With the death at San Diego Zoo Safari Park of a northern white rhino, the species is five rhinos away from extinction.
The death of Angalifu, a 44-year-old male northern white rhino, on Sunday leaves an elderly female at the park, three in a Kenyan preserve and one at a Czech Republic zoo.
Poaching has brought the northern white rhino to the brink of extinction, said Randy Rieches, curator of mammals at the Safari Park.
There were more than 2,000 northern whites in 1960, according to the World Wildlife Fund, but poachers obliterated the population. By 1984, there were about 15 of the rhinos left. That population was doubled by 1993 through aggressive conservation efforts. But heavily armed poaching gangs have now virtually annihilated the species, the WWF says.
Poachers are known to use helicopters, guns with silencers and night-vision equipment to harvest rhinos' horns, which are in huge demand in Asia and sell for as much as $30,000 a pound.
And the penalties are not nearly as severe as for selling drugs, as the International Business Times reported in April. It's a deadly formula for the rhinos.
"We don't like to talk about price," Rieches said, "because we feel by giving out a number it could possibly encourage one more person to think they can make money with rhino horn."
Angalifu died of ailments related to old age, according to the Safari Park, and had been in decline, not eating for days.
"Angalifu's death is a tremendous loss to all of us," Rieches said.
The white rhino -- which has southern and northern subspecies -- is the largest of all the rhino species and ranks as the second-largest mammal on land, after the African elephant, according to the WWF. The white rhino can reach 6 feet in height at the shoulder, with females weighing about 3,500 pounds and males nearly 8,000. The head of the rhino alone can weigh as much as 1,000 pounds.
Conservation efforts with southern white rhinos have been succeeding. The San Diego Zoo Safari Park boasts the most successful captive breeding program for rhinos anywhere on the globe. But the program for the northern white rhino hasn't been so successful.
Efforts at breeding rhinos at the Ol Pejeta Conservancy in Kenya also have failed.
The reproductive system of the northern white rhino is very complex, Rieches said, and gauging the estrus cycle of the female is difficult.
"The rhino is one of the species that we're still working on to perfect artifical insemination."
Some of Angalifu's semen and testicular tissue are being kept at the "frozen zoo" at the San Diego Zoo Institute of Conservation Research. And new methods of insemination are being worked on, Rieches said.
One possibility might be impregnating a female southern white rhino with sperm from a male northern white rhino.
"We're not willing to give up yet," Rieches said.