Lizard Has One-Way Breathing; Hints at How Dinosaurs Breathed?

The savannah monitor lizard (Varanus exanthematicus) doesn’t eat like a bird, but it does breathe like one, a new study has discovered.

Air flows only one way through its lungs—other than the American alligator, it’s the only other known reptile found to have this trait.

A savannah monitor lizard. Photograph courtesy Cheryl Ertelt

That’s surprising, because the unidirectional airflow of birds was thought to have evolved due to the high-oxygen demands of flight: Instead of being partially filled with stale, depleted air like human lungs, avian lungs contain air with a much higher average oxygen content and are much more efficient at getting oxygen to the blood.

So this method of breathing was generally considered to be unique to birds and was thought to be a marker of the evolution of warm-bloodedness as dinosaurs evolved into birds.

That changed in 2010, when Colleen Farmer of the University of Utah and colleagues found that alligators breathe one way.

Now, Emma Schachner, a postdoctoral researcher in Farmer’s lab, has added another species to that list.

“Finding this trait in a [cold-blooded] lizard means that warm-bloodedness and unidirectional airflow have no relationship whatsoever,” Schachner said.

But Schachner has another intriguing question: Did dinosaurs also breathe this way? (See “Superlungs Gave Dinosaurs Competitive Edge?“)

As I Live and Breathe

When humans or other mammals breathe, the air flows through the same lung passages during both an inhale and an exhale. As we inhale, the air flows in through our nose and mouth and down through the windpipe, where it splits and flows into a bronchi, or passageway, that leads to each lung.

The airways divide and divide again, each time getting smaller and smaller. Eventually, the air makes it to tiny alveolar sacs, where oxygen travels from the air into the bloodstream.

When we exhale, the oxygen-depleted air travels back through the same passages, up through the windpipe, and out of our nose and mouth. Because air flows both into and out of the lungs using the same passages, our breathing is said to be bidirectional. (Explore an interactive of the human body.)

Birds, on the other hand, have unidirectional airflow when they breathe. Once air hits their bronchi, it either flows into lung-like structures known as parabronchi where oxygen is extracted, or it flows into a set of posterior air sacs in the lower abdomen where the air is temporarily stored.

As the bird exhales, the muscles around the posterior air sacs contract and force some of the air into the parabronchi. This is where oxygen flows from the air into the bloodstream, functioning very similarly to human alveoli. After this is done, the air keeps moving in the same direction and travels into a set of anterior air sacs, which are located in the mid- to upper abdomen.

Illustrations showing the skeleton lungs and bronchial airflow of the savannah monitor lizard.
Illustrations show the skeleton, lungs, and bronchial airflow (clockwise) of the savannah monitor lizard. Illustration courtesy Emma Schachner

“Birds can get oxygen out of air during both inhalation and exhalation, which is not the case for mammals. Mammals can only get oxygen when they inhale. When they’re breathing air out, they’re not getting any oxygen,” said Schachner, whose results were published December 12 in Nature.

Dinosaur Breathing

Schachner started out interested in dinosaur biology, but shifted her focus to living reptiles after seeing Farmer’s work on American alligators, which belong to a wider group called crocodilians that are among dinosaurs’ closest living relatives. (Also see “Dinosaurs Had Supercharged Breathing Like Birds [2007].”)

Now Schachner wants to know whether dinosaurs might have also breathed one way. Since the remnants of any respiratory systems would have long since degraded in dinosaur fossils, scientists can only study their avian descendants.

She picked the savannah monitor lizard because it branched off from the dinosaur/bird lineage in the distant past. Her rationale: If this species breathed similarly to modern birds, then this trait might have evolved even before the dinosaurs, and could provide insights on how dinos lived and breathed.

As of right now, the jury’s still out, she said.

“Monitor lizard lungs look very different from bird lungs, so we can’t tell yet whether unidirectional airflow evolved in a common ancestor of birds, lizards, crocs, and dinosaurs, or whether it evolved independently until we look at other species.” (Take a dinosaur quiz.)

To more conclusively answer this question, Schachner and Farmer intend to study the respiratory systems of several other species of lizard.

Meanwhile, we can breathe easy that there aren’t any living dinosaurs to study.

Follow Carrie Arnold on Twitter and Google+.

Carrie is a freelance science writer living in Virginia. When she's not writing about cool critters, she's spending time outside, drinking coffee, or knitting. You can visit her website at http://www.carriearnold.com
  • Sarah Schachner


  • reality33

    I love National Geographic .but I realized that they tends to make very Racist remarks towards to Japanese or Japan. My wife is Japanese & she is U.S. citizen & we are very disturbed with the remarks made by this publishing company or the Editor in chief to take full responsibilities by the laws.

  • Timothy Steele

    @Reality33 What the hell are you talking about. Maybe you need to open your eyes wider. There’s nothing even about Japan in this article.


    i like how scientists are finding new things about animals…and thats very very cool.


    Really reality33 i feel ya but tim is right nothing in the page has anything to do with japan so lighten up a bit

  • polistra

    “Mammals can only get oxygen when they inhale. When they’re breathing air out, they’re not getting any oxygen,”

    Not valid as a binary “valve-like” statement. Gas is always being exchanged. It depends on the pressure differentials. If our nose or larynx is partly constricted during exhale, as during speech, the air in the lungs is above ambient pressure, which means gas is being pushed INTO the blood system.

    Think of the technique recommended to break out of hyperventilation. Hold the inhale for a moment, then exhale slowly with constriction. Gives you more oxygen.

  • Michael Morgan

    What a horrible graphic. No labels and very confusing to me. It’s not a medical journal, so I bet others will have a hard time understanding what should have been pretty straight forward flow diagram.

  • LeslieDF

    Another poorly written article on this research, this subject.

    There is a difference in breathing (movement of air) and respiration (distribution and exchange of gases).

    All reports on this discovery have the public thinking inhaled air exits through some mysterious mechanism for this lizard, dinosaurs, etc.

  • carl

    highly doubt unidirectional came AFTER bi-directional. earliest lifeforms like sponges or annelids are based on a constant flow of nutrient-rich environment. lungs are similar but wrapped around the environment as it were.

  • Mark Engstrom

    Schachner reasonably notes “Monitor lizard lungs look very different from bird lungs, so we can’t tell yet whether unidirectional airflow evolved in a common ancestor of birds, lizards, crocs, and dinosaurs, or whether it evolved independently until we look at other species” but Arnold in a preceding statement incorrectly interprets this to mean “scientists can only study their [dinosaurs’] descendants, which include birds and reptiles” The reptiles she refers to (which would include monitors) are not descended from dinosaurs, but represent a separate evolutionary branch from alligators + dinosaurs +birds. Thus if the systems are common to all four then one might conclude that it was present in the common ancestor to this large group including both evolutionary branches (+ other taxa). Alternatively, if the system is different in monitors you might suspect that it evolved independently in the two lines. In any event, with the discovery of unidirectional flow in alligators, you could have quickly asked the same question about dinosaurs since birds and therapod dinosaurs are in the same lineage (in fact birds are members of the Dinosauria). Sorry to ramble, but I’ve seen the above comment by Arnold quoted twice now and it is a misinterpretation of the author’s comments.

  • Patrick O’Hanlon

    So much wrong with this article. Work on your scientific reading comprehension Carrie. Seriously… This is terrible. How does someone with the scientific journalism caliber of Yahoo! News get hired at National Geographic exactly?

    Reptiles are NOT descendants of dinosaurs, they share a common ancestor. Both are diapsids but not only were dinosaurs for the most part warm blooded, but if the uni-directional airflow evolved in dinosaurs, according to the article’s logic EVERY species of reptile would have the same lung structure!

    It’s even stated in the quote from an actual scientist IN YOUR ARTICLE:

    “Monitor lizard lungs look very different from bird lungs, so we can’t tell yet whether unidirectional airflow evolved in a common ancestor of birds, lizards, crocs, and dinosaurs, or whether it evolved independently until we look at other species.”

    Please, first FIX THIS glaring error, and then either work on your scientific literacy or move to a different area of work: this is unacceptable.

  • Kat

    1. “So this method of breathing was generally considered to be unique to both warm-blooded animals and birds.” Birds are “warm-blooded”; everything from that point onward immediately lacked credibility.
    2. What Patrick said.

    Sad face, National Geographic.

  • Vilma

    Siempre resultan interesantes los artículos de Nac. Geog. Muchas gracias, para los legos como yo, es muy instructivo. No es una revista científica, pero tiran muchas inquietudes, creo que no se puede pretender hilar más fino en este tema, a mí me pareció novedosa e interesante la posibilidad de conocer la respiración de reptiles y dinosaurios!

  • darwins puke

    Your all freaking out! Leave the poor girl alone!
    Actually I’m gonna have a moan at her too, her first mistake I spotted before all of you was that she said it like evolved, Ha ha!
    Go look for some fossils that simply don’t exist and puke on a peacocks feather while your at it like your stupid pal Darwin. Then go sail off the edge of the earth and don’t wash your hands first.

  • Carrie Arnold

    Post updated and corrected. Thanks for the eagle-eyes everyone!


  • cbrady

    it looks as if the lungs work in the cool oxygen rich part of the system and the skeleton allows the warm exhaust to exit out direct from them

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