Story highlights Angalifu, 44, has died of old age

Experts warn that the northern white rhino is "teetering on the brink of extinction"

(CNN) A northern white rhino has died at a San Diego zoo, leaving only five worldwide and bringing the species closer to extinction.

Angalifu, 44, died of old age Sunday at the San Diego Zoo Safari Park.

"With Angalifu's passing, only five northern white rhinos are left on the planet, including Nola, our elderly female," the zoo said in a statement.

Of the handful of northern white rhinos left worldwide, a few are at a wildlife conservancy in Kenya. There are no known northern white rhinos left in the wild.

Northern white rhinos and southern white rhinos are different subspecies genetically.

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