In this photo taken Monday, Dec. 1, 2014, female northern white rhino Najin walks in her pen where she is being kept for observation at the Ol Pejeta Conservancy in Kenya.

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."

His death leaves only one northern white rhino at the zoo — a female named Nola — one at a zoo in the Czech Republic and three in a preserve in Kenya.

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.