When coiled around a lizard or throttling a bird, a boa constrictor doesn't actually suffocate its prey. Instead, the snake squeezes the unfortunate victim to death by cutting off the target’s blood circulation. But scientists have often wondered how the snakes themselves keep from suffocating during the process. Somehow boas are able to keep breathing, even while their own lungs are tightly squeezed during constriction and unable to get air normally.
Now a study published today in the Journal of Experimental Biology has uncovered the secrets of boa breathing. In it, researchers show how the adaptable serpents shift to a different style of respiration, using the rear parts of their long lungs and bodies to keep oxygen flowing even while putting a deadly squeeze on their victims. The authors theorize that boas’ incredible breathing adaptations may have evolved far back in their history, and enabled them to later adopt their effective method of subduing and devouring prey.
When not constricting prey, boas breathe by motions of their ribs, expanding and contracting the surrounding muscles in accordion-like fashion to fill their lungs and exhale again. This breathing typically occurs near the front of the snake, around its heart. But the snakes use that same section of body and ribs to squeeze the life out of a bird or small mammal, and when locked down around such prey this part of the snake’s body is unable to move or to provide oxygen as usual.
Scientists had witnessed the snakes’ bodies moving in varied ways during hunting and feeding and wondered what exactly was going on. “Watching animals constrict we were seeing that they were kind of breathing with regions that were different than when they were resting, using those regions further back on the body, so we built out an experiment to kind of tease that apart,” says ecologist John Capano of Brown University, who co-authored the new study.
Capano says scientists had puzzled for years over the snakes’ very long lungs, and the breathing adaptations they enabled, especially since a part of their rear lung is a balloon-like bag without enough blood vessels to be effective in delivering oxygen to the bloodstream. His group suspected that the snakes were able to alternate which parts of the ribcage and lungs they used to breathe, depending on whether they were lying around resting, squeezing the life out of a lizard, or digesting an enormous meal.
To find out how the snakes managed it the group used some constriction methods of their own. The researchers applied blood pressure cuffs to the ribs of boas to prevent them from breathing with certain sections of their ribcage at a given time. “We created a scenario to emulate what happened with constriction, just preventing the rib cage from being able to expand any more, and recreating what we’d seen them do when constricting and eating in a natural setting,” Capano says.
The team then gathered two types of data to get a complete picture of the snakes’ breathing abilities. They used X-rays to visualize how the snakes’ ribs moved during breathing. They also put electrodes on the snakes to record muscles’ electrical activity if and when they were stimulated by nerves.
The group found that the rib muscles in the front of the animal simply stopped working when constricted, only to start firing in the other areas of the snake further back. While a snake was constricting a victim to stop its blood flow, and unable to use the forward parts of their ribs, they did indeed switch to use the hind section of their lungs instead. Using rear ribs to pump the bag-like feature like a bellows, the snake pulled oxygen through the constricted area and that enabled the animal to breathe even while wrapped tightly around its prey.
Michael W. Caldwell, an evolutionary biologist and snake specialist not involved with the study, says breathing can be a real issue for many snakes while they are eating large meals. “In the case of such severe muscle compression, it would seem impossible to avoid not shutting down lung ventilation at the same time,” says Caldwell, of the University of Alberta. On the other hand, because constriction requires a lot of time and effort it’s not surprising that the snakes have evolved a way to keep breathing during the work, Caldwell notes. “The study has uncovered several novel features of lung ventilation that were unknown, but make perfect sense.”
The feisty boa constrictors didn’t necessarily enjoy contributing to the experiment and their opposition actually worked to the team’s advantage. “They would be defensive, and there were huge hissing bouts to try to get me to get away from them,” Capano says. “We were saying, ‘this is so cool!’ because they are so full of air when they hiss their entire body fills up so we were getting the most massive breaths that we could possibly record.”
During the early days of transition to life on land many species evolved chest-based breathing that relies on the motion of the ribs to take in air. Over time, though, some developed other solutions. Mammals evolved a diaphragm to lessen the importance of their ribs in breathing because the ribs are also key for stability when running—it is difficult to run and breathe well at the same time when the ribs are involved in driving respiration. Lizards still grapple with this problem. They use their ribs to breathe and to help to stabilize them while running, and the faster they run the more their breathing is compromised.
Snakes don’t run, but Capano and colleagues suspect that over the long history of snake evolution, boas’ ability to use different sections of their rib cage, to power breathing with different parts of their lungs, likely evolved to help them, and later enabled their ability constrict prey as a means of feeding.
“I really like their hypothesis that modularity in lung ventilation may have been not just an early innovation in snake evolution, but a ‘necessary’ one prior to the evolution of constriction as means of subduing prey,” Caldwell writes in an email.
Capano notes that some unknown aspects of boa breathing need to be untangled. “We have no idea whether or not they can breathe and move at the same time,” he says. If the snakes flexible breathing habits help them to squeeze prey, they might also help them to cover ground. “Their ribs are involved in locomotion,” Capano says, “so their ability to shift where they breathe along their body might possibly help them get around the problem of breathing and moving at the same time.”
And while the research is currently restricted to a single species, it’s possible that boas’ breathing ability might also help non-constricting snakes solve other problems, like breathing with a gut full of a massive meal. Vipers, for example, also have incredibly long lungs, comprising 70 or 80 percent of their body length, and they also eat enormous prey items that fill up their entire bodies and compress their lungs.
It’s possible that these snakes might also shift to breathe with different parts of their bodies, enabling them to digest meals that would fatally choke less adaptable predators who bit off more than they could chew.