in a statement.

The San Diego Zoo has one remaining northern white rhino, an elderly female named Nola. Despite the hopes of conservationists, Angalifu and Nola were unable to breed, the Los Angeles Times reported.

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In addition to Nola, the world's only remaining northern white rhinos are:

Sudan, a male living in the Ol Pejeta Conservancy in Kenya. He's now the final remaining male of his species.

Najin and Fatu, two females living in the Kenya preserve with Sudan.

An elderly female at the Dvur Kralove Zoo in the Czech Republic. The zoo is responsible for the care of Sudan, Najin and Fatu, and sent them to Kenya as part of a last-ditch breeding program.

The species has likely already lost its last chance for natural reproduction. In October, a male called Suni -- the first of his kind ever born in captivity -- died of unknown causes in Kenya, where he was part of the breeding program with Sudan, Najin, and Fatu. At the time, the Dvur Kralove zoo called Suni "probably the last male capable of breeding."

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Last week, the Ol Pejeta Conservancy said publicly for the first time that the breeding experiment has more or less failed. In other words, the last hope for the northern white rhino species now likely lies in "artificial reproductive techniques," the AP reported. The San Diego Zoo has preserved some of Angalifu's testicular tissue and sperm for future attempts to artificially breed new members of the species, as the Times noted. If science can't help, the species is probably doomed to extinction.

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Like other rhinoceros species, the northern white rhino population was decimated by poachers, who take and sell the animal's horns. They're a valuable commodity on the black market; several cultures still maintain that the horns have medicinal value.

For conservationists trying to rebuild the world's rhinoceros population in the wake of the devastating effects of poaching, some efforts have been more fruitful than others.

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Take the southern white rhino, which is a distinct subspecies from the northern white.

The southern white rhino was considered extinct for much of the late 19th century. Then, scientists discovered a small population in southern Africa in 1895. As the World Wide Fund For Nature explains, a century of conservation efforts followed, and now there are about 20,000 southern white rhinos in the world. Poaching is still a threat, and the population is considered "near threatened" by the conservation organization. But compared with the dwindling handful of northern white rhinos, the southern variety seems like a huge success.