Boas Time Their Big Squeeze to Prey's Heartbeat
A Crawl Cay boa constricting around a large rat.
Credit: Scott Boback

During its long, slow squeeze, a boa constrictor will feel every last heartbeat of the animal it has captured. This slithering predator uses its prey's heartbeat to decide when it's safe to stop constricting, a new study has demonstrated.

The snakes are less worried about over-squeezing their prey — which include lizards, birds and mammals — into a mushy meal, and more worried about the energy such a death embrace takes, the researchers say.

"Constriction is an energetically costly and potentially dangerous activity," researchers from Dickinson College in Carlisle, Pa., write in the Jan. 18 issue of Biology Letters. "Thus it would be greatly advantageous for constricting snakes to accurately and precisely determine when prey are incapacitated and no longer capable of retaliation or escape."

Varying the time and effort spent squeezing seems to be an innate skill in these animals, but the study indicates they can even improve upon their natural abilities. [Video of Snake Eating Snake]

Lab-created prey

A boa in the wild tends to constrict a rat for about 20 minutes before feeding on it. To test boas, the researchers say, they inserted fake beating hearts into dead but still-warm rats. When the researchers let the boas loose on the experimental prey, they let the heartbeat continue the whole time, shut it off after 10 minutes, or left it off the entire time.

When given prey with a continuing heartbeat, the snake kept constant pressure for a few minutes beyond the usual 20 minutes, periodically adjusting the coils of its body to ensure adequate squeezing, before it gave up. When given prey with no heartbeat, the snakes squeezed about half as hard, made no visible coil adjustments, and stopped squeezing after about 10 minutes.

A boa constrictor found on Crawl Cay Island, to the northeast of Belize City in the Caribbean. These boas are very unusual and rare: dwarf snakes that max out at about 5 feet long, making them easier to work with in the lab.
A boa constrictor found on Crawl Cay Island, to the northeast of Belize City in the Caribbean. These boas are very unusual and rare: dwarf snakes that max out at about 5 feet long, making them easier to work with in the lab.
Credit: Scott Boback

When the heart stopped beating about 10 minutes into the constriction, the boas kept squeezing for several minutes but then backed off. The boas usually clenched this prey for about 17 minutes altogether, applying pressure that measured midway between that used for the heartbeat and no-heartbeat groups.

"Our results are the first to demonstrate that snakes use the heartbeat in their prey as a cue to modulate constriction effort and to decide when to release their prey," the researchers write.

Perfecting an innate skill

Even snakes that had never eaten live prey before reacted the same way in the heartbeat experiments, which means changing their constriction pressure and duration is probably a skill they are born with. The researchers add that wild-caught snakes squeezed harder for longer than their captive-born relatives did.

"The ability to respond to a heartbeat is innate, whereas the magnitude of the response is guided by experience," the authors write. "We suggest that the capacity to improve performance though learning enables snakes to become efficient predators of variable and unpredictable prey animals."

The researchers say the snake may have evolved this ability to sense its prey's heartbeat as a way of dealing with lizards, as opposed to warm-blooded prey. With a metabolism that's already slow, lizards can survive hours of being squished. Their hearts will beat even after they have stopped moving.

The researchers alternatively think that the snake's extra-sensitive skin could have developed separately, when they lost their legs, to help them sense their environment, and snakes simply use this sensitivity to help dispatch prey.

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