It's hard work being prey. Watch the birds at a feeder. They're constantly on alert, and will fly away from food — from easy nutrition — at the slightest movement or sound. Given that I've never, ever seen a bird plucked from a feeder by a predator, it seems like a whole lot of wasted effort against a small threat.

Assessing and reacting to risk is one of the most important things a living creature has to deal with. The amygdala, an ancient part of the brain that first evolved in primitive fishes, has that job. It's what's responsible for the fight-or-flight reflex. Adrenaline in the bloodstream, increased heart rate, increased muscle tension, sweaty palms; that's the amygdala in action. You notice it when you fear a dark alley, have vague fears of terrorism, or worry about predators stalking your children on the Internet. And it works fast, faster than consciousnesses: show someone a snake and their amygdala will react before their conscious brain registers that they're looking at a snake.

Fear motivates all sorts of animal behaviors. Schooling, flocking, and herding are all security measures. Not only is it less likely that any member of the group will be eaten, but each member of the group has to spend less time watching out for predators. Animals as diverse as bumblebees and monkeys both avoid food in areas where predators are common. Different prey species have developed various alarm calls, some surprisingly specific. And some prey species have even evolved to react to the alarms given off by other species.