One of only six northern white rhinoceros left on earth died over the weekend at a San Diego zoo, bringing the species closer to extinction, zoo officials said.

Angalifu, who was about 44 years old and being treated for age-related conditions, died on Sunday, according to the San Diego Zoo Safari Park.

The world's only known remaining northern white rhinos are an elderly female still at the Safari Park, one at a zoo in the Czech Republic and three in Africa, the California zoo said.

"Angalifu's death is a tremendous loss to all of us," Randy Rieches, curator of mammals for the zoo, said. "Not only because he was well beloved here at the Park but also because his death brings this wonderful species one step closer to extinction."

Northern white rhinos have been nearly eliminated because of poaching in Africa, the zoo said. None of those remaining are reproductive, said Rieches.

In the wild, rhinos are killed for their horns, which some people believe have medicinal value.

Rieches said there are only about 30,000 rhinos of all types left in the world, and a rhino is killed by poachers every eight hours.

Semen and testicular tissue from the male northern white rhino have been stored in the San Diego Zoo Safari Park's "Frozen Zoo" with the hope that new reproductive technologies will allow recovery, the zoo said.

It is possible that semen from northern white rhinos could be used to impregnate southern white rhinos, to eventually produce an almost pure northern white variety, said Rieches. But there are various difficulties to overcome, including the problem of artificial insemination, which is more complex with rhinos than it is with cattle, Rieches said.

"It's crunch time now," Rieches said of rhino preservation efforts.

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