Answer: The Iron Curtain

Researchers tracking the movement of red deer in the Czech Republic discovered something rather curious. Despite the physical ability to travel freely wherever they wished, the deer’s travel routes never crossed the border between the Czech Republic and Germany.

The deer have no political motivation in avoiding international travel, but instead are obeying what amounts to a set of rules passed down, generationally, from doe to fawn. During the Cold War, the entire border between the Czech Republic and West Germany was fenced off with parallel electrified fences and patrolled by armed guards. Crossing the border was a death sentence for the deer and they learned to avoid the entire length of it.

Young red deer spend roughly a year with their mother after birth and it turns out that red deer mothers have, for generations now, been teaching their young to avoid the border. At this point, they could easily cross the border without worry, but the habit is ingrained and although the deer can’t communicate why the area is dangerous to each other, they learn by behavior and avoid it for the rest of their lives—passing the border avoidance on to their offspring.