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Amur LeopardAn Amur leopard named Usi from Nebraska's Omaha Zoo is captured in mid-prowl in this picture by National Geographic photographer Joel Sartore. Found in the Primorye region of Russia, the Amur leopard is a very rare subspecies of leopard: A 2007 census counted only 14 to 20 adults and 5 to 6 cubs. That makes the big cat one of about 2,300 species that are considered critically endangered by the International Union for Conservation of Nature (IUCN). This gallery will highlight some of these animals that are literally on the edge of extinction. (Learn how to help big cats with National Geographic.) By documenting such rare species, Sartore wants to show the public what we would lose if they disappeared forever. He has also written a book called RARE: Portraits of America's Endangered Species. Sartore says he chose to take portraits of his animal subjects because the format gives equal weight to creatures big and small. "Some of the frogs I've photographed are the size of a thumbnail, and this is a way for me to put them on equal footing with bigger animals like lions," he writes on his website. —Ker Than

Photograph by Joel Sartore