You Try Constricting Your Prey and Breathing at the Same Time
Boa constrictors have figured out a way to inflate only parts of their lungs.
By Katherine J. Wu
Oscar Rivera / AFP / Getty
When a boa constrictor coils its midriff around a wriggling rat, it's easy to feel sorry for the soon-to-be-lifeless rodent, its blood supply so blocked that its heart stops pumping.
But consider, too, the plight of the snake. The curly-fry crush of a boa—which can exert pressures of up to 25 pounds per square inch—doesn't just squish the life out of its prey. It also compresses the predator, putting an epic squeeze on the parts of the body that harbor the snake's heart and the upper portions of its lungs and gut, sometimes for up to 45 minutes at a time.
"The entire front third of the body gets involved," which is no fun at all for the organs inside, says John Capano, who studies the reptiles at Brown University. Constricting other animals to death is like trying to win a wrestling match while laced into a corset—a recipe, it would seem, for autoasphyxiation. But boas somehow manage the feat, then go on to swallow their prey whole, smooshing their chests from the outside, then the inside, breathing easy all the while. Capano has a particular way of describing this curious phenomenon, which has mystified snake aficionados for years: "How does one rib cage kill another rib cage without hurting itself?"
The key, he and his colleagues have found, is precise control. Boas are basically accordions of bone: They have hundreds of pairs of ribs, running nearly the entire length of their body. And they can "choose any couple of ribs," Capano told me, "and just use those." The fingerlike bones flare out in isolated clusters, compelling only the bits of lung directly beneath to inflate—in effect adjusting which sections of the organ the snake uses to inhale. Such maneuvering allows the boas to divert the business of breathing away from the parts of the body that are constricting or digesting prey, and toward the unencumbered bits that are free to expand, making it possible for the rib cage to simultaneously squash dinner down and balloon the lung out. Those acts are normally "at odds with each other," says Jennifer Rieser, who uses biophysics to study snake movements at Emory University. But the boa constrictor—and its breath—finds a way through.
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The idea of localized breathing isn't totally new. Over the years, several biologists had come to suspect that boas and their kin might sidestep suffocation by moving their breath all around. That would be a tall order for most animals, but not so for snakes: A boa's lung, for instance, runs about 30 percent of its total body length, creating lots of space for air to be drawn in and expelled back out. (Technically, that's just the right lung; the left one is shriveled and nearly nonexistent, a nonfunctional nub.) Even before Capano started his own experiments, he'd gotten an intuitive sense of lung-localization, just by watching and holding tons of snakes. He could feel their bodies inflating and deflating in sections, while other bits stayed still; he even recalls an instance in which one of his boas started breathing with two regions of its body at once, shutting down the stretch in between. But his observations, and others like them, had all been casual. No one had managed to peer inside the snakes to figure out the how and why of their aspiration antics.
A boa constrictor with a region of its body touching a table breathes with two separate regions of its lung. Credit: Scott Boback and John Capano
To do so, Capano assembled a small cavalry of boas, and simulated the act of constriction by fitting the snakes with blood-pressure cuffs and inflating them. To clinch what was going on underneath, he had to measure the motion of the snakes' ribs—"the most obvious proxy for understanding if ventilation is happening" in the lung within, he said. So he used a combination of X-ray videos and CT scans, blending the footage together to model the shift of bones in real time. Talia Moore, a biomechanics expert and snake researcher at the University of Michigan, told me that Capano's work, which she didn't participate in, includes some of the most "clever biomechanics experiments of the last few years." Thanks to the blood-pressure cuff, no one had to wait around for the snakes to stumble upon a sufficiently hefty meal.
When the cuffs were deflated, the snakes breathed only with the part of their lung closest to the head. That section of the organ would look most recognizable to us: It's laced with blood vessels, ready to soak up oxygen from whatever outside air flows through. The back part of the lung, meanwhile, is wholly lacking in vasculature, resembling "a balloon with nothing in it," Capano said. That bit stays dormant when the snake is just chilling.
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But when Capano inflated the cuff around the ribs encircling the lung's upper half, mimicking the pressure of constriction, the snake's anatomical priorities shifted. "The ribs at the front of the body where we applied the blood-pressure cuff just straight-up stopped moving," Capano said. Only the ribs behind the cuff flared out, yanking the lower part of the lung open. Capano compares the motion to a bellows drawing air in through its nozzle. There's no other way, he told me, to drag air in and hustle it past the blood-rich compartments of the upper lung when the snake's front end is compromised by food pre- and post-swallowing. The pattern reversed when Capano shifted the cuff down, constricting the ribs surrounding the bottom parts of the lung instead—presumably what would happen as the snake's meal traversed through the digestive tract, and freed the top of the body once more.
A boa constrictor with a blood-pressure cuff around its upper body breathes with only its lower lung. Credit: John Capano
This careful finagling of the ribs is so deft, so precise, that the snakes are able to behave almost "as if they have multiple rib cages," says Beth Brainerd, who supervised Capano's research at Brown. The snakes' control over their bones is also voluntary: Each rib is manipulated by an individual muscle, like the string tethered to a piano key; the animal can wiggle just a few units at once. "It's a pretty genius solution" to the battling demands of constricting and inhaling, says Rita Mehta, a biomechanics expert at UC Santa Cruz, who has studied snake constriction but wasn't involved in Capano and Brainerd's work.
A boa constrictor with a blood-pressure cuff around its midsection breathes with only its upper lung. Credit: John Capano
Snakes are probably best known for the bits of anatomy they lack—arms, legs, all the flaily bits that help other animals run and grasp and navigate their world. But maybe that makes what the reptiles can do with the body parts they have, and how versatile their ribs have become, all the more remarkable, Moore told me. Section-by-section breathing may have even made it possible for snake species to go after bigger, stronger prey. Over millennia, constricting and breathing became not enemies, but tag-teaming partners, turning what began as a physiological paradox into a major evolutionary perk.