Large predators don't process humans as "prey" in their neural wiring; they process us as "other apex predators." A lion or tiger can visually assess a zebra's physical condition, but a human doesn't fit that template. Instead, predators rely on learned associations. In ecosystems where humans have existed for millennia (like Africa and Asia), predators possess a cultural memory passed from mother to cub that humans are trouble. In places where humans are new (like the Americas before the 15th century), predators like the now-extinct American lions initially had no such fear—and were wiped out precisely because they didn't avoid us.

(http://www.autoadmit.com/thread.php?thread_id=5875424&forum_id=2Reputation#49947566)