Four of the world's last eight surviving northern white rhinos (Ceratotherium simum cottoni) are now living in Africa for the first time in decades as part of a last-ditch effort to save the subspecies from extinction.

Only eight northern white rhinos exist in the world, all in captivity until recently. Two live in the U.S. at the San Diego Wild Animal Park. Six more resided at Dvur Kralove Zoo in the Czech Republic; four of those animals were crated up on Sunday and flown to Kenya, where it is hoped that living in their natural environment will inspire them to breed.

Northern white rhinos are thought to be extinct in the wild, and have not been seen out of captivity since 2007, when only one sighting was reported. Like all rhino species, their populations were destroyed by rampant poaching for their horns, which are valued in traditional Asian medicine and as ornamental dagger sheaths in the Middle East. The northern white also lived in an area plagued by attacks by Sudan's Janjaweed militia, which made protecting that last wild population almost impossible.

So, why return these rhinos to Africa—and why now? The rhinos sent back to Kenya include a 20-year-old female known as Najin, the last northern white rhino to ever give birth, although that was way back in 2000. According to a report in Sowetan, officials from Dvur Kralove zoo hope "the hormone levels of the female rhinos will get back to normal in Africa, improving chances for breeding."

"We plan to give the remaining individuals with breeding potential their last chance of normal and regular reproduction in a secure location in the wild," zoo director Dana Holeckova told BBC News.

Not everyone thinks this move is a good idea. "It makes no sense to move them at this point in time. It's way too little, too late," Randy Rieches, curator of mammals for the San Diego Wild Animal Park, told the Associated Press.

At this point, there's little to no hope of maintaining the northern white rhino as its own species. Even if the two females moved to Kenya do, eventually, give birth, it is likely that they will breed with their cousin subspecies, the southern white rhino (Ceratotherium simum simum). The goal, now, is to pass on the northern rhino's genes, which could provide the southern cousins with much-needed resistance to diseases carried by the tsetse fly. In fact, crossbreeding the two subspecies is "the only remaining option" to retain their genetic material, according to the International Rhino Foundation.

Whether the rhinos will respond to their "natural" environment remains to be seen. Of the four repatriated rhinos, both of the females and one of the males were born in captivity.

The money to transport the four was donated by Alastair Lucas, vice chairman of Goldman Sachs in Australia. The cost has not been disclosed, although security to protect the rhinos from poachers will cost around $90,000 a year.

Image: A northern white rhino (Ceratotherium simum cottoni) in captivity at Dvur Kralove Zoo. Via Wikipedia