully. Timed insemination protocols have been developed specifically for jennies, using hormone treatments to trigger ovulation predictably. One protocol found that ovulation could be reliably induced when the dominant follicle was in the 31-to-35 millimeter range, and that insemination with cooled semen achieved a pregnancy rate of about 49 percent per attempt.6PubMed. Practical protocols for timed artificial insemination of jennies using cooled or frozen donkey semen Frozen semen produced lower rates. These numbers are helpful for breeders managing small populations of rare donkey breeds, where every successful pregnancy counts.
Interspecies pregnancies add another layer of complexity. When a jenny is bred to a horse stallion, the offspring is called a hinny. When a mare is bred to a jack donkey, the offspring is a mule. In both cases, a jenny carrying a hybrid pregnancy encounters unusual hormonal dynamics. Research has shown that the uterine environment of the mother profoundly alters how the placenta develops in these cross-species pregnancies. The specialized structures called endometrial cups, which produce a key pregnancy hormone, behave very differently depending on whether the hybrid embryo is developing in a horse uterus or a donkey uterus.7Reproduction. Effects of fetal genotype and uterine environment on placental development in equids
Mules, Hinnies, and the Rare Fertile Female
Because donkeys have 62 chromosomes and horses have 64, their hybrid offspring (mules and hinnies, with 63 chromosomes) are almost always sterile. The mismatched chromosome sets cannot pair up properly during the cell divisions that produce eggs or sperm. Male hybrids are effectively always infertile because they produce too few viable sperm cells. Female mules and hinnies, however, have on extremely rare occasions produced offspring. Researchers have proposed that this occasional fertility in females could arise if chromosomes of similar ancestral origin happen to segregate to opposite poles during egg formation, producing a balanced egg by chance.8Cambridge University Press. Does ‘affinity’ hold the key to fertility in the female mule? Detailed chromosome painting studies have mapped out the rearrangements between horse and donkey genomes to help explain both the usual sterility and these surprising exceptions.9PubMed. 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
The distinction between a mule and a hinny matters here. A mule has a jenny’s mitochondrial DNA (inherited from the horse mother) and a hinny has a jenny’s mitochondrial DNA (inherited from the donkey mother). In practice, mules are far more common because jacks are easier to breed to mares than stallions are to breed to jennies, and because mules tend to be larger and more commercially useful for draft work. Hinnies, born from jennies, are rarer and often smaller.
Jenny Milk and Its Unusual Nutritional Profile
One of the most economically interesting things about jennies is their milk. Donkey milk has attracted growing scientific attention because its composition resembles human breast milk more closely than cow milk does in several respects. The protein profile stands out: donkey milk contains a high proportion of whey proteins, roughly 55 to 65 percent of total protein, which is much closer to the whey-to-casein ratio in human milk than what you find in cow milk.10PubMed Central. Nutritional Composition and Biological Activities of Donkey Milk: A Narrative Review This makes it potentially easier to digest for infants or adults who react to cow milk proteins.
Fat content is low, generally well under 1 percent, with a higher proportion of unsaturated fatty acids than cow milk.11Dairy. Milk Properties and Morphological Characteristics of the Donkey Mammary Gland for Development of an Adopted Milking Machine—A Review Lactose content is higher than in cow milk, sitting between about 6 and 7 percent, which gives it a naturally sweeter taste. Donkey milk is also rich in lactoferrin and lysozyme, two proteins with antimicrobial properties, and contains roughly five times as much vitamin C as cow milk.10PubMed Central. Nutritional Composition and Biological Activities of Donkey Milk: A Narrative Review A review of the literature has attributed antioxidant, antimicrobial, and anti-inflammatory properties to donkey milk’s bioactive compounds, including immunoglobulins and cytokines.12Frontiers in Nutrition. Is there sufficient evidence to support the health benefits of including donkey milk in the diet?
Donkey milk is expensive and produced in small quantities, partly because jennies yield far less milk per session than dairy cows and require the foal to be present during milking to stimulate letdown. These practical constraints have kept it a niche product. Still, demand has been rising in parts of Europe and Asia, both for infant nutrition and for cosmetic products. That economic interest has, in turn, given some endangered donkey breeds a reason to be maintained rather than abandoned.
Jennies as Livestock Guardians
A role that surprises many people is the jenny as a livestock guardian. Since the late 1970s and 1980s in North America, ranchers have placed donkeys, particularly single jennies or geldings, in pastures with sheep or goats to deter coyotes and feral dogs. A global review of the practice found that in early surveys of producers, about 60 percent of donkeys were rated fair to excellent at deterring coyotes.13PubMed Central. A global review of Livestock Guardian Donkeys (LGDks) in the prevention of carnivore depredation Jennies are often preferred over jacks for this job because jacks can be aggressive toward the livestock they are supposed to protect. A single jenny bonded to a flock in a small, open pasture tends to work best.
