Sometimes you just get desperate. In a last-ditch effort to help save the Sumatran rhino (Dicerorhinus sumatrensis) from extinction, the Cincinnati Zoo has gathered together the last two members of the species in North America and will now attempt to get them to mate. There's just one catch: the rhinos are brother and sister.

Normally zoos wouldn't even consider captive breeding of related animals. But with the total world population of Sumatran rhinos estimated at fewer than 100, Cincinnati Zoo says the hope they could produce young rhinos outweighs any potential risk. "No one wants to breed siblings," Dr. Terri Roth, Vice President of Conservation and Science at Cincinnati Zoo, said in a press release on July 21. "But when a species drops below 100 individuals, producing more offspring as quickly as possible trumps concerns about genetic diversity. We are down to the last male and female Sumatran rhino on the continent, and I am not willing to sit idle and watch the last of a species go extinct."

Sumatran rhinos are possibly the most endangered large mammals on the planet. Fifty percent of the population has disappeared in the past two decades, the result of human encroachment on their forest habitat and poaching for their valuable horns. The smallest of the rhino species, Sumatrans rhinos have two horns and a light coat of brown fur. They are the only surviving member of their genus and are closely related to the woolly rhinoceros (Coelodonta antiquitatis), which went extinct several thousand years ago.

Cincinnati zookeepers hope that Harapan (a.k.a Harry), a six-year-old male, and his older sister Suci, an eight-year-old female, can help reverse their species’ population decline. Both rhinos were born at the Cincinnati Zoo, which in 2001 became the first institution to breed Sumatran rhinos in captivity. The zoo, which had two adult rhinos at the time, pioneered techniques to assist the species with captive breeding. They have had three successes to date. The first, a male named Andalas, was born in 2001 and shipped to the Sumatra Rhino Sanctuary in Indonesia in 2007, where he sired his first young last summer. Suci and Harapan were born over the next few years; their parents later died of old age. Harapan has traveled the country since his birth in 2007, first to the White Oak Conservation Center in Florida and then to Los Angeles Zoo, where scientists have also been working to restore the species. He returned to Cincinnati this month with the hope of adding a fourth calf to the zoo's breeding record.

As Roth explains in the Cincinnati Zoo video below, the fact that Andalas and a few other captive rhinos in Sumatra are breeding is good news, but it remains important to have a second breeding population in case disease or any other disaster strikes either group. She suggests that the wild population is so small sibling rhinos may also be breeding in the forests, but any potential genetic risks could be bred out in further generations.

To increase the chances of successful breeding, zookeepers will monitor Harapan's testosterone levels and use ultrasound to figure out when Suci is ovulating. The animals—which are solitary by nature—will be kept apart and only introduced to each other during potential breeding windows. "You can't just house them together," Roth explained to the Associated Press. "The only time you can get a successful breeding is if you just put them together when the female is going to be receptive."

Captive breeding programs like this have succeeded before, even when they started with small, interrelated populations. All 300 Mexican gray wolves (Canis lupus baileyi) alive today are descended from just five animals brought into a captive breeding program in 1973. Two island fox (Urocyon littoralis) subspecies were each down to their last 15 individuals in the year 2000; today they each number in the hundreds. All of the kakapo (Strigops habroptila) alive today descended from a handful of captive birds. The Vancouver Island marmot (Marmota vancouverensis) has survived several population crashes, sometimes falling as low as 25 animals. Captive breeding saved the California condor (Gymnogyps californianus) from extinction back when the species was down to just 22 birds; conservationists today carefully move eggs from one nest to another to keep the population genetically healthy. None of the species have completely recovered, and some face potential genetic bottlenecks, but none have disappeared. Hopefully we'll be able to say the same thing about the Sumatran rhino a few decades from now.

Photos: One of the Sumatran rhinos at Cincinnati Zoo, photographed by Mark Dumont on July 21. Mother Ratu and her un-named calf, sired by Cincinnati Zoo-born Andalas in Indonesia, courtesy of the International Rhino Foundation. Both used under Creative Commons license

Previously in Extinction Countdown: