One of six northern white rhinos in the world died Sunday at the San Diego Zoo Safari Park from old age, park officials announced in a statement.
The death of the 44-year-old male leaves just one elderly northern white rhino, a female, at the Safari Park. Three are at a preserve in Kenya and one is in the Czech Republic.
"Angalifu's death is a tremendous loss to all of us," said Rancy Reiches, curator of mammals at the Safari Park.
The rhino had been in decline for days, refusing to eat. He was under veterinary car for the last few days of his life.
"Angelifu's death is a tremendous loss to all of us," said Randy Rieches, a curator of mammals at the Safari Park, according to the Los Angeles Times. "Not only because he was well beloved here at the park, but also because his death brings this wonderful species one step closer to extinction."
Zoo keepers had been trying to mate Angelifu with a female counterpart at the San Diego zoo, but they were not successful. Researchers in Kenya have been dealing with a similar issue, having accepted the fact that their one male and two female rhinos wouldn't reproduce naturally.
Scientists have turned to in vitro fertilization, or IVF, where the egg is fertilized outside the body and then placed in the female.
There were more than 2,000 of the species remaining as of 1960. By 1984, just 15 remained, according to the Ol Pejeta conservancy.
The Ol Pejeta conservancy named the northern white rhino the most critically endangered rhino subspecies and the most endangered mammal species in the world.
"Until recently, the only known wild northern white rhino population was clinging to survival in Garamba National Park in North-East Democratic Republic of Congo, but continuous civil war and armed conflict in the area have resulted in depletion of wildlife populations," said Ol Pejeta in a statement.
The southern white rhino is in "near threatened" status with only 20,000 remaining, according to the WWF.