(CNN) -- A northern white rhino has died at a San Diego zoo, bringing the rare 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.
He was one of a handful of northern white rhinos left worldwide, including a few 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.
Last year, the northern white rhino was declared as "teetering on the brink of extinction" by the International Union for Conservation of Nature.
Rhinos are killed by poachers almost exclusively for their horns, which sell for tens of thousands of dollars.
Experts say the rhino horn is becoming more lucrative than drugs. The demand is driven primarily by buyers in east Asia, who believe it cures a series of ailments.
Organized crime syndicates are using military-grade helicopters, night-vision equipment and guns fitted with silencers, taking rhino poaching to a whole new level and leaving conservationists struggling to keep up.