Northern White Rhino
The only remaining male northern white rhino on the planet is Sudan (pictured on May 23, 2015), who lives in Kenya. Getty Images/AFP/Tony Karumba

Being a northern white rhinoceros just got lonelier. Earlier this week, a rhino named Nabire died in the Czech Republic, zookeepers said. The rhino, who was 31 years old, died from complications of a ruptured cyst.

Nabire was born on Nov. 15, 1983, according to the Telegraph , and marked a rare case of the northern white rhino breeding in captivity. Only four other northern white rhinos remain: One is housed at the San Diego Zoo and the others are at the Ol Pejeta Conservancy in Kenya.

Northern white rhinos were once found in southern Chad, the Central African Republic and the Democratic Republic of Congo, according to the World Wildlife Fund . In the late 1960s, there were estimated to be more than 2,000 northern white rhinos. However, that number dropped dramatically, and by 1984 there were only 15 left after widespread poaching. While efforts were made to grow the population, poaching effectively killed all wild northern white rhinos, leaving only those few that were in captivity.

Rhinos are one of the most critically endangered animals, according to the African Wildlife Foundation . Rhino horn is believed to treat hangovers, impotence and cancer, although these benefits are not scientifically proven. The poaching of rhinos has been an increasing problem, according to Save the Rhino International , which says rhino poaching is growing across Africa and Asia.

Earlier this week, it was revealed that Walter Palmer, an American dentist, allegedly paid $55,000 to poach a lion -- another vulnerable species -- in Zimbabwe. The lion turned out to be the famed Cecil, who was the subject of an Oxford University study.

After Palmer’s name was released, he was bombarded with death threats and Yelp reviews critical of his dentistry practice. Zimbabwe has already charged professional hunter Theo Bronkhorst and landowner Honest Ndlovu in Cecil’s death.

While it might be too late for the northern white rhino, we can certainly work to keep alive what species do have.