A northern white rhinoceros that zoo officials said was only one of six left in the world died Sunday at the San Diego Zoo Safari Park.
Angalifu, who was about 44 years old, apparently died of old age.
"Angalifu's death is a tremendous loss to all of us," safari park curator Randy Rieches said in a statement. "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."
Rhino horns are valued as dagger handles and are mistakenly seen as an aphrodisiac. As a result, poaching has pushed the critically endangered rhinos to the brink of extinction.
Attempts to mate Angalifu with Nola weren't successful.
Just last week, preservationists at the Old Pejeta animal sanctuary in Kenya conceded that their one male and two female northern white rhinos will not reproduce naturally. The animals were flown from the Czech zoo to the Kenyan conservancy in December 2009 in hopes the natural environment could be easier for them to breed there than in captivity.
Efforts will now be made to keep the species alive through in vitro fertilization. That experiment could take place with a southern white rhino surrogate mother. Southern white rhinos almost went extinct at the end of the 19th century, plunging down to only 20 at one point. Decades of conservation efforts gradually brought them back to life. There are now some 20,000 in the world.
As The Christian Science Monitor reports, efforts are under way to save the last remaining white rhinos:
The greater population of the southern white rhino could aid in reproduction of its northern counterpart. Scientists who have studied the critically endangered animal claim that dramatic population loss has reached the point at which inter-crossing the two subspecies will likely be necessary for the survival of the northern white rhino. Geneticists view breeding rhinos across subspecies as a last resort of sorts, because of the possibility that the genes of the northern white rhino would not be preserved.
Other options are even less appealing. The six remaining animals in the subspecies would have to inbreed to produce any pure-bred offspring. But the negative effects of inbreeding, like reduced fertility and higher infant mortality rates, decrease the likelihood that the population of the subspecies could rebound this way.
"One can always believe in miracles but everything leads us to believe that hope they would reproduce naturally has gone," Dvur Kralove Zoo spokeswoman Jana Mysliveckova told Agence France-Presse.