Unable to stand and sick from old age, the last male northern white rhino was put down on Monday. The sad demise of Sudan, whose keepers loved him as a “gentle giant,” means that only two members of the species are still alive — and both are female.

Not long ago, such a tragic situation would have meant certain extinction for a species. But thanks to advances in assisted reproduction, that may not necessarily be the case with northern white rhinos or any other endangered species.

Scientists have been collecting and storing animals’ sperm and egg cells, as well as bits of skin and other tissue, that one day may be used to revivify species in danger of extinction as a result of hunting or habitat loss. These are part of a grand rescue plan for the northern white rhino that was set in motion long before Sudan’s death. And similar plans might possibly be used to save polar bears, elephants, and other beloved animals whose futures are very much in doubt.

Sudan is survived by his daughter and granddaughter, Najin and Fatu respectively. Jan Stejskal

“I would prefer that we would really care about animals to stay in their habitats and have, let’s say, free lives in a place where they belong,” said Jan Stejskal of Dvůr Králové Zoo in the Czech Republic, where Sudan lived before he was transferred to the Kenyan wildlife preserve where he died. “But if we get to a situation where we did with the northern white rhino, then I would still say we should try and not just let them go.”

Fertility clinics for endangered species

Beyond preserving their habitats and protecting them from poaching and other threats, the most obvious way to help endangered animals is to use the same in vitro fertilization (IVF) methods used in human fertility clinics: Collect sperm from males and eggs from females, combine them in a lab to produce embryos, and then transfer these embryos to females to grow into babies.

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IVF has been used with cattle and horses, as well as for tigers, ocelots, and antelopes, said Pierre Comizzoli, a research biologist at the Smithsonian Conservation Biology Institute in Washington D.C., who isn’t part of the rhino rescue mission. But the odds of success with IVF are slim because, as Comizzoli told NBC News MACH in an email, “We don’t understand the basic biology in many species.”

Luckily, some of Sudan’s semen was collected and frozen years ago, along with that of other northern white rhino bulls. Scientists say it should be possible to use sperm from these samples to fertilize eggs harvested from the last two females of the species, Fatu and Najin, who remain at the same Kenyan preserve where Sudan died.

As neither animal is healthy enough to carry a pregnancy, researchers will have to look to the northern white rhino’s cousin — the southern white rhino — to find surrogate mothers.

Things would be easier if rhino eggs could be frozen and then thawed, as needed, to create embryos, but for unknown reasons, many animal eggs tend not to be viable once thawed. “In the future, we may be able to freeze the [egg cells], but now we have to work around it,” says Stejskal.

So it’s a race against time to hone the technique while Fatu and Najin are still alive. “The main thing is those two cows mustn’t die, because they can still produce the eggs,” said Morné de la Rey, a veterinarian and director of the company Embryo Plus, who is working on refining these techniques in southern white rhinos.

Growing animals from skin

The good news is that it might be possible to save Sudan’s kind even if his survivors die before doing their part in the IVF process. Scientists say that it may be possible to create new eggs and sperm cells from different types of animal tissue, like skin — and several institutions around the world have been stashing tissue samples from the northern white rhino and other species.

Oliver Ryder, a scientist at the San Diego Zoo, examines a frozen biological sample. Ken Bohn / San Diego Zoo Global

Some northern white rhino skin cells have already been successfully grown into stem cells. “The next steps will take us, we hope, to sperm and eggs,” said Barbara Durrant, director of reproductive sciences at San Diego Zoo, which is among the institutions storing animal tissues. These would then be used to create embryos to be implanted into a surrogate mother.

So far, this approach has been used successfully only in lab mice, but Durrant is undeterred. “We know it can be done, the principle is there, but we do realize the mouse to the rhino is a huge step,” she says.

“This is the only way to go, right now, said Comizzoli. “This is a really ambitious project. But if we really want to regenerate and resurrect that species, this is the kind of approach that has to be taken.”

Of course, bringing new animals to life in the lab won’t mean much if habitats disappear and poaching continues. “We’ve been messing with nature in the past so much by killing these animals and slaughtering them and using their rhino horn [as] an aphrodisiac, that is messing with nature,” said de la Rey. “If we don’t rectify that, the animal will remain extinct.”

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