Story highlights Female northern white rhino dies

Untreatable ruptured cyst caused death

Nabire's ovary harvested

Sole surviving male under 24-hour armed guard

(CNN) And then there were four.

Nabire, a female northern white rhino, died of a ruptured cyst on Monday at a Czech Republic Zoo, leaving only four known northern white rhinos on Earth.

The 31-year-old rhino's cyst was so huge, it was untreatable, a rhino curator at the Dvur Kralove zoo said in a statement.

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"It is a terrible loss. Nabire was the kindest rhino ever bred in our zoo. It is not just that we were very fond of her. Her death is a symbol of the catastrophic decline of rhinos due to senseless human greed," the zoo's director Premysl Rabas said in a statement.

The northern white rhino has been hunted to the brink of extinction for its horn, fueled by the belief in Asia that it cures various illnesses. The highly prized horn is made of keratin -- the same material that makes up human hair and fingernails.

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