The boa constrictor got its name for a reason. To kill its prey, a boa will squirm around it, squeezing hard enough to keep the prey’s blood from flowing, and then, stretching its jaws open, they devour it whole. They are known to eat opossums and rats. Some of their larger relatives, anacondas, can eat capybaras and deer, and there have been some cases of pythons eating humans.
But constricting and taking in prey is no small — or quick — feat. “They do this for 10, 15, to 45 minutes,” said Elizabeth Brainerd, an evolutionary biologist at Brown University. “And that takes quite a bit of energy, so they have to breathe.”
dr. Brainerd and her colleagues tried to understand how boa constrictors breathe under such cramped conditions and found that they are able to move exactly the area of their rib cage that expands to draw air into their lungs. Their work was published Thursday in the Journal of Experimental Biology. Their findings shed light on the anatomy of snakes and how these slippery predators have come to thrive in so many parts of the world.
“Large prey intake really opened up all these new avenues for snakes to evolve that wouldn’t have been possible otherwise,” said John Capano, an evolutionary biologist at Brown University and an author of the study.
If you look over the tree of life of vertebrates, it turns out that breathing in and breathing out air is more complicated than it seems.
“Breathing is one of those things that seems super simple,” said Allison Hsiang, a computational paleobiologist at Stockholm University who was not involved in the study. “But if we look at all the major vertebrate groups, we actually all breathe with completely different systems.”
For example, humans have a diaphragm, while birds use air sacs. But snakes are completely dependent on their rib cage and additionally have a large, long physique that sits behind a relatively minuscule head and requires a lot of food to maintain.
“In fact, snakes are all ribs,” said Dr. Brainerd. To breathe, the animals slowly expand part of their rib cage, creating a pressure change that pulls air in.
As they squeeze, those ribs are compressed. Ingesting prey also expands the ribs to the limit. Exactly how boas could breathe while contracting or swallowing remained a mystery.
“Something had to happen to the evolution of their lung ventilation system for them to become these elongated, small-headed animals that eat large meals,” said Dr. Brainerd.
Based on previous observations in the field, scientists had theorized that when snakes constrict and ingest their prey, they most likely change the specific area of their rib cage that expands. But another option would be for them to use every uncompressed part of their rib cage to suck air into their lungs.
To test these hypotheses, the research group visualized the rib cage of a boa constrictor during constriction using 3D X-ray technology. With the snakes sedated, the team implanted metal markers no larger than half a millimeter into the ribs and vertebrae they wanted to image. They then filmed these regions with X-ray video and, using a blood pressure cuff, limited the movement of the ribs in specific regions, simulating what happens in nature when these snakes constrict their prey.
At rest, boa constrictors breathe with ribs near the top third of their lungs. But when the blood pressure cuff was wrapped around those ribs, a specific set of ribs further down the tube’s body began to expand to draw in air. “The snake just turns off one part of the rib cage and then turns on another part,” said Dr. Capano, who added that once the cuff was removed, the ribs that breathed during rest were immediately reactivated.
The team believes that this ability to modulate rib pull emerged while or before snakes developed the ability to constrict, and perhaps before a snake’s ability to eat large prey. Without this adaptation to breathing, they say, snakes might not have become such a diverse class of reptiles scattered around the world.
