Boa constrictors are notorious for their deadly grip, squeezing their next meal until it expires. But scientists have long wondered whether this fatal hug kills prey by suffocation or by obstructing blood flow in the snake's victims.

Now, a new study finds that rats attacked by boas don't die from a lack of air. Instead, the boa's tight coils block the rat's blood flow, leading to circulatory arrest. That deadly grip helps to more quickly subdue rats and other prey that might be clawing back, allowing the snake to quickly end the struggle and preserve its energy, the researchers said.

"This is such an efficient behavior, and it allows us to realize that this behavior was really important in snake evolution," said lead researcher Scott Boback, an associate professor of biology at Dickinson College in Pennsylvania. "[Constriction] is extremely efficient at killing their prey and allowing them to be successful." [Watch a Boa Constrictor Subdue and Devour a Howler Monkey (Video)]

Old idea, new test

It's no surprise that people suspected that the Boa constrictor used suffocation to kill prey, Boback said. Oftentimes, it looks like prey is gasping for breath as it fights against the snake's hold, he said.

But two studies — one published in 1928 and the other in 1994, the latter written by Dr. David Hardy, an anesthesiologist who studies snakes — suggested otherwise. Suffocation can take minutes to kill a rat, whereas circulatory arrest can lead to death within 60 seconds, Boback told Live Science.

"What Hardy saw was the speed at which the animals were dying ... They were dying way too quickly for it to be suffocation," Boback said in a statement. "He suspected that it was circulatory or cardiac arrest because of the speed at which death was occurring."

To investigate, Boback and his colleagues tested how anesthetized rats responded to boas' constriction. But first they had to perform rat surgery: They implanted electrocardiogram electrodes to measure the rats' heart rates, and inserted blood pressure catheters into a major artery and vein in each rat.

"We did that with an artery and a vein because we wanted to see both sides of the circulatory system," Boback said. (Arteries carry oxygenated blood away from the heart to the organs, and veins return de-oxygenated blood to the heart.)

They also inserted a pressure probe, and took a blood sample from each of the 24 rats, before placing them, anesthetized, next to the hungry snakes. After the rats died, the researchers removed them — before the snakes had time to devour the rodents — to take another blood sample. Then, they gave the snakes more dead rats to eat.

"They actually exert a lot of energy to constrict, and we wanted to allow them to recoup their energy," Boback said.

Rat results

The snakes struck quickly, biting the rats' heads and wrapping their bodies around the prey. The sensors embedded in the rats showed that the rodents' circulation shut down within seconds of the attack, Boback said.

The rats' arterial pressure dropped, meaning their hearts had trouble pumping oxygenated blood to the rest of their bodies. Meanwhile, their venous pressures rose, suggesting that the snakes' constriction applied a pressure that was too high for the blood to return to the heart.

Without oxygenated blood, rats' hearts went haywire. Blood samples also showed that the rats' potassium levels skyrocketed, likely leaking from burst cells and thus indicating cardiac arrest. The researchers didn't measure the rats' brain activity, but the lack of blood to the brain also likely hastened the rats' deaths, Boback said. [Check Out the World's 6 Deadliest Snakes]

"The blood is not moving, and therefore, the cells in the body are dying from the inability to get blood to them," he added.

Scientists think that even early snakes during the Cretaceous period had the ability to constrict — an advantage that set them apart from their lizard ancestors and allowed them to pursue larger prey, as long as they could wrap their bodies around victims, Boback said.

Boback said he hopes to study how other types of prey respond to Boa constrictor's tight grasp. For instance, iguanas can hold their breath underwater for more than 4 hours, but it's unclear whether this ability would help them during a Boa attack, he said.

The study is "detailed and carefully done," said Brad Moon, an associate professor of biology at the University of Louisiana at Lafayette who was not involved in the research but has also studied the amount of pressure snakes exert on prey.

"If you go to basically any reference material on snakes and giant snakes and constrictors, they typically say that constriction suffocates the animal and they squeeze a little harder each time it exhales," Moon told Live Science.

But the new study shows that snakes are squeezing hard enough to interfere with the prey's circulation, Moon said. This technique allows it to kill quickly, conserves its energy and keeps it alert of potential dangers in its environment, Moon said.

The study was published online today (July 22) in The Journal of Experimental Biology.

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