Nola, one of four northern white rhinos remaining, dies at Safari Park

But her conditioned began rapidly deteriorating on Saturday, and the decision was made to euthanize her.

The 4,500-pound rhino was under veterinary care for a bacterial infection as well as age-related issues when her condition took a turn for the worse. She says she saw Nola in May.

Nola was a favorite of visitors to the zoo and to fans from afar.

Supporters were invited to share memories of the rhino on social media, using the hashtag #Nola4Ever.

“It’s been a very hard day for all of us, and particularly or animal care staff”, said Christina Simmons of San Diego Zoo Global. Widespread poaching of rhinos in Africa and regular civil wars, driven mainly by demand in Asia for ground-up horns as an ingredient in medication, has caused their numbers to plummet. According to Save The Rhino, more than 20,000 of the creatures exist in the wild after being hunted nearly to extinction a century ago.

The last surviving male northern white rhino, Sudan, grazes at the Ol Pejeta Conservancy in Laikipia national park, Kenya June 14, 2015.

These rhinos are heavily guarded by 24-hour armed security and are restricted to a 700-acre enclosure. White rhinos are thought to be able to live up to 40 or 50 years and Sudan is 42-year-old.

Unfortunately, the two remaining females are infertile, which makes the outlook for the species pretty bleak. But good fortune and local governments favored the southern white rhino: a combination of legal protections, breeding efforts and regulated trophy hunting initiatives helped bring the subspecies back from the brink.

On Facenook, the zoo added: ‘It is with profound sadness that we announce that 41-year-old Nola, one of only four remaining northern white rhinos, has died.

The zoo is working with the sanctuary in Kenya to try to save the species. In 1895, less than 100 southern white rhinos were left after poachers had almost eliminated the population.

Previously, researchers had hoped to save the endangered northern white rhinos by artificially inseminating females.

The conservancy has been trying to breed the northern white rhino.