Sadly there are now only 5 northern white rhinos in the world after the passing of a male at the San Diego Zoo, who died, apparently of natural causes. Efforts have been under way to breed the northern white rhino without success so far. In vitro fertilization will be tried in Africa after exhausting natural methods: http://www.sfgate.com/news/science/article/Rare-northern-white-rhino-dies-at-California-zoo-5958027.php