Last year, a rare endangered horse named Ollie was born.

Following his brother, Kurt, delivered in 2020, Ollie became the newest Przewalski’s (pronounced “sheh-vahl-skees”) horse at the San Diego Zoo Safari Park. This was noteworthy on its own. Although herds of this wild equine—known for its stocky body, zebralike mane, and ocher coat—once roamed throughout Europe and central Asia, by 1969 their numbers had dwindled so low that they were declared extinct in the wild. While this is no longer true—thanks to conservation, approximately 2,000 are alive today, half of which live in Mongolia and China—the species is genetically bottlenecked. Nearly all living individuals descend from just 12 animals. As clones, Kurt and Ollie offer much-needed diversity to the population.

These two Przewalski’s colts are copies of a male named Kuporovic, whose cells were collected in 1980. At the time, Oliver Ryder, director of conservation genetics at San Diego Zoo Wildlife Alliance, visited Minnesota to examine Kuporovic. Part of his job was to analyze the pedigrees of Przewalski’s horses and recommend how to best preserve genetic diversity, as inbreeding had become a serious concern. Ryder traveled North America to examine hundreds of individuals of this unique species and asked breeders to contribute biological samples, like hoof trimmings, to the Frozen Zoo. In a laboratory next to the 1,800-acre Safari Park, researchers grew cultures of Kuporovic’s DNA, which they cryogenically preserved and stored in pressurized tanks at -320 degrees Fahrenheit. The cultures remained frozen in suspended animation for 38 years.

In 2018, a partnership between San Diego Zoo Wildlife Alliance, the conservation group Revive & Restore, and the company ViaGen Pets & Equine led to the effort to clone Kuporovic. Increasingly, cloning is being used as a tool for conservation of endangered animals. This is different from de-extinction, the push to try to bring back long-dead creatures like the dodo or the woolly mammoth from fossils or other remains. Rather, this process uses living material to help a species on the brink of extinction continue to survive. Of the 450 Przewalski’s horse samples in the Frozen Zoo, Kuporovic provided the most genetic variation, making him the best candidate to copy.

To clone Kuporovic, scientists harvested an egg cell from a female quarter horse and stripped it of its DNA. Then they took the nucleus from one of Kuporovic’s somatic cells and inserted it into the empty egg. An embryo formed, which was implanted into a quarter horse’s womb to proceed as in a normal pregnancy. The results, Kurt and Ollie (who is named after Ryder), are genetically identical copies. When they reach sexual maturity, their potential offspring could help revitalize the Przewalski’s species.

“I’ve been thinking about that concept [of cloning endangered species], but I tell you it’s a very different thing to see a living, breathing, beautiful young colt. It’s like a dream realized. You know, you didn’t dare to dream, but there was this possibility, and now it’s manifest. And it’s very astounding.”

– Oliver Ryder, San Diego Zoo Wildlife Alliance

The Przewalski’s horse (Equus przewalskii) is a critically endangered species that once ranged throughout Europe and Asia. Formerly extinct in the wild, today’s Przewalski’s horses are all descendants of just 12 individuals. Since 2018, Revive & Restore and its partners have worked to restore genetic diversity in the Przewalski’s horse through strategic conservation cloning.

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