Only five white rhinos of the northern subspecies (northern square-lipped rhinoceros) remain in the world after one died at the San Diego Zoo Safari Park yesterday (14 November).
Angelifu was estimated to be 44-years old when he died of old age. He had been under veterinary care for the last few days of his life.
Randy Rieches, a curator of mammals at the Safari Park, said: "Angelifu's death is a tremendous loss to all of us. 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."
His passing means that there are only five known northern white rhinos left, all in cpativity - a female in the San Diego zoo, one in a zoo in the Czech Republic, and three in Africa.
Zoo keepers had been attempting to mate Angelifu with his female counterpart Nola at the San Diego zoo, but their efforts proved unsuccessful.
Scientists in Kenya have been facing a similar problem. They recently became resigned to the fact that their one male and two female white rhinoceroses would not reproduce naturally. They had previously been in a Czech zoo but were sent closer to home in 2009, in the hope that their natural environment would help them to reproduce.
With only five left, scientists have turned to in vitro fertilization, or IVF, where the egg is fertilised outside of the body and then placed in the female, in a last ditch attempt to keep the subspecies alive.
As late as 1960, there were more than 2,000 of the species remaining. By 1984, only about 15 individuals had survived.
The northern white rhino (Ceratotherium simum cottoni) is the most critically endangered rhino subspecies and the most endangered mammal species in the world, according to the Ol Pejeta conservancy. They used to range parts of Uganda, Chad, Sudan, Central African Republic and the Democratic Republic of Congo.
"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 recent decades the northern white rhinos in Garamba population had increased to about 30 individuals, but then reduced drastically to 4 individuals in 2005. Sadly, there has been no sign of the four since 2007 and it is thought that the wild population may now be extinct."
The southern white rhinoceros (Ceratotherium simum simum) subspecies is currently in 'near threatened' status, with just over 20,000 remaining, according to WWF.
Rhinos are killed for their meat, hide, and horn, widely used in Vietnam and China in traditional medicine.