One of only four northern white rhinos believed to be left in the world has died at the San Diego Zoo Safari Park.
Nola, a 41-year-old female who has been at the park since 1989, was euthanised after her health took a turn for the worse, a zoo statement said.
The geriatric rhino had arthritis and other ailments and was being treated for a bacterial infection linked to an abscess in her hip.
Nola had surgery on 13 November to drain the abscess but her health began to deteriorate about a week ago; her appetite faltered and she became listless. She worsened over the weekend and vets decided they had to euthanise her, according to the zoo.
“Nola was an iconic animal, not only at the San Diego Zoo Safari Park, but worldwide,” the park statement said.
The remaining three northern white rhinos, all elderly, are in a closely guarded preserve in Kenya.
The subspecies has been decimated by poachers, who kill the rhinos for their horns. The horns are in high demand in parts of Asia where some people claim they have medicinal properties for treating everything from hangovers to cancer.
In an effort to preserve the species, the San Diego zoo took possession earlier this month of six female southern white rhinos from South Africa.
Zoo researchers are working on developing northern white rhino embryos to be implanted in the six new arrivals, who will serve as surrogate mothers.
Researchers have said they hope a northern white rhino calf could be born from a San Diego surrogate mother within ten to 15 years.
The northern white rhino once roamed parts of Uganda, South Sudan, the Central African Republic and the Democratic Republic of the Congo.
Poaching saw the population plunge from around 500 to just 15 in the 1970s and 1980s but improved throughout the 1990s. In 2000, surveys confirmed there were 30 animals, and possibly six others.
Since 2003, poaching intensified and the population fell again
The San Diego Zoo Safari Park had three northern white rhinos that were wild-caught. A female named Nadi died in 2007 and a male named Angalifu died last December.