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TIME OF DEATHA boa constrictor takes the pulse of a dead rat implanted with an artificial heart, letting go only when the faux vital sign disappears.Scott Boback

A boa constrictor knows to stop squeezing a juicy rat by sensing the heartbeat of its prey, easing up only when the pulse stops, a new study finds.

Detecting heartbeats may give snakes like the boa constrictor an edge for hunting iguanas and other large cold-blooded animals that can cling to life for a long time when cut off from oxygen, researchers report online January 18 in Biology Letters. Taking the pulse of such creatures would be a surefire way to know when to let go. 

To pinpoint the snake’s sensitivity to this particular vital sign, researchers at Dickinson College in Carlisle, Pa., started with rat corpses lacking any signs of life. The scientists then implanted pressure sensors and artificial hearts, small bulbs pumped with fluid that produce the illusion of a regular pulse.

Wild boa constrictors attacked the carcasses with or without the simulated heartbeat. But the snakes hugged harder and for about twice as long when the pulse was switched on. If the pulse stopped, the squeezing also stopped. Lab-raised snakes never exposed to live prey responded the same way, suggesting the behavior is innate, not learned.

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