How boa constrictors avoid self-suffocation

Now to another piece of animal news. When snakes like boa constrictors wind themselves around their potential lunch, their aim is to squeeze sufficiently hard that blood cannot get back to the prey animal’s heart, so it passes out, making it easier to swallow whole. But the snake’s got a problem: by squeezing that hard, it’s also potentially stopping its own lungs working, meaning it should be at risk of asphyxiation, shouldn’t it? From Brown University, John Capano explains to Chris Smith…

John - We were really interested in understanding whether or not snakes could actually breathe when they're constricting, because they're using their ribcage to kill another ribcage. Similarly, when they eat something, their ribcage expands. We've all seen snakes with really big food items in them and we were wondering, 'can they actually breathe when their body is swollen that wide?'

Chris - It sounds like a pretty obvious thing, but how do snakes breathe?

John - Snakes breathe with their ribcage. We as mammals are actually special in that we also use a diaphragm, but snakes and lizards only use their ribcages. They rotate the ribs, they make their chest bigger which then causes a pressure change, which then causes air to rush into their lungs.

Chris - And so how did you investigate what the snakes were doing then?

John - We originally were just doing some observational work. We were just watching snakes, constrict and feed, and we noticed that they were breathing with a different part of their body when they were constricting or feeding relative to when they were just hanging out in the cage at rest. It started with some electromyography called EMG. It's a technique that lets us measure the electrical pulses, running through a muscle. We found that the muscles that control breathing and pull the ribs, snakes can turn them on and off in different sections of their body with pretty good control. But that really didn't give us the resolution to answer the question we wanted. Then I actually applied to grad school and went to Brown in order to use this technique, X-ray reconstruction of moving morphology. It lets me look inside of an animal and see how the bones are moving inside a living animal. When I did that, I was able to use a kind of an experimental setup to prevent rib motions in one part of the body and then see if the snake would then shift it somewhere else. Analogous to what would happen during constriction or eating something really big. But we did a really controlled experiment where we could replicate it really easily too.

Chris - So what do they do then in order to avoid themselves asphyxiating when they're constricting things? How do they do it?

John - What we found is that snakes have pretty long lungs. In boa constrictors, their lungs are about 30% of their body length. In other snake species, they can go up to like 70 or 80% of their body length. But in these boas they have the front part of the lung where gas exchange happens, and then the back part, which is just kind of a bag. They normally will breathe in the front part. But then when we put the cuff on the front part and we compressed it down, they stopped using those ribs entirely. They just started breathing with the back part, which is really amazing because they're actually ventilating with the back region. They just switch and start breathing further back down the tube and drawing air through that part of the lung in the front, even if that part of the body is doing something else.

Chris - Is there not a sort of 'a chicken or egg' situation here then? Because in order to do this sort of behaviour that had to evolve for them to be able to do that in the first place, which then meant they could feed this way. How do you think this behaviour appeared in the first place?

约翰,这才是真正的难题study. It's really hard to tell which came first, the chicken or the egg in this scenario. Because you're right. Constriction and large spray ingestion require you to be able to breathe while you're doing them, so it would be really difficult for early snakes to exaggerate these behaviours to eat prey that's 100% their own body size. It's like me eating a cheeseburger that weighs 180 pounds in one bite. That would be really difficult if you couldn't already breathe while doing it. We think that this modular lung ventilation mechanism either preceded constriction in large prey ingestion or evolved in concert with it. Then the feedback of being able to breathe a little bit better, allows you to maybe constrict something a little bit bigger and then eat something a little bit bigger and then the feedback of having that ability would then allow you to exaggerate these traits even further. Considering that snakes are literally just a tube of ribs and that they really have this fine rib control, it may also be possible that this rib control may be one of the earlier traits of snakes. This ability to move ribs all along your body in order to push into the environment in different ways; That may have been turned into a ventilation mechanism, that may have come from a ventilation mechanism. But just because of the way snakes work, I think that this rib control thing was probably a really early trait within snake evolution. It's hard to tell which one came before the other ones, but maybe some future work will figure that out for us.