The behaviors that make donkeys effective are deeply instinctive. Donkeys have a low-flight response, meaning instead of running from a predator they stand their ground, face it, and often charge. They bray loudly as an alarm, chase intruders, and can strike with their front hooves. These traits are effective against mid-sized canids, though they are not reliable against larger predators like wolves or bears. The guardian donkey approach is low-cost compared with livestock guardian dogs and requires no training, but it works best in specific conditions and is not a universal solution.
Social Behavior of Jennies
Donkeys are often mischaracterized as stubborn loners, but research paints a very different picture. Their social behavior is dominated by affiliative interactions, friendly behaviors like mutual grooming, standing close, and resting together, which far outnumber aggressive encounters. A study of domestic donkey groups found that these affiliative behaviors were concentrated within consistent pairs, supporting the idea that pair bonding is a defining feature of donkey social life.14Applied Animal Behaviour Science. Characterization of social behavior in a group of domestic donkeys (Equus asinus)
Among feral donkeys on the Caribbean island of St. John, the most common social grouping for adult females was one or more jennies with their offspring. Beyond the bond between a jenny and her most recent foal, though, these groupings were fluid and unstable.15Applied Animal Behaviour Science. The social organisation of feral donkeys (Equus asinus) on a small Caribbean island (St. John, US Virgin Islands) This contrasts with horses, where mares tend to form long-lasting harem bands around a stallion. Donkey social structure is looser, built more around individual friendships and mother-offspring bonds than around stable multi-adult groups. For anyone managing jennies in a domestic setting, this means separating bonded pairs can cause real stress, and introductions between unfamiliar donkeys should be handled gradually.
Origins and Domestication
Every domestic donkey alive today descends from the African wild ass, a species that once ranged across the arid regions of northeast Africa. Genetic studies tracing mitochondrial DNA, which is passed down exclusively through the maternal line, from jenny to foal, have identified two major lineages in domestic donkeys corresponding to two subspecies: the Nubian wild ass and the Somali wild ass.16PubMed Central. Ancient DNA provides new insight into the maternal lineages and domestication of Chinese donkeys Both lineages are present in donkey populations worldwide, indicating that domestication drew on at least two distinct wild populations.
Ethiopian donkey populations are genetically diverse and may sit close to one of the original centers of donkey domestication. A study of six morphologically different donkey populations across Ethiopia found high haplotype diversity, and network analysis of their mitochondrial sequences placed the Ethiopian lineages at the center from which most other worldwide domestic donkey populations appeared to radiate.17Livestock Science. Genetic diversity and matrilineal genetic signature of native Ethiopian donkeys (Equus asinus) inferred from mitochondrial DNA sequence polymorphism This is consistent with archaeological evidence placing donkey domestication in northeast Africa roughly 5,000 years ago, making the donkey one of the earliest domesticated animals.
Conservation of Endangered Donkey Breeds
Donkey populations worldwide have declined dramatically since mechanization replaced animal draft power. The Martina Franca donkey of southern Italy illustrates the pattern. Once famous for producing high-quality mules, the breed was driven to the brink of extinction in the 1980s when agricultural machinery eliminated demand for draft animals. By the time conservation efforts began in earnest, the breed register held fewer than 500 animals.18Journal of Animal Science. Monitoring of genetic diversity in the endangered Martina Franca donkey population
More recent pedigree analysis of the same breed showed that despite some demographic recovery driven partly by interest in donkey milk and donkey-assisted therapy, genetic diversity remained critically low. Inbreeding had increased from about 7 percent in 2009 to 10 percent by 2020, and the effective population size had dropped well below the threshold that conservation geneticists consider safe for a population’s long-term survival.19Frontiers in Animal Science. Conservation and genetic analysis of the endangered Martina Franca donkey using pedigree data The Martina Franca is not unique in this respect. Dozens of traditional donkey breeds across Europe, Asia, and Africa face similar genetic bottlenecks, and in many cases the jennies that remain represent the last carriers of maternal lineages stretching back thousands of years.
Finding new economic roles for jennies, whether through milk production, therapy programs, or livestock guardianship, has become a practical conservation strategy. When a breed has a reason to exist in the modern economy, farmers are more likely to maintain breeding herds. The irony is that the very traits that made donkeys seem obsolete, their small size, low speed, and stubbornness compared with horses, are precisely the traits that make jenny milk, guardian donkeys, and therapy animals viable. Slow, calm, and food-efficient turns out to be an advantage when the job is no longer pulling a plow.