A New Cloned Horse Offers Hope for Endangered Species

The technique may finally be emerging as a way to preserve species at risk of extinction.

BLAKE RUSSELL WAS still asleep when he got an alert on his phone. His mare was giving birth. He sprang out of bed and headed to the barn, about 50 yards from his house in Gainesville, Texas.

Russell is used to getting woken up for a late-night delivery. But this foal was special. It was a clone of a rare Przewalski’s horse, a now-endangered species that once roamed the grasslands of central Asia. Crouched in the corner of the barn stall, Russell waited for its birth with anticipation. “When I saw toes and nose, I thought, ‘Whew, this is going as planned,’” he recalls.

You might be surprised that cloned animals exist—they do, but the procedure is mostly used for domesticated animals. Russell’s company, ViaGen Pets, clones around 100 horses a year, as well as cats and dogs.

Yet the technology has rarely been used for endangered animals. Up until that moment, cloning had only successfully produced a single animal of any such species. The new Przewalski’s horse, born in February and still unnamed, is the second of his kind. He’s a genetic copy of the world's first cloned Przewalski’s horse, Kurt, who was born in August 2020. Both were made from cells frozen more than 40 years ago at the San Diego Zoo. The scientists behind the effort say this second birth is evidence that cloning could be a viable strategy for saving endangered species.

“It’s certainly a milestone in conservation,” says Oliver Ryder, director of conservation genetics at the San Diego Zoo Wildlife Alliance, which worked with ViaGen and the nonprofit Revive & Restore to clone the foal. “It offers a new chance for reducing extinction risk and preserving the genetic diversity of species.”

Sandy-colored and with large heads, Przewalski’s horses are shorter and stockier than their domesticated relatives. After centuries of hunting and habitat disruption, the horses became extinct in the wild in the 1960s. Luckily, many were still living in zoos.

Starting in the 1990s, captive-born Przewalski’s horses were reintroduced into the wild to establish herds in Mongolia, China, and Kazakhstan. Today, there are about 1,900 left. Nearly all of them are descended from just 12 animals captured from native habitats between 1910 and 1960.

As a species’ numbers dwindle, so does its genetic diversity—the range of inherited characteristics within its population. Generally, the more diverse the gene pool, the longer animals live and the more offspring they have, boosting their chances of survival.

But once their population has dramatically shrunk, even if the species rebounds, genetic variation does not. “About half of the gene pool of the wild horses had been lost,” Ryder says. So scientists took matters into their own hands.

The idea of breeding livestock for desirable traits is nothing new—and for the past few decades, some ranchers have turned to cloning their most prized cattle, pigs, and sheep. The team chose the Przewalski’s horse partly because of ViaGen’s experience with cloning domestic horses, and partly because they already know a lot about how horses reproduce and how to care for foals. And perhaps most importantly, the San Diego Zoo already had stored cells from a Przewalski’s horse that was genetically different from the horses living today. Introducing that DNA into the current population could help restore lost genetic variation. “We were looking for a species that had gone through a bottleneck and could use a boost,” says Ben Novak, lead scientist at Revive & Restore.

TYPICALLY, CLONING STARTS by removing a small piece of tissue—usually a skin sample—from a living animal and isolating cells from it in the lab. For the Przewalski’s horse clones, scientists used cells that had been collected from a stallion in 1980 and then cryopreserved.

Taking one of these donor cells, the scientists transferred its nucleus, where the DNA resides, into an egg from a surrogate mother that had been hollowed out to remove its own genetic material. The egg and donor cell joined together, and the embryo grew in a test tube until it matured enough to be transferred to the surrogate mother’s womb. (Domestic horses were used to carry the pregnancies for both Kurt and the new foal.) None of the animal’s genes changed in the process, so the resulting foal is an identical twin of the original horse—just born at a later time.

The birth of Dolly the sheep in 1996 was a breakthrough for cloning technology. Dolly was the first mammal cloned from a mature cell—in this case, from a donor sheep’s mammary gland. Previously, cloned animals had only been produced using cells from embryos. But this was a big limitation, because it required knowing which animals you'd want to clone, and obtaining embryos from them in advance. The ability to use mature cells meant that cloning was suddenly possible using any cell from an animal at any age.

