Head caretaker Mohammed Doyo feeds Sudan, the last male northern white rhino left on the planet, on June 12, 2015. Sudan lives in a 10-acre enclosure at Ol Pejeta Conservancy in Kenya, where he is protected from poachers 24 hours a day by armed guards.

The last male northern white rhinoceros is wallowing in the mud and grazing again after an age-related infection nearly spelled his end earlier this month. But although 45-year-old Sudan is still standing — for now — conservationists are debating whether his subspecies has a chance at survival.

There are only three northern white rhinos (Ceratotherium simum cottoni) left in the world. Sudan, the eldest, is the only male. The other two, Najin and Fatu, are his daughter and granddaughter, respectively. They all live under armed guard at Ol Pejeta Conservancy in Kenya.

None of the rhinos will ever reproduce again naturally. Sudan is too old, and his sperm quality is dismal, keepers discovered when they last collected his semen in 2014. Najin is too old to bear the weight of a mating male or to carry a pregnancy, while Fatu has uterine problems. [See Photos of the Last Standing Northern White Rhinos]

Regarding saving the subspecies, Sudan is already functionally obsolete, said Jan Stejskal, director of communications and international projects at Dvůr Králové Zoo in the Czech Republic, which technically owns the three rhinos. Najin and Fatu, both of whom still produce eggs, are the more valuable rhinos for the future, as their donor eggs could be used in in vitro fertilization with stored sperm from a deceased male. The resulting pregnancy could then be implanted into a southern white rhinoceros (Ceratotherium simum simum), a closely related subspecies. The question becomes — is saving the northern white rhino worth it?

Rhinoceros reproduction

Sudan is cared for around-the-clock by a team of veterinarians and protected from poachers by armed guards. Caregivers at Ol Pejeta conservancy reported on Feb. 28 that a leg infection was threatening the elderly rhino's life and that Sudan was responding poorly to treatment. Over the next week, however, Sudan began to stand, graze and even wallow in the mud.

Still, the last male's death is likely not far in the future, given his advanced age. This vision of extinction has spurred debate in conservation circles, with some arguing that saving the northern white rhino is a poor use of resources. "[P]revention is better than cure," according to Save the Rhino. The organization argues that the northern white subspecies is functionally extinct already; saving it with IVF and surrogacy is more akin to reviving dead woolly mammoths than saving a critically endangered subspecies.

"Much of the sub-species’ former range has lost rhinos in its entirety, with limited conservation programmes or expertise for managing a rhino population, and large-scale habitat loss," according to Save the Rhino's position statement on northern white rhino IVF. Focusing efforts on poaching prevention — rhino horn is coveted for its "medicinal properties," which are just a myth — and saving habitat would be more beneficial for the other rhino species that have a better chance at survival, according to the organization. [6 Extinct Animals That Could Be Brought Back to Life]

Stejskal and other researchers active in the IVF project see it differently. A substantial portion of the donations for saving the northern white rhino come from parties more interested in the development of IVF technologies for different species than in rhino conservation in particular, Stejskal said.

"So we actually brought to conservation resources that would be likely spent on a different subject," he told Live Science.

He also said that knowledge gained through developing IVF for the northern white rhino could potentially help breeding programs for other endangered rhino subspecies, particularly the Javan, Sumatran and black rhinos.

Slow progress

The process of gaining this knowledge is painstakingly slow, however. In December 2015, experts from around the world met in Vienna to hash out a plan to save the northern white rhino. In May 2016, the research group published their plan in the open-access journal ZooBiology. Their goals included developing a way to collect eggs from females, a hard-enough task in itself, Stejskal said, because a rhino's ovaries are tucked a good 5 feet (1.5 meters) inside her body, and the egg follicle is a mere millimeter or two in diameter. Trying to puncture the follicle to collect an egg at such a distance, with only ultrasound to guide you, is "not really easy," Stejskal said.

In the nearly two years since the meeting, researchers have made some progress in egg-harvesting, though, Stejskal said. So far, they've tried it only on southern white rhinoceroses, because northern white females are too rare to take any risks.

Avantea, a veterinary assisted-reproduction company in Cremona, Italy, has also successfully taken an ovary posthumously from Nabire, a northern white rhino who died at Dvůr Králové Zoo in 2015, and coaxed eggs out of it that were healthy enough to be fertilized, Stejskal said. The eggs were old and in bad shape, Stejskal said, so researchers knew from the start that they would never be the basis for a viable pregnancy. However, they did nudge them through the very early stages of embryonic development.

"It gave us some initial information on how the rhino embryo evolves," Stejskal said.

Deepening the genetic pool

Even if researchers are successful at both creating viable northern white rhino embryos and getting them to thrive in surrogate mothers, genetic diversity would be a concern. With just two living sources of oocytes and semen stored from five males, researchers would have to reboot an entire subspecies from a mere seven animals. Similar bottlenecks have been overcome before — the 20,000-plus southern white rhinos alive today descend from around 30 animals that survived at the end of the 19th century — but the lack of genetic diversity could cause problems with development or fertility in potential offspring.

That's why another line of work, spearheaded by the San Diego Zoo Institute for Conservation Research, is attempting to turn regular rhino cells into sperm and egg. Using methods that start with tissue cells called fibroblasts, researchers can engineer stem cells, which are cells capable of becoming any kind of body cell. Body tissue has been banked for an additional five northern white rhinos, Stejskal said, so stem cell technology could boost the size of the founding population to 12.

While progress is slow, Najin and Fatu are likely to survive and continue to produce eggs for some time, Stejskal said. As long as they're moving forward, the research team is hopeful.

"It's a normal scientific process," Stejskal said. "If you look at the history of IVF for people or horses, it was always that it was many, many trials before it was successful."

Original article on Live Science.