Long-Eared Lovin'

Donkeys and mules have played, and continue to play, an important role on the equine stage. Mules in particular are used for everything from pleasure riding and packing to dressage, racing, and jumping. Donkeys are family pets in many instances and, of course, are a necessary ingredient in the production of mules. While there are some similarities between donkeys and horses, there also are some significant differences, particularly in the reproductive department.

Mule foal


A great discussion on donkeys and mules took place at last December's American Association of Equine Practitioners meeting in Orlando, Fla. Presenting papers on the subject were Suzanne Burnham, DVM, of Burnham Veterinary Hospital in Brenham, Texas; D.G. Pugh, DVM, MS, Dipl. ACT., Dipl. ACVN, of Auburn University and a breeder of donkeys; Tex Taylor, DVM, Dipl. ACVS, of Texas A&M University and also a breeder of donkeys and mules; and Nora Matthews, DVM, Dipl. ACVA, of Texas A&M University. Much of what follows is taken from their presentations as well as from interviews.

"The mule," said Burnham in her opening remarks at AAEP, "has been described as the animal 'without pride of ancestry, nor hope of posterity,' or by the muleskinners as '900 pounds of free enterprise.' "

The breeding of donkeys and mules in this country has been going on for some time. When Meriwether Lewis and William Clark made their celebrated trip to the Pacific Ocean back in the early 1800s, they found that some of the Native Americans along the way had mules and, in some cases, preferred them to horses. In outlining the history of donkey and mule breeding, Burnham pointed out that a major player in the game in the late 1700s was none other than George Washington, the country's first president.

She said that Washington in 1785 investigated the purchase of a quality jack from Spain, but was told the exportation of jacks was illegal in Spain. However, when the King of Spain learned that the celebrated American leader was seeking a jack, he made a gift to Washington of both a jack and a jennet.

The jack was named Royal Gift and stood at Mt. Vernon. He was of the variety of donkeys called Mammoth, and he stood between 14.2 and 15 hands. Burnham said that his progeny were so valued that his stud fee soon was higher than that of some celebrated stallions of that era.

The next year, the Marquis de Lafayette, Washington's young French protegé during the Revolutionary War, sent his mentor a black Maltese jack that was called Knight of Malta. He, too, became an excellent sire of mules.

By 1840, Burnham said, a quality jack used for mule breeding could fetch up to $5,000 in Kentucky, a state that was, and still is, a leader in mule production.

The donkey, or domestic ass, Burnham said, is an equine species consisting of many breeds, just as there are breeds of horses and zebras. Donkeys which are under 36 inches (nine hands) at the withers are known as miniatures, while those measuring 14 hands or more are known as mammoths. Donkeys in between often are referred to as standards.

A basic difference between the horse and the donkey is that the horse has 64 diploid (having two sets) chromosomes, while the donkey has 62, and the Asiatic wild ass has 54 to 56.

There also is a difference in gestation length. For the horse it is 335-346 days; for the donkey it is 360-375 days or more.

Reproductive Variances

Pugh drew on his experience as both a scientist and a breeder in discussing the reproductive traits of the donkey as compared to the horse. There were positives and negatives. On the positive side, he said, donkeys tend to be more fertile than horses, with the conception rate averaging about 78% ( with mares averaging 65% when diagnosed at 15 days of gestation). On the negative side, however, is the fact that jennets suffer from dystocia (difficult births) more often than mares.

Some of the differences are more interesting than significant. For example, jacks generally have larger testicles and a larger penis than do stallions. To compensate, Pugh said, jennets generally have a longer cervix than mares (more on this in a moment).

Because of the size of a donkey's or mule's testicles, Burnham said, care must be taken when they are castrated. "Castrating a donkey or mule by the same method used in horses, without ligation (stitches), may result in excessive bleeding, and some have died as a result."

