True to their name, boa constrictors squeeze the life out of their prey. But how does a boa know it's snuffed out a rat? The snake listens for a heartbeat. When it stops, that's the cue to let go, according to a study in the journal Biology Letters. Researchers outfitted rat cadavers with artificial beating hearts. They used dead rats to control for other signs of passing, like muscle spasms. Then they warmed up the rats, set the hearts pumping, and dangled them in front of hungry boas. The snakes attacked. And as long as that rat heart kept thumping, the boas kept tightening their coils and applying bursts of pressure, sometimes for more than 20 minutes. But as soon as scientists killed the heartbeat, the boas loosened up. Even captive-born boas who'd never hunted live prey paid attention to the pulse—suggesting the behavior is innate. And for good reason. The authors say constriction takes a lot of energy. And it can be dangerous, say, if an enemy strikes while the snake's coiled around its quarry. But by following the telltale heart, boas can keep the pressure on just long enough. Before a relaxing meal.