One of the last northern white rhinos in the world has died, leaving only five remaining. Thankfully, it was a natural death, with the reportedly comfortable San Diego Zoo resident passing on at the ripe age of 44 years old.

"Angalifu's death is a tremendous loss to all of us," safari park curator Randy Rieches said in a statement Sunday, just after the animal's death had been confirmed by zoo staff. "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."

According to the World Wildlife Foundation (WWF), the northern white rhino, one of the most endangered sub-species in the world, often lives between 35 and 50 years of age, making Angalifu's passing not unusual.

These rhinos officially became critically endangered in 2006, after field surveys revealed that there were only four wild northern white rhinos left in the world, grazing along the African savannah woodlands south of the Sahara. However, that wild population has not been seen since 2005, and experts believe that it is safe to assume that the sub-species is now extinct in the wild - a victim of excessive trophy hunting and poaching.

After the male rhino Suni died at the Ol Pejeta Conservancy reservation in Kenya back in October, only six captive northern whites remained in all the world, with only four of that population remaining sexually active. All attempts to artificially inseminate the remaining female population, two of which could be found in Kenya, had failed.

Back in the San Diego Zoo, efforts to have Angalifu mate with his sole companion, a female named Nola, also proved unsuccessful.

And that's what makes this male's death particularly upsetting. Just last week, conservation officials in Kenya conceded that efforts at breeding the Czech Republic's rhinos with their own have not been successful.

The animals had been shipped to Ol Pejeta Conservancy back in 2009 with the phrase "Last Chance to Survive" stamped on their containers, according to The Associated Press. With their own efforts failed, and San Diego's Nola now alone, the time of the northern white rhino may be rushing to its end.

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