Nairobi, Kenya. Researchers say a rhinoceros was impregnated through embryo transfer in the first successful use of a method that they say might later make it possible to save the nearly extinct northern white rhino subspecies.

The experiment was conducted with the less endangered southern white rhino subspecies. Researchers created an embryo in a lab from an egg and sperm collected from rhinos and transferred it into a southern white rhino surrogate mother at the Ol-Pejeta Conservancy in Kenya.

“The successful embryo transfer and pregnancy are a proof of concept and allow (researchers) to now safely move to the transfer of northern white rhino embryos -- a cornerstone in the mission to save the northern white rhino from extinction,” the group said in a statement Wednesday.

However, the team learned of the pregnancy only after the surrogate mother died of a bacterial infection in November 2023. The rhino was infected when spores from the clostridium strain were released from the soil by floodwater, and the embryo was discovered during a post-mortem examination.

Still, the scientists were optimistic about their finding, though some conservationists are skeptical that the breakthrough has come in time to save the northern white rhino.

“Now we have the clear evidence that an embryo that is frozen, thawed, produced in a test tube can produce new life and that is what we want for the northern white rhino,” said Thomas Hildebrandt, the lead researcher and head of the Department of Reproduction at BioRescue.

Roughly 20,000 southern white rhinos remain in Africa. That subspecies as well as another species, the black rhino, are bouncing back from significant reduction in their populations due to poaching for their horns.

However, the northern white rhinoceros subspecies has only two known members left in the world.

Najin, a 34-year-old, and her 23-year-old offspring, Fatu, are both incapable of natural reproduction, according to the Ol-Pejeta Conservancy where they live.

The last male white rhino, Sudan, was 45 when he was euthanized in 2018 due to age-related complications. He was Najin's sire.

Scientists stored his semen and that of four other dead rhinos, hoping to use them in in vitro fertilization with eggs harvested from female northern white rhinos to produce embryos that eventually will be carried by southern white rhino surrogate mothers.

Some conservation groups have argued that it is probably too late to save the northern white rhino with in vitro fertilization, as the species' natural habitat in Chad, Sudan, Uganda, Congo and Central African Republic has been ravaged by human conflict.

Skeptics say the efforts should focus on other critically endangered species with a better chance at survival.

“News of the first successful embryo transfer in a rhino is an exciting step, however it sadly comes too late to recreate a viable population of northern white rhinos," said Dr. Jo Shaw, CEO of Save the Rhino International.

Shaw said her group's focus remains on addressing the two main threats to the five species of rhino around the world -- poaching of rhinos for their horns and their loss of habitat to development.

“Our best hope remains to work with the range of partners involved to give rhinos the space and security they need to thrive naturally,” she said.

Her group said it continues to encourage natural breeding to boost numbers. It cited the example of the Sumatran rhino, which has fewer than 80 animals left. Last year, two calves were born through natural reproduction, the group said.