The migrations of many large hoofed mammals are in serious decline worldwide, says a new report led by Grant Harris of the Center for Biodiversity and Conservation at the American Museum of Natural History. In fact, a quarter of large migrating species may no longer migrate at all. Fences, agriculture, and other human changes are to blame, as they block natural migration routes and limit access to food and water. As a result, the populations of many of these mammals are shrinking.