It also opened up the possibility of cloning as a way to preserve endangered species. Collecting embryos from endangered species could waste precious genetic material if the cloning attempt failed. Collecting mature cells, which are available throughout an animal’s lifetime, is much less risky.

And cloning has a notoriously low success rate. Most cloned embryos never result in live births. Embryos may die in the lab, or fail to implant in the uterus of the surrogate, or develop abnormally. In Dolly’s case, it took 29 embryo transfers into surrogate ewes to get a successful pregnancy.

Cloning can also give rise to health issues: large birth size, organ defects, and premature aging. Researchers think the procedure may create random errors in how genes are expressed. Many of the first clones of endangered animals died young. In 2001, scientists cloned the first member of an endangered species, a type of wild cattle called a guar. But the animal died soon after birth from an infection. In 2003, a pair of endangered banteng calves—a species of wild cattle in Asia—were born at the San Diego Zoo, but one had to be euthanized shortly after birth due to health problems. The surviving banteng was later put on display at the zoo.

The cloning process has gotten more efficient since the days of Dolly, but it still doesn’t work all the time. ViaGen scientists created and transferred seven embryos into as many mares to create the new foal. Four of them had pregnancies that advanced into the first trimester, but three miscarried. Russell says that’s in line with the company’s typical success rate for producing cloned domestic horses.

Because of this history, Novak says scientists waited to announce this latest horse’s birth until he survived infancy. Even now, they’ll have to monitor his health for the rest of his life. As for Kurt, he’s in “great health,” Novak says. He now lives at the San Diego Zoo Safari Park with a female Przewalski’s horse, Holly.

Even if the two clones remain healthy, they won’t be released into the wild—but their children or grandchildren will. Novak says they will become breeding stallions when they reach maturity at age 3 or 4. “Their purpose in life is to breed as much as possible, so we want them to live as long as possible,” he says. The team also plans to continue cloning more Przewalski’s horses.

NOT EVERY ENDANGERED species is suitable for cloning. The technology relies on having cell samples from animals, which aren’t always easy to obtain. (And to take it a step further, the lack of a complete genome is one of the reasons why efforts aimed at the “de-extinction” of long-ago animals like the woolly mammoth aren’t using cloning, but are instead aiming to edit the DNA of a related species, like the Asian elephant, to create a hybrid.)

Plus, a domestic species often needs to serve as a surrogate—this ameliorates the risks of taking an endangered species out of its natural habitat and putting it through the surrogacy procedure. But for many endangered species, there are no domestic animals that are genetically similar enough to carry a successful pregnancy.

David Jachowski, associate professor of wildlife ecology at Clemson University, says cloning alone won’t save endangered species. “As a scientist, it’s intriguing. We’re going from science fiction to reality,” he says. “But the reality is, if we don't address the threat the species faces in the wild, making more of them to release in the wild isn't going to move the needle on their recovery.”

The real problems that threaten most species, he says, are environmental, and cloning can’t fix those. Jachowski previously worked at the US Fish and Wildlife Service, where he helped coordinate the recovery of the black-footed ferret, an endangered North American animal. The species was close to extinction after its main food source—prairie dogs—were decimated by disease, habitat loss, and poisoning campaigns.

In 2020, the same team behind the horse clones collaborated with the agency to clone a black-footed ferret named Elizabeth Ann. But so far, that effort has only produced a single animal, and she has not reproduced. The Fish and Wildlife Service’s broader efforts have focused on more conventional conservation techniques, like restoring prairie dog populations while releasing captive-born black-footed ferrets into the wild.

Noah Greenwald, endangered species director at the nonprofit Center for Biological Diversity, doesn’t think cloning will be a major part of endangered species recovery because of its limitations. He thinks more traditional strategies, like addressing habitat loss and competition from invasive species, will remain the most effective tactics. He sees it as a last-ditch effort: “For the species that really are down to such a small number of individuals, it’s a possible way to increase the gene pool,” he says.

For the Przewalski’s horse, at least, cloning offers hope for future survival of the species. The team that created the new foal didn’t say what kind of animal they’ll clone next, but there are plenty of options. The San Diego Zoo’s frozen repository contains cell lines from more than 1,100 species and subspecies—some of them critically endangered. Russell is looking forward to the next conservation project. “Hopefully they have more animals in their bank that they allow us to produce in the future,” he says.

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