An animal can be considered critically endangered if there are dozens or hundreds of them left. But in this case, the only survivors are Najin and Fatu, a mother and daughter. And scientists discovered in 2014 that even artificial insemination using frozen sperm was unlikely to be an option for them, since neither seemed physically capable of carrying an embryo to term.

Any hope of natural mating vanished entirely last year, when the only male northern white rhinoceros died.

That rhino, called Sudan, had spent most of his life in the Dvur Kralove Zoo in the Czech Republic. He had been captured from the wild in 1975, which might have saved him. In 1960, there were about 2,000 northern white rhinos in Africa. But the population has since been decimated, in large part by habitat loss and poaching.

Sudan moved to the conservatory in Kenya in 2009 and died at the old age of 45 in March 2018, leaving the two females — his daughter, Najin, and his granddaughter, Fatu — alone at the Ol Pejeta Conservancy in Kenya’s Laikipia County .

“When he died, it was a sad moment for all of us,” said Stephen Ngulu , the veterinarian in charge at the conservancy. “We knew that we had some sperm that had been collected from him and several other males. So we knew that the only hope for the species was to get the eggs from the female.”

In order to extract the eggs from the two rhinos — using a probe guided by ultrasound — the animals had to be put under general anesthesia. That procedure is never risk free, so the scientists and veterinarians involved knew they had to be exceedingly cautious.

Dr. Frank Göritz, the head veterinarian at the Leibniz Institute in Germany, was in charge of administering the anesthesia during the operation, which was also overseen by David Ndeereh of the Kenya Wildlife Service and Thomas Hildebrandt of the Leibniz Institute.