Popular Science published this article online on May 1, 2018. It will also appear in the magazine's summer 2018 edition. To read the article on PopSci.com, click here.  Thomas Hildebrandt first saw the inside of an elephant in 1990. With the mammoth carcass laid across his lab bench at the ­Leibniz Institute in Berlin, where the German veterinary student was working that summer, he pondered his thesis on using human-fertility techniques to save endangered wildlife. Hildebrandt, then 27, was taken aback by the mammal’s bizarre reproductive tract. The passage was 10 feet long and concealed by a folded vaginal opening as narrow as a sunflower seed. The task of artificially inseminating an elephant, he learned in that moment, would mean getting shoulder-deep in many a cavernous nether region.

“I’ve always loved to solve problems other people cannot,” recalls Hildebrandt, now 54, of his 26-year career as an animal-­fertility expert and pioneer of endangered-species insemination.

The procedures he’s developed take hours and demand a steady hand. Today, as the lead reproduction specialist at the Leibniz Institute for Zoo and Wildlife Research, he has more practice than anyone. He’s helped conceive more than 50 elephant calves, performed CPR on rhinos sedated for surgery by jumping on rib cages, and patented a slew of techniques and devices for making babies in these behemoths.

This year, he’ll attempt his greatest feat yet: the first ever successful in vitro fertilization (IVF) of a rhino. Despite superficial similarities to its trunked cousins, the beast’s anatomy poses a new, high-stakes biological puzzle. If he can crack it, however, Hildebrandt could pull the northern white back from the brink.

Colonial-era hunting, poaching, and habitat loss have put all rhino species at risk. Though people have used the creature’s horn in ­traditional medicine for thousands of years, a recent surge in demand in Asia sent populations crashing. South Africa lost 13 individuals out of some 15,000 to poaching in 2007; in 2014, the number hit 1,215. Three of the world’s five rhino species are now critically endangered, a designation marking them as at an extremely high risk of extinction. But the northern white rhino occupies a uniquely precarious position.

Only two—both females incapable of carrying calves— remain at the Ol ­Pejeta Conservancy in Laikipia, Kenya. Najin, age 28, has Achilles tendons that could rupture under the weight of a pregnancy. Fatu is younger but barren, thanks to a uterine infection. The world’s last male, Sudan, died in March at the age of 45.

But the beloved bull could still sire a calf. Sperm from Sudan (and four other males) is on ice, and Hildebrandt hopes to harvest Fatu and Najin’s eggs. The vet and his international partners will fertilize the ova in an Italian lab, and return the embryo to Ol Pejeta, where the kid-carrying will fall to a family friend.

With their robust population of 20,000 living individuals, Hildebrandt believes that the southern white rhino—technically the same species as its endangered northern cousin but so distinct from years of separation that many experts argue otherwise— can stand to spare a few females to serve as surrogates.

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In 2016, Millie had the pleasure of meeting Sudan while on a reporting trip to Ol Pejeta.