If you had asked Thomas Hildebrandt a decade ago whether the northern white rhinoceros could be saved, his answer would have been grim. The rhino’s numbers had dwindled to single digits, and the few remaining individuals all had severe reproductive issues.
“We thought, ‘The story’s over,’” said Dr. Hildebrandt, a wildlife reproductive biologist at the Leibniz Institute for Zoo and Wildlife Research and the Free University of Berlin. His prognosis got even bleaker when Sudan, the last male of the subspecies, died in captivity last spring.
But on Wednesday, Dr. Hildebrandt and a team of colleagues reported in the journal Nature Communications that the story of the northern white rhino is not, in fact, over.
Using frozen sperm from northern white rhinos and eggs from closely related southern white rhinos, the scientists created hybrid embryos that can potentially be implanted into surrogate southern white rhino mothers.
This lab achievement is a very early step toward the much longer-term goal of resurrecting a population of full-blooded northern white rhinos, said Jan Stejskal, director of international projects at the Dvur Kralove Zoo in the Czech Republic and an author of the paper, in a press briefing on Tuesday.
Multiple teams around the world are now working collaboratively on high-tech options for bringing back the northern white rhino, which is now functionally extinct. Only two females, a mother-daughter pair named Najin and Fatu, are still alive at Ol Pejeta Conservancy in Kenya.
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One avenue is in vitro fertilization with northern white rhino eggs and sperm. This would involve producing early embryos in petri dishes that could then be transferred to develop in surrogate southern white rhinos, another subspecies.
Dr. Hildebrandt and his collaborators started to investigate this possibility. They had frozen sperm samples from four northern white males, and successfully used one sample to fertilize eggs from two southern white females, creating four hybrid embryos in total. Additionally, they created three full southern white rhino embryos.
The next step is to implant the southern white or hybrid embryos into surrogates, which should happen in the coming months, said Cesare Galli, founder and managing director of Avantea, a reproductive biotechnology company based in Italy, and an author of the paper.
If that succeeds, the researchers will appeal to Kenyan authorities to let them harvest eggs from Najin and Fatu, then fertilize the females’ eggs with stored northern white rhino sperm. The team hopes to see the first purebred northern white rhino born to a surrogate through this method in three years, Dr. Hildebrandt said.
One major drawback to this tactic is that the genetic pool from just two northern white cows and four bulls is extremely limited, and would likely lead to severe inbreeding.
Because of this, Dr. Hildebrandt’s team and a group led by researchers at San Diego Zoo Global are also exploring another approach, using so-called induced pluripotent stem cells (also known as iPS cells). Such cells have been reprogrammed into blank canvases — they can become any other cell type in the body, including egg and sperm cells.
The strategy is promising because researchers have already generated iPS cells (though not egg and sperm cells yet) from northern white rhino skin cells, and because the San Diego Zoo has a genetically diverse collection of skin cells from 12 northern white rhinos.
The trade-off is that the iPS cell technology will take longer to develop — perhaps a decade or so, Dr. Hildebrandt estimated.
In the meantime, it’s important to pursue the more immediate possibility of in vitro fertilization with Najin and Fatu’s eggs, he said, particularly if potential calves are to be raised around or socialized by the two northern white females in their lifetimes.
Conservation scientists widely applauded the technical sophistication of the new study, but several expressed concerns about relying too much on high-tech solutions.
It’s still “a long road from creating an embryo to having a viable birth — and then an even longer path from succeeding once to creating a herd of rhinos,” said Susie Ellis, executive director of the International Rhino Foundation.
She worried that technology can be “the shiny object in the room” that takes focus away from protecting habitat or supporting crucial, on-the-ground conservation efforts for remaining rhinos.
Terri Roth, director of the Center for Conservation and Research of Endangered Wildlife at the Cincinnati Zoo, said that saving the northern white rhino — or any species — will ultimately require multiple facets.
“We should all be working on as many strategies as possible,” she said. “Let’s not throw any one out, because you just can’t predict what’s going to happen. We need all the help we can get.”
