They are distinguished from their relatives by hairier ears, differing dental structures, and smaller body size. Some researchers have argued that the northern white rhino should be considered a separate species.
War, habitat loss and poaching for rhino horn have decimated populations, and by 2008 researchers could no longer locate northern white rhinos in the wild. But a number of the animals — including Sudan, who was captured in 1975 — remained at zoos around the world.
“Sudan is an extreme symbol of human disregard for nature,” said Jan Stejskal, director of international projects at the Dvur Kralove Zoo in the Czech Republic, where Sudan spent most of his life. “He survived extinction of his kind in the wild only thanks to living in a zoo.”
A multidisciplinary team of scientists spanning five continents has turned to an innovative mix of classic reproduction techniques and cutting-edge stem cell technology in hopes of resurrecting the subspecies.
While banked northern white rhino sperm is available from several males not related to Najin and Fatu, neither female is able to carry a pregnancy to term. Conservationists hope to extract Najin and Fatu’s eggs; fertilize the eggs in vitro with banked sperm; and then implant the embryos in surrogate southern white rhino females.
In vitro fertilization is commonly performed in humans and livestock, but the procedure has never succeeded with a rhino. So far, the researchers have perfected egg-extracting techniques on southern white rhinos, and soon they plan to attempt the first extraction on Najin and Fatu.
Even if their eggs do lead to a number of healthy northern white calves, however, a population originating solely from a mother and a daughter will be dangerously lacking in genetic diversity.
So scientists also plan to use frozen cell cultures from 12 northern white rhinos, including Sudan, stored at the San Diego Zoo to create stem cells, which in theory might be coaxed into becoming egg and sperm and united to create an embryo.
Recently advances have made the plan less far-fetched than it might seem. The potential to make stem cells from cell cultures earned Kyoto University medical researcher Dr. Shinya Yamanaka a Nobel Prize in 2012. The technique has since been used to create healthy, fertile mice.
Encouraged by those findings, rhino researchers have so far succeeded in reprogramming stored skin cells from five northern white rhinos into specialized stem cells.
“It’s going to be a long process, but it’s an incredible story in scientific study and analysis,” said Oliver Ryder, director of conservation genetics at the San Diego Zoo Institute for Conservation Research. “And the backdrop, of course, is the imminent extinction of this form of rhinoceros.”
Some conservationists fear that any such advances will come too late.
Every rhino species is under threat, said Cathy Dean, chief executive of Save the Rhino, an advocacy group, while the technical advances researchers are discussing may take 15 years to bear fruit.
“It may be too late for the northern white rhinos, but we still have time to save all the other species,” Ms. Dean said.