Angalifu, one of the last six northern white rhinos, dies at the San Diego Zoo

SAN DIEGO, Dec. 15 (UPI) -- Only five known northern square-lipped rhinoceroses, better known as northern white rhinos, remain on Earth after a male housed at the San Diego Zoo Safari Park died Wednesday.

Zoologists estimate Angalifu was 44 when he died of natural causes.

"Angalifu's death is a tremendous loss to all of us," San Diego Zoo Safari Park mammal 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."

In the 1960s, over 2,000 white rhinos remained alive, but by 1984 the species' population had been poached to less than 20. There are no remaining known northern white rhinos in the wild, CNN reported.

"Until recently, the only known wild northern white rhino population was clinging to survival in Garamba U 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," a spokesperson for the Ol Pejeta conservancy told the International Business Times.

"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."

Efforts to mate Angalifu with female white rhino and fellow San Diego Zoo Safari Park resident Nola were never fruitful.