Answer Man

Ever wonder why mules can’t reproduce? Here’s the explanation

Donkeys and Mules 2018

Sights from donkeys and mules at the Fort Worth Stock Show and Rodeo
Up Next
Sights from donkeys and mules at the Fort Worth Stock Show and Rodeo

Q: I know the conclusion, but not the biology, of this question: Why can’t two mules produce baby mules? Why is a horse necessary in the process? Is it a problem of the male or the female, or are there no female mules? Horses get a lot of credit, but without mules we would never have conquered the West.

Scott Townsend, of Marissa

A: It’s a numbers game, Scott, and, like most people who dream of winning that big lottery prize, mules are forever doomed to lose the procreating jackpot.

Of course, the numbers we are talking about here are chromosomes, not powerballs. Mules, you see, result from a freak of nature. When people go looking for a mate, we choose someone from our own species — i.e., another human. We don’t tend to mess around with giraffes or elephants.

That’s not always the case with donkeys and horses, which are of two different species. Usually, it’s the male donkey (jack) mating with a female horse (mare) to produce a mule, which can come in either sex. Less frequently, a male horse (stallion) will mate with a female donkey (jenny) to produce a hinny, which also come in both varieties. But while mules have turned out to be extremely valuable work animals, neither mules nor hinnies can mate among themselves to produce their own offspring because of their odd origins. Here’s why:

Humans have 23 pairs of chromosomes, which enable your genetic information to be passed on to your children. When human eggs and sperm collide, you have 23 chromosomes from each parent. So your dad’s chromosome No. 1 lines up with mom’s No. 1, No. 2 lines up with No. 2, etc. Each pair is nearly identical, so when egg and sperm merge, the pairs usually combine smoothly to create another healthy baby that eventually will be capable of carrying on the species.

Not so with mules and hinnies. Horses, you see, have 32 pairs of chromosomes while a donkey has only 31. As a result, their offspring will inherit 32 horse chromosomes and 31 donkey chromosomes, which, as you can see, leaves one chromosome that can’t match with anything. If you know a little about genetics, you know that some humans do occasionally wind up with an extra copy of chromosome 21. The result is a person with Down syndrome. Other extra chromosomes can produce miscarriages in humans. In mules, this extra chromosome poses a problem when it comes to creating sex cells.

But an extra chromosome is not the only problem. As mentioned previously, human chromosomes are similar whether coming from the mom or dad. Without getting into all the mumbo jumbo of how sex cells are created (a process known as meiosis), let’s just put it in simple terms: Horse and donkey chromosomes are so dissimilar that mules and hinnies only rarely can produce the cells critical for reproduction — eggs and sperm. Hence, as we will see, the chances of mules and hinnies having babies without a little outside help is infinitesimally small.

So how can such creatures exist in the first place? Having an odd number of chromosomes doesn’t matter for everyday life — only in sexual relations, says Monica Rodriguez, a Stanford genetics student who wrote on the topic a few years ago.

“A mule’s cells can divide and make new cells just fine,” she said of the animals, which are generally more patient and longer lived than horses yet less obstinate and more intelligent than donkeys. “A mule’s cells can divide and make new cells just fine. (But) chromosomes sort differently in regular cells than they do in sperm and egg. Instead of matching up, they just sort into two new cells. So for the mule, each cell ends up with 63 chromosomes. Nothing on the extra or missing chromosome causes it any harm.”

But combined with the dissimilarities in the horse-donkey chromosomes, the end result is usually — but not always — sterile offspring. While there are no recorded cases of fertile mule stallions, there have been a few dozen cases of mule mares giving birth after mating with a horse or donkey. In fact, the ancient Greek historian Herodotus saw a mule giving birth as an ill omen for Xerxes’ invasion of Greece in 480 B.C.:

“There happened also a portent of another kind while he was still at Sardis — a mule brought forth young and gave forth a mule.”

And, in 2002, the BBC ran a story about how a 14-year-old mother mule in Morocco surprised everyone by giving birth.

“The mule’s aged owner did not realize the mule was pregnant and rode her (12 miles) to market the day before the birth,” according to the report. Soon, streams of people were flooding the woman’s farm with gifts to celebrate the miracle.

“Miracle” is probably an apt description. According to the BBC report, only 60 cases of mules giving birth were recorded from 1527 to 2002, nearly 500 years. In recent times, mules produced a filly in China in 2001 and colts to mules in Morocco in 2002 and Colorado in 2007. However, according to the American Donkey and Mule Society, only one hinny mare has ever been known to give birth, in China in 1981. On the other hand, male mules and hinnies apparently shoot nothing but blanks when it comes to producing offspring.

Today’s trivia

What famous mule team made its debut appearance at the 1904 St. Louis World’s Fair?

Answer to Sunday’s trivia: Cahokia Mounds near Collinsville is famous for its Monks Mound, the largest man-made earthen mound in North America. But if you’d go to Effigy Mounds National Monument near Harpers Ferry, Iowa, you’d see mounds of earth in the shapes of birds, bear, deer, bison, lynx, turtles and panthers. Archaeologists believe the effigy mounds delineated territories of choice gathering and hunting grounds, but much of the data remains inconclusive, according to the National Park Service. The effigy-mound culture extends from Dubuque, Iowa, north into southeast Minnesota and across southern Wisconsin to Lake Michigan.

Roger Schlueter: 618-239-2465, @RogerAnswer