Richard Vigne, CEO of Ol Pejeta Conservancy in Kenya, admits that there's not a huge amount of hope of saving the northern white rhino (Ceratotherium simum cottoni) from extinction. "You have to be realistic," he says, referring to the conservancy's efforts to breed the last four northern white rhinos in Africa. "The reality is that the chances of it working are pretty small, and have been since we started. But nevertheless we're going to do what we can."

The four northern white rhinos at Ol Pejeta came there a little over four years ago from Dvůr Králové Zoo in the Czech Republic. About three years earlier the last wild members of their subspecies had been killed by poachers. At the time there were only eight of these rhinos left in the world: two at San Diego Wild Animal Park and six at Dvůr Králové. All of the rhinos were already getting old, and few had ever bred, but in 2009 the four youngest and healthiest animals were crated up and shipped to Kenya in the hope that living a more natural life would encourage the females to start going through a menstrual cycle and perhaps give the subspecies one last chance at survival. (One of the two rhinos that remained at Dvůr Králové died of old age in 2011.)

From the beginning there were two anticipated outcomes: The best would involve the four northern white rhinos—two males and two females—breeding with one another and producing progeny. The other possible scenario involved introducing the northern whites to the closely related southern white rhino subspecies (C. s. simum) and possibly getting hybrid calves—not an ideal development, obviously, but enough to preserve some of the northern white's genetics into another generation.

The first step was to get the rhinos healthy. Despite the high quality of care they received at Dvůr Králové, "it was obvious to us that they'd been living in unnatural conditions," Vigne says. Their horns were gray and their toenails—an indication of a rhino's health—were in "a pretty poor state." Things turned around pretty quickly once they reached Ol Pejeta and had 285 hectares on which to roam (plus round-the-clock armed guards to keep them safe from poachers). "Within six months of their being here their skin had improved, their toenails had improved, the whole condition of the animal had completely changed," Vigne reports. "They became semi-wild very quickly."

The next step was to create a more natural social environment. The four northern whites were carefully introduced to several southern whites as well as black rhinos. "Experience has shown that if female rhinos are presented with a social environment akin to something they would encounter in the wild—other rhinos around them and competition from other females—they would be expected to cycle and more likely to reproduce," Vigne says.

It worked, to a degree. Although the northern whites didn't display much interest in sex, there were a few unsuccessful mating attempts in 2011, including one between a northern and southern white. Then, in March 2012, the magic happened. Two northern white rhinos named Najin and Suni entered a courtship ritual and then mated. It was about as exciting a moment as you can get in conservation.

Alas, nearly two years later, we can now say that the mating did not result in a pregnancy. Rhino gestation takes about 450 days, so that window has passed.

And so, now we come to plan B. In the next few days a male southern white rhino will be brought over from nearby Lewa Wildlife Conservancy and introduced to the two northern white females, Najin and Fatu. The male hadn't yet been selected when Vigne and I spoke last week, but he did identify several necessary characteristics: "He's got to be a dominant bull, a prime breeding bull with calves already on the ground," he explained. "There's not a lot else you can do."

If mating happens, there's hope that it could produce a hybrid northern-southern rhino. There's some precedent: a hybrid white rhino named Nasi was born at Dvůr Králové in 1977; she died in 2008.

Although a hybrid rhino would keep some of the northern white's genetic material "on the ground" for a bit longer, it's not a foolproof plan. Hybrids often lack certain key characteristics necessary for long, healthy lives. "There is a chance that the animals won't be viable from a reproductive perspective," Vigne says, "or that they won't have the kind of vigor necessary to survive in a semi-wild environment. It's all up in the air. We'll just have to wait and see."

Meanwhile, it's possible that the four animals just aren't healthy enough to breed. The males are both over 30; mounting a female could be too difficult for them at that age or their semen could be defective. (There was some inbreeding in their family tree.) The females, Vigne suspects, could be suffering from a syndrome called persistent corpus luteum, which prevents the animal from cycling properly. Fully diagnosing and treating this, however, would be too invasive and dangerous for a critically endangered species. The females would need to be anesthetized before being examined, which is very risky for such a large animal. Rhinos, particularly older animals, sometimes die under anesthesia. "We're trying to avoid that," Vigne says. "To recover a species from four individuals is asking a lot in the best of times," he acknowledges. But that won't stop them from trying—or from hoping.

No matter what happens, the four last northern white rhinos in Africa are, at least, safe and spending their final days under their native sun. "These animals were living in a zoo," Vigne points out. "They're now living in a much more natural environment. They're happy animals. And if they don't breed, we've done something good for the last four individuals in the wild, if nothing else."

Photos © Ol Pejeta Conservancy. Used with permission

Previously in Extinction Countdown: