All animals need to breathe to exchange incoming oxygen with outgoing waste gases, like carbon dioxide. Take a look at the wonderful animation below, by illustrator Eleanor Lutz, of breathing in humans.
Humans and other mammals suck air into their bodies by lowering a muscle just below the lungs called the diaphragm. When the diaphragm raises up, air is squeezed back out. Scientists call this exchange tidal flow, because air simply whooshes in and out of the lungs, like sea tides. Breathing like this means that old outgoing air mixes with new incoming air—not the most efficient way to get oxygen into the body.
Not all animals breath the same, though.
Birds, for example, avoid the mixing problem by moving air through their lungs in one direction via a series of 7 to 9 air sacs, connected by loopy tubes. Birds take oxygen into their body tissues when they breathe in and when they breathe out. So, for every one bird breath, humans would need to take two. This makes birds super-efficient breathers. Amazing!
When a bird inhales, it’s air sacs inflate and oxygen-rich air from the outside is sucked into the body while waste air is drawn from the lungs. When a bird exhales, the air sacs deflate and the oxygen-rich air in the rearmost air sacs is squeezed into the lungs while waste air is expelled from the body by the front-most air sacs. It sounds complicated (and it is).
Research published in the past year has shown that reptiles are also capable of this kind of one-way breathing. While this might not come as a huge shock to you or me, lung scientists were very surprised! When told, they reacted by saying things like "absolutely transformational", "truly shocking" and "just ridiculous…in a good way". These scientists had assumed one-way breathing was unique to birds and had evolved relatively recently. Evidence that reptiles may share this peculiarity smashed that assumption and raised questions about the evolution of one-way breathing. It's possible that one-way breathing has been around for hundreds of millions of years!
Previously, scientists assumed that one-way breathing required air sacs, so if an animal didn't possess air sacs, it was presumed that the animal was not a one-way breather. They looked everywhere for air sacs. They looked for them in the direct ancestors of modern birds—the dinosaurs. And, they looked at their more distant relatives, too—crocodiles, lizards and other reptiles. Having found nothing, they concluded that birds' air sacs were completely unique within the animal kingdom.
Scientist, Colleen Farmer, however, didn’t think animals needed air sacs for one-way breathing at all. To prove it, she decided to directly track how air moved through three species of living, breathing reptiles: iguanas, monitor lizards and alligators. After a lot of trial and error, Farmer hit upon a winning combination: Froggy's Swamp Juice theatrical fog and an endoscope. With these tools, Farmer watched as the fog traveled in one direction through each reptile’s body. She proved air sacs are not required for one-way breathing. All that is needed, apparently, is some cleverly angled tubing.
Farmer's findings pose so many questions like: what advantage do reptiles get from breathing like birds?
Birds are warm-blooded and live a high energy, acrobatic lifestyle. It makes sense, then, that they have a breathing system adapted for maximizing their oxygen uptake in order to feed their rapid metabolism. Reptiles, on the other hand, are cold-blooded and have a slow metabolism. Farmer thinks they need to maximize their oxygen uptake in each breath to save energy overall. Many reptiles spend much their lives sitting very still for a long time, waiting for food. They need to use every trick in the book to conserve energy during this time.
Curious about the inner-workings of animals? Don't miss our latest feature exhibition Body Worlds: Animal Inside Out, on now until March 28, 2016.