When Sudan, the world’s last male northern white rhino, died in March at a wildlife preserve in Kenya, it seemed to signal the end of his kind. But it might not be over yet. Scientists on four continents are working with rhino eggs, cell cultures, tissues, blood and frozen sperm samples to produce a live northern white rhino birth—and maybe even repopulate the entire subspecies. If they succeed, it will be the first time a virtually extinct mammal with no living males has been brought back from the brink.
One strategy depends on the subspecies’ two known females: Sudan’s 29-year-old daughter and 18-year-old granddaughter. Because neither is healthy enough to deliver a calf, scientists hope to extract some of their eggs, combine each egg with previously collected frozen sperm and implant the resulting embryo in another female.
Most likely the surrogate would be a southern white rhino, another subspecies, which has a longer horn, more hair and some 21,000 members still living in southern Africa. But it’s not impossible that a horse could serve as a surrogate; rhinos and horses belong to the same class—Perissodactyls, or odd-toed ungulates—and have similar reproductive systems.
At the San Diego Zoo Safari Park, which holds the world record for the most rhinos born in captivity (176 to date), researchers are studying how to best extract viable eggs from southern white rhinos. The procedure involves guiding a probe through the rhino’s rectum into an ovary, then drawing eggs out through a catheter. Because the two-ton animal’s ovaries are three to four feet from its rump, the process is difficult at best, says Barbara Durrant, the zoo’s director of reproductive sciences. They’ve managed to gather a few eggs so far, but they’ll need to improve their success rate if they hope to employ the procedure on one or both of Sudan’s descendants.
Another approach would take advantage of recent breakthroughs in manipulating a kind of stem cell called induced pluripotent stem cells, which can be generated from other adult cells. Researchers in San Diego, the Czech Republic, Austria, Italy and Japan are working on transforming northern white rhino skin cells into such stem cells and then into egg cells. (The Japanese scientists have accomplished the feat with mouse cells.) The rhino egg would then be combined with sperm to form an embryo, which would be implanted in a surrogate.
One potential upside of using stem cells is diversity, Durrant says. The San Diego Zoo has 12 northern white rhino cell lines, and working with them on the gene level can furnish eggs with enormous genetic variety—an insurance policy against future inbreeding and genetic bottlenecks.
Some experts speculate that in vitro fertilization and surrogate gestation could lead to a live birth within ten years. Durrant is more cautious: “I hesitate to give a timeline because we simply don’t know.”
Technology is only part of the puzzle, says Dino Martins, a Smithsonian research associate and director of the Mpala Research Centre in Kenya. Attitudes have to change, too, because the forces that wiped out the northern white rhino—war, poverty, poaching—remain in place. “We need young people in East and Central Africa to feel like these rhinos belong to them,” Martins says. “We’re only going to save what we care about.”