• Tag Archives crocodilians
  • New study shows that gators are one-way breathers too.

    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.


  • Mechanics of bipedalism suggest dinosaurs had to be warm-blooded. Or: Why the aerobic capacity model needs to be retired.

    The old "cold blooded or warm blooded" argument once again rears its ugly head.

    [Editor’s note: A response from the authors can be found here. It answers many of the questions I had about the paper, though I feel the biggest question remains open for debate. I appreciate the authors taking their time to answer my questions, and PLoS ONE for allowing this type of open communication.]

    This post has taken an inordinate amount of time to write up. Mostly because it required finding enough free time to sit down and just type it out. So I apologize ahead of time for bringing up what is obviously old news, but I felt this paper was an important one to talk about, as it relied on a old, erroneous, but very pervasive, popular and rarely questioned hypothesis for how automatic endothermy (mammal and bird-style “warm-bloodedness”) evolved.

    Back in November, a paper was published in the online journal: PLoS ONE. That paper was:

    Pontzer, H., Allen, V. & Hutchinson, J.R. 2009. Biomechanics of Running Indicates Endothermy in Bipedal Dinosaurs. PLoS ONE.Vol 4(11): e7783.

    Using muscle force data for the hindlimbs of theropods, and applying it to a model based on Pontzer (2005, 2007), the authors were able to ascertain the approximate aerobic requirements needed for large bipedal theropods to move around. Their conclusion was that all but the smallest taxa had to have been automatic endotherms (i.e. warm-blooded).

    Time to stop the ride and take a closer look at what is going on here.

    In 2004, John Hutchinson – of the Royal Veterinary College, London UK – performed a mathematical study of bipedal running in extant taxa. He used inverse dynamics methods to estimate the amount of muscle that would be required for an animal to run bipedally. He then tested his models on extant animals (Basiliscus, Iguana, Alligator, Homo, Macropus, Eudromia, Gallus, Dromaius, Meleagris, and Struthio). The predictive capacity of his model proved to be remarkably substantial and stable (Hutchinson 2004a). A follow up paper in the same issue (Hutchinson 2004b) used this model to predict bipedal running ability in extinct taxa (Compsognathus, Coelophysis, Velociraptor, Dilophosaurus, Allosaurus, Tyrannosaurus and Dinornis). Results from this study echoed previous studies on the running ability of Tyrannosaurus rex (Hutchinson & Garcia 2002), as well as provided data on the speed and agility of other theropod taxa.

    The difference between effective limb length and total limb length in the leg of Tyrannosaurus rex

    Meanwhile in 2005, Herman Pontzer – of Washington University in St. Louis, Missouri – did a series of experiments to determine what was ultimately responsible for the cost of transport in animals. To put it another way: Pontzer was searching for the most expensive thing animals have to pay for in order to move around. One might intuitively assume that mass is the ultimate cost of transport. The bigger one gets, the more energy it requires to move a given unit of mass, a certain distance. However experiments on animals found the opposite to be the case. It actually turns out that being bigger makes one “cheaper” to move. So then what is going on here?

    Pontzer tested a variety of options for what could be happening; from extra mass, to longer strides. In the end Pontzer found that the effective limb length of animals, was ultimately the limiting factor in their locomotion. Effective limb length differs from the entirety of the limb. Humans are unique in that our graviportal stance has us using almost our entire hindlimbs. Most animals, however, use a more crouched posture that shrinks the overall excursion distance of the hindlimb (or the forelimb). By taking this into account Pontzer was able to find the one trait that seemed to track the best with cost of transport in animals over a wide taxonomic range (essentially: arthropods – birds).

    This latest study combines these two technique in order to ascertain the minimum (or approx minimum) oxygen requirements bipedal dinosaurs would need in order to walk, or run.

    As with the previous papers, the biomechanical modeling and mathematics are elegant and robust. However, this paper is not without its flaws. For instance in the paper the authors mention:

    We focused on bipedal species, because issues of weight distribution between fore and hindlimbs make biomechanical analysis of extinct quadrupeds more difficult and speculative.

    Yet this did not stop the authors from applying their work on bipeds, to predicting the maximum oxygen consumption of quadrupedal iguanas and alligators. No justification is ever really given for why the authors chose to do this. Making things even more confusing, just a few sentences later, it is mentioned (ref #s removed to avoid confusion):

    Additionally, predicting total muscle volumes solely from hindlimb data for the extant quadrupeds simply assumes that the fore and hindlimbs are acting with similar mechanical advantage, activating similar volumes of muscle to produce one Newton of GRF. This assumption is supported by force-plate studies in other quadrupeds (dogs and quadrupedal chimpanzees)

    The force plate work cited is for quadrupedal mammals. However, mammals are not reptiles. As Nicholas Hotton III once mentioned (1994), what works for mammals, does not necessarily work for reptiles. This is especially so for locomotion.

    In many reptiles (including the taxa used in this study) the fore and hindlimbs are subequal in length; with the hindlimbs being noticeably longer and larger. Most of the propulsive power in these reptiles comes from the hindlimbs (which have the advantage of having a large tail with which to lay their powerful leg retractor on). The result is that – unlike mammals – many reptiles are “rear wheel drive.”

    The last problem is by far the largest, and ultimately proves fatal to the overall conclusions of the paper. The authors operated under the assumptions of the aerobic capacity model for the evolution of automatic endothermy.

    It is here that we come to the crux of the problem, and the main subject of this post.

    Continue reading  Post ID 548


  • The return of fruit eating crocodylians.

    Back in 2002, a short paper came out that commented on the observation that captive caimans would eat fruit left in their cage. When I initially read the paper, I found it interesting. In the end, though, I assumed this to just be a fairly anomalous incident.

    Now Darren Naish of Tet Zoo has followed up on this story with further evidence of frugivory in crocodylians.

    As one can see, this observation has been filmed at least once.

    So does this mean that crocodylians are not as completely carnivorous as once thought? It’s hard to say. All observations made so far have been from alligatorids (alligators and caimans). This might be an apomorphic trait to this group. Only more observations will say for sure.

    Another option that Darren pointed out, is that this was a learned trait of these captive animals. In each case, observed animals were found to be sharing their enclosures with herbivorous animals (usually tortoises). This type of operant learning is rather rare, and would be amazing if found to be true.

    However, as evidenced by the comments of St. Augustine Alligator Farm park director, John Brueggen, fruit eating has been observed in wild animals too; so this is not simply a case of bored captives.

    Whatever the case, these observations do illustrate just how adaptaptable crocodylians are as a group.

    ~Jura