Boas sense when prey's heart goes still

Boa constrictors don't like to waste their hugs. A study involving dead mice and fake hearts shows that the snakes loosen their deathly grip once their prey's pulse stops.

This reaction allows the snakes to save energy, say Scott Boback and his team at Dickinson College in Carlisle, Pennsylvania.

Boback and his team fed sixteen wild and captive-born snakes on dead rats rigged with artificial hearts that either beat continuously, pumped for ten minutes or were still.

They warmed the rats with an electric blanket to mimic live prey, and implanted two small water-filled bladders in their chest cavity and abdomen to monitor the boas' squeeze. The bladder in the chest cavity was placed right next to the rats' actual heart and hooked up to a piston pump to simulate heartbeats.

The team found that the boas squeezed all rats, but would frequently adjust their hugs and periodically give a tighter squeeze if they sensed a heartbeat. Rats with no pulse were released after 10 minutes and squeezed with half the pressure of those whose heart beat continuously through the 20-minute trial. In experiments where the heartbeat was switched off halfway through, it was a matter of minutes before the snakes released their coils.

Innate behaviour

Captive-bred snakes that had never hunted behaved in the same way as wild ones, so sensing and responding to the prey's heartbeat is likely innate, says the team. Just how much force is necessary may be something learned through experience, though?? wild boas used a looser and shorter grip.

Being economical with their squeeze makes sense for the boas. Studies show that constriction requires a seven-fold increase in aerobic metabolism, so cutting it short means saving energy.

"It's a tremendous expense in terms of onboard fuel to search, find, grab, kill and swallow a prey item?? not to mention throughout all of it you are very, very vulnerable," says Charles Zwemer, another member of the team. "It makes a lot of sense to know when to cut your losses, swallow the food and run."

While rats succumb after a minute or two of being trapped in a boa's grip, this pulse-sensing ability could also help the snakes check the vital signs of more tenacious prey like lizards, which can survive an hour and a half without oxygen.

Journal reference: Biology Letters, DOI: 10.1098/rsbl.2011.1105

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