Field mice may not carry passports, and nobody ever asked a tree frog for a green card, but do animals care about imaginary lines on a map? In fact, national borders can become natural borders over time, with significant consequences for the nonhumans living on either side.     

After Israel normalized relations with its neighbor Jordan in 1994, University of Haifa biologist Uri Shanas studied how the fauna on either side of the border differed. He found that rodents in Israel were much more cautious than their counterparts in Jordan -- a distinction Shanas attributes to Israel's more modern agricultural development.     

Such differences can persist for decades. Scientists have observed that red deer in the forests on the Czech-German border are still stuck in the Cold War, avoiding the old boundary because a long-dismantled electric fence once ran along it. But the effect isn't always negative. The 390-square-mile demilitarized zone between North and South Korea has been a no-go area for humans for decades, which makes it a great place for rare red-crowned cranes and Chinese water deer. Now there is talk of making the de facto wildlife preserve permanent: a very rare case of animals benefiting from human conflict.