While the onset of puberty in the donkey is similar to that of the horse--one to two years--the donkey seems to display less seasonality than the horse, Pugh said. Although the estrous cycle in the jennet has been reported to range from 20 to 40 days, Pugh said, it usually lasts from 23 to 30 days (compared to 21-25 days in the mare). Estrus, he explained, normally lasts between six and nine days for jennets (three to eight days for mares), with ovulation five to six days after the onset of estrus (and 12-24 hours before the end of estrus in mares). Foal heat usually occurs between five and 13 days postpartum, while a mare's first day of foal heat varies from Day 7 to Day 12 and is dependent on the season of year. Mares which foal early in the year typically start foal heat later.

If natural breeding systems are used, Pugh advised, the jennet should be mated the second day of estrus, then bred every other day until the end of "standing heat."

One peculiarity of donkey reproduction is the longer time it takes for the jack to achieve an erection and ejaculate when compared to the horse, Pugh said. Jacks usually need five to 30 minutes to complete the breeding, while a stallion normally completes the task in 10 to 11 minutes.

The teasing behavior, he explained, usually includes vocalization, sniffing the vulva of the female, and one or more mounts before penile exposure and erection.

A personal anecdote: Some mares do not like to be approached by a braying jack. At one point, this writer took two mares to be bred to a Mammoth jack. The jack was extremely vocal during the pre-breeding stage. With one of the mares, according to the jack owner, this was no problem, but with the other one it was. As the jack approached, braying and snorting, the mare, though in "standing heat" would not let the jack mount her. The jack owner resorted to a blindfold and breeding hobbles to get the job done. With those restraints in place, the mare was bred and she conceived.

It can also go the other way, according to Taylor, a breeder of Mammoth donkeys. If a jack is to be used to breed mares, he advised that the animal "should grow up in a horse environment." If he doesn't, Taylor said, the jack might refuse to breed mares.

As is the case with a stallion, Pugh said, a jack can be trained to service an artificial vagina and semen can be collected and used in fresh- or cooled-semen artificial insemination. Pugh also noted that although some seasonal differences in libido are observed in the jack, there seems to be little or no alteration in seminal parameters during the winter months.

Reproductive behavior of jennets also is different than horses. Pugh said that researchers at the University of Wisconsin reported prolonged estrus, a lower incidence of ovulations during December, shorter estrous cycles from May to September, and shorter estrus from May through October with donkeys.

Jennets can be pasture- or hand-bred, he said. "Whenever a hand breeding program is employed--either with natural breeding or artificial insemination--an effective teasing program will enhance its success."

If palpation or an ultrasonography evaluation is conducted, Pugh said, follicles of 25-30 millimeters in the donkey should be considered potentially ovulatory. Interestingly, Pugh said, the incidence of multiple ovulations in Mammoth jennets appears to be higher than in standards or miniatures.

When donkey semen is collected in an artificial vagina, Pugh said, it can be handled similar to that of the stallion, with skim milk extenders. Donkey semen also can be frozen for later insemination.

While the jennet's cervix is usually longer than that of the mare, Pugh said, it is smaller in diameter. The donkey cervix, he said, protrudes into the vagina, and this protrusion might make intrauterine ejaculation impossible as normally occurs with horses, thus making artificial insemination of donkeys more difficult. This peculiarity, Pugh said, might be associated with a high incidence of cervical adhesions in jennets, which causes the jennet to have a difficult time giving birth.

Take-Home Message

While handling and breeding donkeys to either their own kind or to horses can be exciting and rewarding, the breeder would do well to obtain as much information as possible about the peculiarities of the animals/species involved. That information should include not only physiological differences, but psychological differences as well. An excellent source for this information would be a knowledgeable veterinarian, or someone who has spent years working with donkeys and mules.


To set the stage for a discussion of the breeding of donkeys to produce other donkeys, or the crossing of donkeys and horses to produce mules, we must understand the nomenclature. Here is a basic glossary:

  • Donkey--A unique equid of special qualities, with long ears, a distinctive bray, hard hooves, and a great deal of stamina and strength.
  • Jack--A male donkey.
  • Jennet or Jenny--A female donkey.
  • Mule--A cross between a jack and a mare. A mule normally is sterile.
  • Hinny--A cross between a
    stallion and a jennet. Like the mule, a hinny normally is sterile.
  • John--A male mule.
  • Molly--A female mule.

About the Author

Les Sellnow

Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.

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