We've already lost one rhino subspecies—the western black rhino—and now it seems another, the northern white rhino, will follow. Yesterday, a 44-year-old northern white rhino named Angalifu died at the San Diego Zoo. Angalifu was one of just six of his kind known to remain in the world.

Related Content Only Four Northern White Rhinos Are Left on Earth

Northern white rhinos, like all species of rhino, are threatened with extinction, mostly due to poaching. In 2008, the last four northern white rhinos disappeared from a park in the Democratic Republic of the Congo. A few animals still lived in zoos, but even then, "the number [was] so low that the species is regarded as biologically unviable," the Sunday Times reported.

Although Angalifu lived a long, healthy life since arriving at the San Diego Zoo in the 1980s, he never managed to breed with a Nola, the San Diego Zoo's female northern white rhino, the Los Angeles Times reports. Other breeding efforts around the world have been equally disappointing.

With Angalifu's death, just five northern white rhino are known to survive. All live in zoos, and most are exceptionally elderly. Although there is still talk among some conservationists about extreme interventions such as de-extinction, at this point, it seems that, realistically, it's just a matter of time before the sub-species' end arrives. As Richard Vigne, chief executive of the Old Pejeta Conservancy in Kenya where three rhinos live, told the Associated Press, "We always knew from the very beginning that the chances of this working were small even if they bred."