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According to a new study in Biology Letters, boa constrictors listen to their prey’s heartbeat for an indication when to stop squeezing them, or continue squeezing.

From the study:

Many species of snakes use constriction—the act of applying pressure via loops of their trunk—to subdue and kill their prey. Constriction is costly and snakes must therefore constrict their prey just long enough to ensure death. However, it remains unknown how snakes determine when their prey is dead. Here, we demonstrate that boas (Boa constrictor) have the remarkable ability to detect a heartbeat in their prey and, based on this signal, modify the pressure and duration of constriction accordingly.

That is to say, thanks to an awesome example of evolution, constrictors will not stop squeezing you until your heart stops beating (terrifying, I know).

How do you prove such a thing? Quite simple, really. The researchers outfitted a dead rat with an artificial device that would simulate the beating of a heart. Dead rats were used so that the only factors signifying death would be the beating heart and not some other factor like muscle spasms.

The dead rats were warmed, the artificial hearts were set beating, and the whole apparatus was dangled in front of hungry boas. What the researchers found that was as long as the fake 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.

[Via Scientific American]