Photograph by Lenny Ignelzi, Associated Press
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Seen here in December 2014, a rare northern white rhino named Nola passed away at the San Diego Zoo this week.

Photograph by Lenny Ignelzi, Associated Press

Only Three Northern White Rhinos Remain

Sunday's death of Nola raises the stakes for saving the rare subspecies.

The death of one of the four remaining northern white rhinos has further stoked fears that the subspecies is doomed for extinction.

After a series of illnesses, Nola, a 41-year-old female, had to be euthanized by San Diego Zoo staff Sunday. Vets had been treating the rhino for a bacterial infection and had recently performed surgery to drain an abscess, but Nola’s condition worsened.

“This is a very difficult loss for the animal care staff who worked with her, our volunteers, guests, and to her species worldwide,” the zoo wrote on its Facebook page, in announcing Nola’s death. “Nola’s legacy will live forever as her death leaves just 3 northern white rhinos on the planet,” the zoo added.

Rhinos in the wild have a life expectancy of 35 to 40 years. Nola had been at the San Diego Zoo since 1989, when she arrived from a zoo in what is now the Czech Republic. Only three more northern white rhinos (Ceratotherium simum cottoni) remain, all at the Ol Pejeta Conservancy in Kenya, where they are watched over by armed guards 24/7 in response to intense poaching pressure.

A female northern white rhino named Nabiré died in a Czech zoo in July.

The three remaining rhinos (two females, one male) are considered too old to reproduce naturally. However, scientists have harvested sex cells from the individuals and are planning in vitro fertilization in a related southern white rhino surrogate in a last-ditch effort to save the subspecies. That effort is expected to take more than a decade.

Rapid Decline

Rhinoceros populations in both Asia and Africa have suffered for decades due to habitat loss and poaching, which is spurred mainly by demand for their horns as an ingredient in many traditional Asian medicines (which Western scientists say do not work). (See a map of the international illegal trade in rhinos.)

The northern white rhinoceros subspecies was once robust throughout central Africa, with populations ranging from southern Chad to the Democratic Republic of the Congo. Their numbers declined precipitously from more than 2,000 in 1960 to only 15 individuals by 1984.

Like their northern relatives, the southern white rhino went through previous periods of decline, and only a handful remained at the turn of the century. However, conservation efforts by the South African government have helped the subspecies rebound to a population of around 20,000, through breeding and relocating rhinos, as well as allowing controlled sport hunting, which is controversial but has encouraged some landowners to raise rhinos. (Related: "Why African Rhinos Are Facing a Crisis.")

“Let this be a warning of what is happening to wildlife everywhere,” the San Diego Zoo wrote in regards to Nola’s passing.

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Watch: One of the world's rarest rhinos filmed.