And then there were five. The death by old age this past weekend of Angalifu, a 44-year-old northern white rhino (Ceratotherium simum cottoni) that lived at San Diego Zoo, reduces the world population of this critically endangered subspecies to just five, all of which live in captivity and none of which are breeding.

One more elderly female—42-year-old Nola—remains at San Diego Zoo. Another male lives at Dvůr Králové Zoo in the Czech Republic. The final three rhinos—two females and a male—live under 24-hour armed guard at Ol Pejeta Conservancy in Kenya to protect them from the poachers that wiped out their kin in the wild.

"Angalifu's death is a tremendous loss to all of us," San Diego Zoo Safari Park curator Randy Rieches said in a prepared release, "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."

Angalifu's passing follows less than three months after that of Suni, a 34-year-old male that spent the last five years of his life at Ol Pejeta, where conservationists hoped the rhinos would start to breed under their native skies.

That hoped-for breeding never happened, although there were a few furtive attempts at mating, some with the related southern white rhino subspecies. Earlier this month the conservancy announced that it would finally move ahead with exploring artificial reproductive techniques, something they had avoided because it would require anesthetizing the animals, a dangerous procedure for large animals, two of which are also of advanced age.

Now, with so few northern white rhinos remaining, the time for one last-ditch effort may have arrived. This month Ol Pejeta did go ahead and anesthetize their three remaining rhinos to give them thorough medical examinations. All three, they found, had medical problems that will make reproduction difficult even with artificial techniques. The male, Sudan, has low sperm count with "low motility and some morphological abnormality," according to a conservancy press release, although they note that this does not rule out his potential for fatherhood. Of the two females, the youngest one, Fatu, has degenerative lesions in her uterus that will prevent her from conceiving. Her mother, Najin, is now the subspecies's best bet. Her reproductive organs are fine but she suffers from weak back knees that would make mounting by a male a dangerous experience. Her bad knees could also be a problem during pregnancy from the extra weight.

Will we ever see another northern white rhino? The experts at Ol Pejeta and Dvůr Králové still have some hope and they don't plan on giving up. But these animals aren't getting any younger or healthier, and their remaining time on Earth gets shorter with each day. Their countdown to extinction is truly on its way.

Photo: Nola, the remaining northern white rhino in California, courtesy of San Diego Zoo Global

Previously in Extinction Countdown: