SSS 2012-01-18（在线收听）

 True to their names, boa constrictors squeeze the life out of their prey. 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 outfitted 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 dangled them in front of hungry Boas. The sankes attacked, and as long as that rat heart pumping, the boas kept tightening 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 innate 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.