New study shows that gators are one-way breathers too.

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I would be remiss not to talk about this amazing discovery published last week in Science.

Farmer,C.G. & Sanders,K. 2010. Unidirectional Airflow in the Lungs of Alligators. Science. vol.327:338-340

The anatomical similarities of alligators and birds has been known for quite some time (at least 100 years), and this anatomical similarity extends down into the lungs. Though alligators lack the pneumatic carvings of the post-cranial skeleton (air sacs) that are seen in birds, saurischian dinosaurs and pterosaurs; their lungs and bronchi do share the same structural features.

Birds have a unique lung design that allows air to pass through it in a single direction. Unlike mammals, there is no “dead end” to the avian lung. This provides the benefit of a constant supply of highly oxygenated air to the lung tissue; which allows for more efficient gas exchange. Up until last week, this lung design was thought to be a hallmark of birds, and possibly saurischian dinosaurs, and pterosaurs.

Well it turns out that this unique avian synapomorphy is a heck of a lot older than we thought.

Dr. Colleen Farmer, and Kent Sanders M.D. of the University of Utah, considered the uncanny anatomical similarities of the avian and crocodylian lung, and wondered if these similarities extended to the physiology too. In other words: If it looks like a unidirectional lung, does it also function like one?

Farmer & Sanders set to work by removing the lungs of four dead alligators donated to her lab. They pumped air through them, and monitoring the direction in which it traveled (using flowmeters). They then surgically inserted flowmeters into anesthetized alligators, and measured the airflow direction in living animals. Lastly, to drive the point across completely, they filled up an excised lung with fluid that contained fluorescent beads, and proceeded to pump the water in and out. This last test was recorded, and three movies of it, were made available to the public. They can be viewed here. Three was probably overkill though, as once you’ve seen fluorescent beads move one way in a gator lung, you’ve seen them all. : )

The results showed conclusively that alligator lungs pump air through them in one direction only. The repercussions of this find are actually pretty enormous. For starters, the similarity in anatomy and physiology of avian and crocodylian lungs, suggests that they are homologous. This would mean that both groups inherited these lungs from a common ancestor. This means that it was highly likely that all dinosaurs, pterosaurs, rauisuchians, aetosaurs, phytosaurs and the myriad of other archosaurs that graced this planet some 200 million years ago, housed this particular flow-through style lung.

It also helps put to rest arguments about air sac functions. It has long been argued that the presence of a unidirectional lung, necessitates the presence of air sacs to “pump the air in.” (air sacs offer zero, or next to zero gas exchange potential, so there is no actual breathing going on in them). A lack of air sacs in ornithischian dinosaurs, has been used to suggest that their pulmonary physiology was more like mammals and lizards, than it was like birds (Ruben et al 1999). Data from previous research (O’Connor & Claessens 2005) has cautioned that the presence of air sacs does not guarantee the existence of a flow through system. These latest data now show us that a flow-through system can, and likely did, evolve without the “need” for an air sac pump.

CT scan of alligator, with 3D reconstruction of lungs. For more details on what the colours mean, click the picture.

Exactly how all of this works, is still not understood. The “hepatic piston” diaphragmatic pump of crocodylians is well known, and is likely the ultimate driver of respiration in these animals, but the nuts & bolts of how all this unidirectional flow takes place (the fluid dynamics of the lung) remains a mystery. One question that would be worthy of a follow up study (which the author’s have hinted at doing) is whether, or not a cross-current, or counter-current system (where deoxygenated blood flows perpendicular, or opposite the direction of highly oxygenated air) is present in crocodylians too.  A cross-current system is found in birds. Is that unique to them, or was this also a phylogenetic “hand-me-down?” Hopefully now, with this new discovery, future research will be done on the crocodylian lung, to further understand how it actually works.

Ultimately that is the biggest piece of news to come out of this paper. For well over 100 years, the crocodylian lung was just assumed to be a “dead-end” space that worked in a manner similar to that of mammals. It wasn’t until someone actually thought “what do we really know about this structure” did we find something quite the opposite taking place. This is hardly the first time that this has happened either (for instance). As I have mentioned (ranted/harped on) before, reptiles tend to get the short end of the stick when it comes to a lot of biological and paleontological studies (especially if they involve comparison between broad animal groups [classes]). I’m always amazed (though rarely surprised) when a study that actually looks into commonly held assumptions about these critters, finds said assumptions to be quite off the mark. Here’s hoping that we continue to see future studies like this, go on.

In the end, all of this brings us closer to the truth about how life really works; which is why we do all of this stuff in the first place.

~Jura

References


Farmer,C.G. & Sanders,K. 2010. Unidirectional Airflow in the Lungs of Alligators. Science. vol.327:338-340

O’Connor, P.M.& Claessens, A.M. 2005. Basic Avian Pulmonary Design and flow-Through Ventilation in Non-Avian Theropod Dinosaurs. Nature. Vol. 436:253-256.

Ruben, J.A., Dal Sasso, C., Geist, N.R., Hillenius, W.J., Jones, T.D. 1999. Pulmonary Function and Metabolic Physiology of Theropod Dinosaurs. Science. Vol.283(5401):514-516.

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5 Responses to New study shows that gators are one-way breathers too.

  1. Avatar Michael O. Erickson
    Michael O. Erickson says:

    My thoughts: Farmer and Sanders’ work destroys the link between unidirectional air-flow and endothermy, and calls for people to quit throwing the presense of flow-through lungs and air-sacs in non-avian saurischians up in the faces of those such as yourself and myself*, who consider dinosaurs to have been ectothermic.**

    Another excellent post, Jura!

    *I was once an endothermy advocate. No so anymore.

    **Or, who quite simply fail to be swayed by the purported evidence “clearly” favoring endothermy and prefer to view the evidence as equivocal.

    • Thanks. I’ve always felt that the push to make dinosaurs “warm-blooded” was specifically to allow us to imagine them running around, taking care of their children, or getting into epic battles with each other. All traits that have generally been considered a hallmark of “warm-blooded” animals only. What I hope, is that data from Farmer, Sanders, Somma and others will help to remove this old stereotype of what “cold-blooded” animals can and cannot do. Who knows, there might even come a day when calling a dinosaur a “good reptile” is no longer considered an insult.

  2. Avatar
    Louis Bérubé says:

    Speaking of endothermy…

    If it took us this long to figure out gator breathing direction when living specimens are all around us, what are the odds of actually coming to any solid conclusion about something as complex as dinosaur metabolism?

    Good writeup, Jura.

  3. As a comment to the comments (the post is very nice):
    Actually, on the contrary, what this paper does is further support the possibility that unidirectional lungs DO support endothermy, and in fact like many features of crocodylian anatomy, suggests that modern crocs simply “downgraded” their physiology to match their lifestyle. Aside from that, there are so many lines of evidence that support dinosaur endothermy, that the unidirectional lung is by no means necessary (e.g. feathers).

  4. Avatar Michael O. Erickson
    Michael O. Erickson says:

    Well, for one, not everyone is convinced that crocodilians are only secondarily ectothermic. The idea is still highly controversial, though it may not seem so as the proponents of the hypothesis have been more vocal than the detractors.

    Also, despite what many have claimed, feathers do not necessarily support full-blown endothermy, as Jura has touched upon here:

    http://reptilis.net/2009/04/13/a-critical-evalution-of-tianyulong-confiusci-part-3-plucking-at-the-idea-of-feathered-dinosaurs/