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 True to their names, boa constrictors squeeze the life out of their prey¹. But how dose a boa know it’s enough for a rat? The snake listens, for a heart beat. When it stops, that’s a cue to let go, according to a study in the Journal biology letters. Researchers outfitted² with artificial beating hearts. They used dead rats to control for other signs of , like muscle. Then they warmed up the rats, set the hearts pumping and dangled³ them in front of hungry Boas. The sankes attacked, and as long as that rat heart pumping, the boas kept tightening⁴ their coils and p of pressure, sometimes for more than 20 minutes. But as soon as the scientists kill the heart beat, the boas loosened up. Even captive boas, who had never hunted a live prey paid attention to the pulse, suggesting the behavior is innate⁵ and for good reason. The authors say constricion takes a lot of energy, and it can be dangerous, say, if an enemy strikes while the snake’s coiled around its  . But by following heart, boas can keep the pressure on just long enough before a relaxing meal.