01/18/2012 10:36 am ET Updated Feb 04, 2013

Boa Constrictors Know When To Stop Squeezing By Monitoring Heartbeat Of Prey

Boas and other constrictors kill by literally squeezing the life out of their prey--no secret there. But how do they know when to stop squeezing?

With the help of a bunch of hungry boas and a tempting smorgasbord of rats, scientists at Dickinson College in Carlisle, Pennsylvania have found the answer: Through their tightly wrapped coils, the snakes can feel the heartbeat of their prey--and they stop squeezing once the hearts stop beating.

It's lucky for that the snakes have this ability, the scientists point out in their macabre study. Why? Because the snakes expend huge amounts of energy when they squeeze--and stopping squeezing as soon as possible helps prevent precious calories from being wasted. What's more, the snakes are more vulnerable to attack by predators--or even the prey the're trying to subdue--when they're busy squeezing.

For the study, published in the journal Biology Letters, Dr. Scott Boback and his team put wild and captive-born snakes through their paces. To make their measurements as accurate as possible, the biologists offered the snakes not wildly thrashing live rats but cadaveric (dead) rats fitted with simulated hearts (water-filled bulbs made to pulsate via pump).

What happened next was pretty dramatic.

"I couldn't believe my eyes the first time we tested a snake with a rat with a simulated heart," Dr Boback told BBC Nature. "It was writhing and squeezing the rat in an apparent effort to kill it."

And the longer the artificial hearts kept beating, the longer the boas kept up the pressure. According to Discover magazine, the pressure is now on biologists to explain exactly how the snakes evolved to have this heart-sensing ability.

CORRECTION: An earlier version of this article misstated the institution where the research was conducted as Dickinson University.