Only 4 northern white rhinos left on Earth

We are witnessing an extinction.

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

According to CNN, 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.

The only living male – Sudan – is under 24-hour armed guard at the Ol Pejeta Conservancy in Kenya. His horn has been cut off to deter poachers. Due to his age, he may not be able to naturally mate with a female. Sudan also suffers from a low sperm count.

Of the three females left, two live with Sudan in the reserve, but also are old and potentially unable to support the weight of natural mating. The fourth, Nola, lives in the San Diego Zoo.

According to CNN, the dead rhino's potentially healthy ovary and other tissue has been taken to an Italian laboratory, where her eggs may be harvested for future in vitro fertilization. A cousin, the southern white rhino, could be used as a surrogate.

Read more from CNN.