In the 1960s, an estimated 2,300 northern white rhinos still lived in the wild. But in the following decades, pseudoscientific beliefs about the medical properties of rhino horn created a lethal demand that poachers rushed to fill. By the early 1990s, there were just a few dozen left. In 2008, the creature was believed to be extinct in the wild.

Sudan escaped the fate of his wild peers. In 1975, he was captured by animal trappers at the age of 2, and moved to Dvůr Králové Zoo in the Czech Republic. In 2009, he and three others—Najin, Fatu, and an unrelated male, Suni—were moved to the Ol Pejeta Conservancy to take part in a last-ditch breeding program. It didn’t work. Even when rhinos mated, none of the females achieved a confirmed pregnancy. In 2014, Suni died. Now, Sudan has joined him.

It’s important to note that the white rhino is not extinct. The northern white is one of two subspecies that are separated by around a million years of evolution. And the other subspecies—the southern white—is something of a success story.

At the end of the 19th century, it was down to just a few dozen southern white rhinos, clinging to existence in a single South African reserve. Now, there are more than 20,000 of them in the wild—more than any other rhino subspecies. Still, that incredible comeback is precarious. Most of the southern whites live on private lands in South Africa, but as their numbers have grown, their value has fallen. An increasing number of landowners are “seeking to get rid of their rhino,” according to the International Union for Conservation of Nature. This, combined with the ongoing threat of poachers, threatens to reverse the successes of the last century.

In 2010, three researchers argued that the northern and southern white rhinos are physically and genetically distinct enough to justify classifying them as two distinct species. But defining a species is controversial. There are dozens of ways of doing so, and working out whether two groups are subspecies or species can be like working out whether a mound of sand qualifies as a pile or a heap. In the case of the northern and southern white rhinos, the conservation community still largely counts them as subspecies.

In many ways, this is an academic debate with little real-world import. Classifying something as its own species can influence how you might go about protecting or breeding an animal, but the northern white rhino is already too far gone. Even when there were still four living individuals, conservationists argued that the survivors were so inbred and genetically narrow that they couldn’t give rise to their own viable population. The only way to preserve some northern white-rhino genes is to artificially interbreed them with southern whites.

That work is now underway—but it’s tricky. A rhino’s ovaries lie two meters within its body, so collecting eggs requires shoulder-length gloves and a lot of dexterity. Scientists have perfected the technique on southern-white females, and they’re now ready to try on Najin and Fatu, whose eggs could then be fertilized in a laboratory. Another possible approach involves taking frozen cells from Sudan and other northern whites, transforming them into stem cells, and nudging those into becoming sperm and eggs.