How do boa constrictors avoid suffocating when they squeeze their prey?

Boa constrictors famously hunt by ambushing their prey and then squeezing the captured animals to death with their muscular coils.

But as a boa constricts its body around a victim and cuts off blood flow to that animal's brain, how does the snake avoid squeezing all the air from its own lungs and suffocating itself in the process?

It turns out, a boa constrictor can rapidly adjust which section of its ribcage it uses to breathe, according to a study published March 24, 2022, in the Journal of Experimental Biology (JEB). So if a boa entraps a squirrel or rat using the front half of its body, the constrictor will then use the ribs farther down its noodle-like body to continue breathing as it crushes the rodent. And likewise, the ribs closer to the animal's head will take over if the back ribs are currently pressed up against an immobilized animal.

"Constriction is an incredibly energetically taxing behavior and almost certainly requires high oxygen demands," said David Penning, an assistant professor of biology at Missouri Southern State University, who was not involved in the study. The new research "helps to unpack some of the confusion around how oxygen intake occurs during this taxing process." 

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In addition to revealing how boas breathe during constriction, "I think this work can be used to make larger inferences beyond just the boa constrictor," Penning told Live Science in an email. "Not only do we know very little about how snakes function, we know equally little about the real metabolic demands of most of their activities." 

Snake lung evolution 

This ability to control which section of their ribcage is involved in breathing likely allowed boas to evolve to their present forms, said study first author John Capano, a postdoctoral research associate in the Department of Ecology, Evolution, and Organismal Biology at Brown University. "It doesn't seem like you could evolve constriction to kill really big things if you're compromising lung ventilation," Capano said.

This precise breathing strategy likely also helps boas survive the process of swallowing and digesting large prey, since these hefty meals restrict the movement of the animals' ribs from the inside, Capano told Live Science. In their report, the study authors theorize that other snake species likely use this same breathing method, and that the method likely evolved in tandem with snakes' highly mobile skulls, which contort so the animals can wrap their jaws around enormous prey and swallow it in one gulp, he added. 

Unlike humans, snakes lack diaphragms, the large, dome-shaped muscles that contract and flatten to allow a person's lungs to expand and fill with air and then relax and compress the lungs to push air out. Instead, snakes use muscles attached to their ribs to alter the volume of their ribcage and allow air in and out of the lungs.

When animals breathe with their ribcage, they typically use small muscles called intercostals that run between adjacent ribs, Capano said. These animals use the intercostal muscles to move entire "blocks" of ribs at one time, rather than having fine-tuned, independent control of individual rib bones.

By comparison, boas and other snakes primarily use levator costae muscles to breathe; each levator costa runs from the vertebral column to one of the snake's more than 400 ribs. In their new study, the team revealed how each levator costa "can basically control motions a lot more discreetly," Capano said. "It can just lift that individual rib." When a levator costa contracts, it pulls the rib back, like a door on a hinge, while also causing the bone to slightly rotate; these delicate motions control when and where the snakes' lungs can inflate.  

All snakes have fully-developed right lungs, but depending on the species, a snake may either have a puny left lung or no left lung at all, according to a 2015 report in the journal PLOS One. Boa constrictors fall into the first group, in that they have a teeny-tiny left lung and a lengthy right lung that's roughly one-third as long as the snake's body, the JEB report notes. 

The front one-third of the long lung, closest to the snake's head, contains tissue that can perform gas exchange, meaning it can pass oxygen into the bloodstream and remove, or exhale, waste products, like carbon dioxide. The back two-thirds of the lung cannot perform gas exchange and are essentially "just a bag," Capano said.  

This image highlights the different rib regions that a boa constrictor uses to breathe while resting, constricting prey and digesting a meal. (Image credit: Scott Boback)

Scientists have different theories as to the function of this bag-like region, but the new study supports the idea that it acts as a kind of bellows that helps draw air through the front, gas-exchanging part of the lung, Capano said. So when the front of the lung can't fully expand — when the boa is busy subduing a snack — the back of the lung can still pull air through the tissue and allow gas exchange to occur.

"Even if your front [lung] can't move, or even if something's squishing it, you can still draw air through it," Capano said. "And then by doing so, you're still pulling oxygenated air through your vascular tissue."

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The team figured out that boa constrictors used this unique breathing method by placing blood pressure cuffs on adult boas in their lab, in order to restrict the movement of some of the snakes' ribs. The team used various techniques to measure air flow in and out of the snakes' lungs and the electrical activity of different muscles. They also used a technique called "X-ray reconstruction of moving morphology" (XROMM) to track how the snakes' ribs were moving, in real-time. 

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Using XROMM involved placing small metal markers on a few of the snakes' ribs and then scanning the animals from the side and from above as they moved. By combining the footage taken from both viewpoints, the team captured how the ribs moved in three dimensions and created detailed models of the ribcage in motion, Capano said. 

The new study nicely captures how the movement of boas' ribs changes in response to the blood pressure cuff, which presses in on the animal from all sides, Penning said. That said, when a snake actually constricts an animal, the side of the snake that makes contact with the prey is "likely doing the bulk of the work exerting force," while the other side of the snake may be less compressed, by comparison, he noted.

So there may be slight differences in how the snakes adjust their breathing to accommodate pressure from the cuff, as compared with when they're throttling prey; Penning said he'd be interested in seeing those differences investigated in the future. Looking forward, Capano said that he's interested in studying how boas and other snakes move their ribs during different dynamic behaviors, such as slithering. 

Originally published on Live Science.