<p>An Amur <a href="http://animals.nationalgeographic.com/animals/mammals/leopard/">leopard</a> named Usi from Nebraska's <a href="http://www.omahazoo.com/">Omaha Zoo</a> is captured in mid-prowl in this picture by <a href="http://www.joelsartore.com/">National Geographic photographer Joel Sartore</a>.</p> <p>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 <a href="http://www.iucnredlist.org/">International Union for Conservation of Nature (IUCN)</a>. This gallery will highlight some of these animals that are literally on the edge of extinction. (<a href="http://animals.nationalgeographic.com/animals/big-cats-initiative/get-involved/">Learn how to help big cats with National Geographic</a>.)</p> <p>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 <a href="http://www.joelsartore.com/store/rare-portraits-of-americas-endangered-species/"><i>RARE: Portraits of America's Endangered Species</i></a>.</p> <p>Sartore says he chose to take portraits of his animal subjects because the format gives equal weight to creatures big and small.</p> <p>"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.</p> <p><i>—Ker Than</i></p>