Born to Move: Astonishing Photos of Great Migrations

: Every summer when the ice melts, male and female walruses of the Pacific go their separate ways. The males head for crowded beaches, while the females and their young head north, through the Bering Strait, struggling to find ice. Researchers said global warming has melted much of the Arctic ice and often females are forced to stay on the beaches where food and space is scarce. At one point, 20,000 walrus bodies are packed in together, some on top of each other. "Availability of ice is absolutely critical," Wilson said. "The bunching together of males and females is going to have profound, profound implications for walruses." (Courtesy National Geographic)