How boa constrictors squeeze their prey without strangling themselves

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JoAnna Wendel

The boa constrictor’s choke hold is an iconic animal attack. Once coiled around its prey, in mere minutes a snake can squeeze the life out of a victim. The boa then gulps down its dinner whole. Now, X-ray videos show just how these snakes squeeze so hard — or swallow something as big as a monkey — without suffocating.

When one part of a Boa constrictor’s rib cage is compressed, the part of its lungs enclosed here cannot draw air. But the new videos reveal that a snake can simply move another section of its ribs to inflate its lungs there. That allows a boa to keep breathing even while one part of its body is squeezing.

Researchers shared their finding March 24 in the Journal of Experimental Biology.

Some people had reported seeing this behavior in snakes before. “But no one’s ever empirically tested that,” says John Capano. He’s a biologist at Brown University. That’s in Providence, R.I.

Capano and his colleagues wanted to take a closer look at how boas breathe. So, they implanted metal markers on the ribs of three boa constrictors. One set of markers was placed about a third of the way down the animals’ bodies. The other set was placed about halfway down the snakes. Those metal markers showed up in X-ray videos of the animals. This allowed the researchers to map rib motions over different parts of the snakes’ lungs.

The team wrapped a blood-pressure cuff around different parts of the boas’ bodies. The cuff’s pressure slowly increased until a snake’s rib cage could not move in that area. This mimicked the effect of a snake using that part of its body to grip prey or gulp it down.

Some snakes reacted to the cuff better than others. “One was really, really calm. Never had to worry about her,” Capano says. “The other two, I had to watch my back quite a bit more. But they were all pretty amenable to it, once the cuff was on.”

Snakes at rest breathed by moving ribs near the front of their lungs. When gripped by a cuff about one-third of the way down its body, a snake breathed by moving ribs closer to its tail. When gripped by a cuff about halfway down their length, the snakes breathed by moving ribs closer to their heads.

“They can basically just breathe wherever they want,” Capano says. This ability was probably crucial for early snakes to start throttling and swallowing large prey, he adds. That’s important. Why? Snakes’ ability to eat big prey is thought to be a key reason these animals have adapted to so many habitats. Snakes are some 3,700 species strong. And they’re found on six continents.

Controlled breathing may be “one of the key innovations within snake evolution that allowed this group of animals to explode and become one of the most successful groups of vertebrates we’ve ever had,” Capano says.

Panel 1. Image: A boa constrictor wrapped around a tree branch against a blue background. Text: How boas keep breathing while squeezing, Written by Maria Temming, Illustrated by Joanna Wendel
Panel 2. Image: A drawing of a man wearing a blue shirt and brown pants. He has short dark hair and a beard and mustache. He is looking at a boa constrictor wrapped around his arm. The snake is wearing a shirt that says 'Free Hugs' and looking up at the man's face. There are hearts next to the snake. The man is saying 'I love you too, buddy, but this is getting a little tight.' Text (top image): The boa constrictor's grip is one powerful flex. Coiled around it's prey, a snake can squeeze the life out of a victim in mere minutes. Text (bottom image): But how do these snakes squeeze so tight - or swallow large prey - without cutting off their own air supply?
Panel 3. Text at top: To find out, researchers wrapped a blood pressure cuff around three boa constrictors. The cuff's squeeze mimicked the effect of a snake using part of its body to grip or gulp down prey. It stopped a snake's ribs from moving - and lungs inflating - in that area. Image: Boa constrictor with a blood pressure cuff. The snake says 'That'ssssss a sssssssnug fit'
Panel 4. Top text: Not all the snakes were thrilled about this plan. 'One was really, really calm,' says John Capano. He's a biologist at Brown University in Providence, R.I.  Image of John, the man seen in panel 2, putting a cuff on a snake resting calmly on a desk. The snake says 'This is fine' Middle Text: 'The other two, I had to watch my back quite a bit more,' Capano says. Image below: Capano holds a blood pressure cuff towards a snake and says 'Please?" The snake is coiled up and glaring at Capano 'Don't even thhhhhhink about it' Bottom text: But all the boas settled down once the cuff was on.'
Panel 5. Top text: A snake's lungs extend throughout much of its body. X-ray videos showed that boas use different parts of their lungs to breathe while being squeezed in different places. When gripped by a cuff closer to their heads, the snakes breathed by moving ribs closer to their tails.... Image: An x-ray view of a snake abdomen from above. The pressure cuff is on the snake to the left of the image. To the right, towards the tail, the abdomen is wider and is labeled 'Ribs expanding'
Panel 6. Top text: ... And when gripped by a cuff closer to their tails, the snakes breathed by moving ribs closer to their heads. Image: X-ray view from above showing the cuff to the right of the x-ray, towards the tail. To the left, toward the head, ribs are expanding.
JoAnna Wendell

Maria Temming is the assistant editor at Science News Explores. She has bachelor's degrees in physics and English, and a master's in science writing.

JoAnna Wendel is a freelance science writer and cartoonist in Portland, Ore. She loves to make comics about all types of science, but she especially loves drawing planets, invertebrates and sea creatures. When she's not drawing, JoAnna is probably reading, hiking or hanging out with her cat, Pancake.

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