Boa constrictors are not picky eaters. Though the snakes typically snack on small mammals, lizards, and birds, they don’t shy away from hunting larger prey.
A large boa can devour entire pigs and monkeys, stretching its body around an undigested carcass with ease. Once in the stomach, the snake’s meal is side-by-side with vital organs like the lungs. But snakes, like any animal with lungs, need to expand their ribcage to breathe — and scientists weren’t sure how they managed to keep air flowing when digesting dinner.
It turns out boas don’t have to hold their breath in order to swallow a big meal. Researchers writing Thursday in the Journal of Experimental Biology set out to uncover how they can devour animals whole without suffocating. It’s a skill that may have evolved in tandem with its other predatory traits, allowing it to eat a wide variety of meals.
Here’s the background — The elongated, legless form of a snake is extremely common in the animal kingdom. Reptiles alone have evolved from having lizard-shaped bodies, with four legs and a tail, to snake-like bodies at least 26 separate instances across the evolutionary tree.
Today, there are more than 3,000 snake species on the planet, and they’re found on every continent except Antarctica. “They live in the water, they live on land, they live underground, they can climb trees, they can glide to some extent and kind of be aerial,” says John Capano, an organismal biologist and co-author of the new study. “It’s just insane that they're everywhere.”
The success of snakes can be attributed to certain key traits developed over millions of years of evolution. For boas, their ability to constrict prey and open their jaws extremely wide to swallow allowed them to hunt even larger animals and adapt to new environments.
But those skills came with trade-offs. “In order to constrict something, you have to utilize your body, and then that becomes disengaged for other behaviors in order to crush something to death,” Capano says. “And then when you kill something really big [and eat it], you fill yourself up in this immense way. You’re completely full to the gills, literally.”
Performing either of these feats makes it tough for boas to expand their rib cages in order to breathe. So Capano wanted to explore how snakes deal with so much pressure on their lungs, an adaptation that may have been evolutionarily necessary for them to become successful hunters and consumers of large prey.
What they did — The researchers recruited several captive boa constrictors and measured their lung and rib functions in different scenarios, taking X-rays of the snakes’ bodies to get an inside look at their movements.
Most snakes have a single, narrow lung that runs through their body. Boas actually have two lungs, but the right lung is much larger than the left, and spans about 30 percent of its length.
The right lung also overlaps with organs like the liver and stomach. “When they eat something big, it's for sure going to be pressing on the lung for the entire time that it goes through the digestive tract,” Capano explains.
So, Capano and his colleagues wanted to see what would happen if they blocked off one part of the lung with a blood pressure cuff. That would restrict the snake’s rib movements in a manner similar to constriction or digestion of a large piece of food.
More importantly, they wanted to see how the snakes would respond when the front part of their right lung was cut off. There are two sections to a boa lung, Capano explains, and the front part, closest to the heart, holds vascular tissue that is essential to converting oxygen into CO2. That’s normally the section of lung that the snakes would use to breathe when unrestricted.
The back part, on the other hand, does not have that kind of tissue. “It’s just a bag,” Capano says, and the chemical conversion process can’t be completed without air passing back through the front of the lung. So, the question remained: what would snakes do when a part of their lung essential to breathing wasn’t able to move?
What they discovered — When the researchers placed pressure on the front part of a snake’s lung, they watched as it simply turned on the back part of its lung and continued to breathe. And when they took the cuff off, it switched back to breathing in the front.
“I took the cuff off and just de-pressurized it, and it literally just stopped breathing there and just started breathing in the front again,” Capano says.
So why was the snake able to breathe in the back without expanding its lungs in the front? Capano says it comes down to two separate processes at play: ventilation and respiration.
The snakes are able to ventilate — expand and contract their lungs — in the back, while respirating, or converting oxygen to CO2, in the front. As long as the air they inhale touches the tissue in the front of the lungs, which it does in order to enter and exit the snakes’ airways, gas exchange can still happen. “But normally they'll just ventilate with the part that they can do gas exchange, by just moving air right in there and back out,” Capano explains.
It makes sense considering the boa’s long lungs and stomach. As food moves down the pipe, the snake can turn on whichever section of the lung is easiest to ventilate, while still passing air in and out of the top of its lung with little to no movement.
The evolution question — The boa’s ability to control its ventilation may have been an important aspect in its development of other traits as well.
Capano says it’s unclear which ability came first — constriction, jaw features, or lung modulation. It’s possible that they all evolved in tandem. Since the traits complement each other, they could have shown up at similar points in the snake’s evolution and strengthened each other as time went on.
“I think that they all kind of evolved in concert,” he says. “They were all probably extant in some form, and then just kind of kept growing over time.”
What’s next — Though the study clearly showed the snake’s ability to change what part of the lung it uses to breathe, researchers still aren’t sure how it pulls off this feat. “We don't know how they're controlling this necessarily from a neuromuscular standpoint,” Capano says.
Snakes also use some body parts for multiple tasks. Their ribs, for example, are used for locomotion as well as breathing, which could come with its own trade-offs. Many species of lizards, which are evolutionary cousins to snakes, cannot breathe and run at the same time because they use the same muscles to complete both tasks.
A similar limitation could exist in snakes as well, though it hasn’t been thoroughly studied. “Looking at whether or not snakes can ventilate while slithering is an interesting idea” for future research, Capano says.