Three guards stand by a female northern white rhino called Najin inside an open enclosure at Ol Pejeta Conservancy in Nanyuki, Kenya, April 17, 2015. (Xinhua/Sun Ruibo)

A female northern white rhino called Najin is seen with a southern white rhino inside an open enclosure at Ol Pejeta Conservancy in Nanyuki, Kenya, April 17, 2015. (Xinhua/Sun Ruibo)

Female northern white rhino called Najin walks past a southern white rhino inside an open enclosure at Ol Pejeta Conservancy in Nanyuki, Kenya, April 17, 2015. (Xinhua/Sun Ruibo)

Guards stand near the female northern white rhino called Najin inside an open enclosure at Ol Pejeta Conservancy in Nanyuki, Kenya, April 17, 2015. (Xinhua/Sun Ruibo)

Sudan, the last male northern white rhino, feeds himself inside its enclosure at Ol Pejeta Conservancy in Nanyuki, Kenya, April 18, 2015. (Xinhua/Sun Ruibo)

Richard Vigne, Chief Executive Officer of Ol Pejeta Conservancy, introduces the differences between northern white rhino and southern white rhino during an interview with Xinhua at Ol Pejeta Conservancy in Nanyuki, Kenya, April 17, 2015. (Xinhua/Sun Ruibo)

Photo taken on April 17, 2015 shows a female northern white rhino called Najin at Ol Pejeta Conservancy in Nanyuki, Kenya. (Xinhua/Sun Ruibo)