Model organisms are a staple of biology. They are taxa that are used to answer larger questions about that group as a whole, or some general biological problem. Model organisms are chosen for their ease of handling, cheap acquisition, generally “generic” structures, or all of the above. Every major class has a model organism to represent it. Just among vertebrates we have:
Mammals with mice (Mus musculus), dogs (Canis familiaris [or Canis lupus familiaris if you lean that way]), cats (Felis catus [or Felis sylvestris catus for the same reason as dogs]), guinea pigs (Cavia porcellus) and rhesus monkeys (Macaca mulatta).
Birds with chickens (Gallus gallus), pigeons (Columba livia), and zebrafinch (Taeniopygia guttata).
Ray finned fish with zebrafish (Danio rerio), swordtails (Xiphophorous) and cichlids (Cichlidae).
Amphibians with the African clawed frog (Xenopus laevis), and axolotol (Ambystoma mexicanum).
Reptiles with anoles (Anolis), fence lizards (Sceloporous), painted turtles (Chrysemys picta) and finally, the American Alligator (Alligator mississippiensis).
Alligators are relatively new to the model organism realm, but they have proven to be extremely informative. They seem to the be most even tempered of extant crocodylians, making them “more safe” for researchers to work with. Hatchlings start off as miniscule 68 gram (0.15 lbs) animals that later can grow to 363 kg (800 lbs) adults, passing through an enormous size range throughout ontogeny. This growth rate is very food dependent, making it possible to raise alligators almost as bonsai trees. Also, with their unique position on the organismal family tree, alligators are one of the closest living relatives to dinosaurs. Along with birds, they have the potential to help constrain our assumptions about dinosaurs; thus making them very popular subjects for paleontological research as well.
Today, alligators get to make one more stamp on human knowledge with the release of the 3D alligator project from the Holliday and Witmer labs.
Researchers from both labs went through the painstaking process of digitizing the skulls of an adult and a hatchling American alligator, and then digitally separated each bone. The result is a 3D model that can have each bone turned on and off at will. The neat thing is that both labs have made these data freely available for anyone to look at, and download as 3D pdfs, wirefusion models, and multiple movies.
So if one every wanted to know just how many bones make up a crocodylian skull, or how each bone aligns to each other, I highly recommend downloading the 3D pdfs of the adult and hatchling. Not only will one learn all the different bones that compose the skull, but by comparing hatchling to adult, one can see just how radically these bones change throughout ontogeny.
It’s neat, free, informative and reptilian. What more can one ask for. ðŸ™‚
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.
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.
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.
Two new papers have recently hit the journal circuit. Both of them involve using living crocodylians to gain a better understanding of paleo-life.
The first one comes from Denver Museum of Natural History paleontologist, Dr. Kenneth Carpenter:
Carpenter, K. 2009. Role of Lateral Body Bending in Crocodylian Track Making. Ichnos. Vol.16:202-207. doi:10.1080/10420940802686137.
The study used an adult Caiman sclerops (first use of a large adult reptile for a locomotion study; at least as far as I know) placed in a small room with two 30cm walls placed on either side of it. This restricted any lateral movement, and “funneled” the animal out the singular opening. At this opening, a camera was placed. It would photograph the animal as it left the room. The room itself, had a smoothed mud covering. This muddy floor would record the tracks of the C.sclerops as it walked by.? Several runs were done, and photographs were taken for each run.
This is the first study I have seen that gave a front view shot of an adult crocodylian as it walked along. As Carpenter mentioned in the paper:
This front view is in contrast to most photographic studies which only capture pro?le and top views….
Carpenter also mentioned the potential of there being an ontogenetic change in limb stance as animals move from hatchling to adult. This is something that I have hinted at previously Hatchling crocodylians seem to have weaker femoral adductors than adults. This is understandable given the greater weight that adult femora need to bear. This can result in a skewed view of crocodylian erect stance; with most authors tending to underestimate the degree of “parasagittality.”
That said, I was surprised to read that Carpenter had found the adult Caiman sclerops to have a hip adduction angle of approximately 65? from the horizontal. Judging from figure1B, the hindlimb appears to be much closer to the midline than the forelimb. Fig1D seems even closer to, if not 90?. It is important to point out that much of the hindlimb is blocked by the body in this shot, as the animal is fully laterally extended. A concurrent shot from behind would have been very useful here; as would an x-ray series of shots throughout the walk phase (for instance: see this long video of a Crocodylus acutus walk cycle. Pay special attention to the position of the femur).
Alas, that is not what the paper is about.
The paper is about how lateral movements during locomotion, have substantial effect on trackways. Dr. Carpenter points out how, despite the semi-erect stance of the forelimbs, the track evidence would suggest an animal with a much narrower (parasagittal?) stance. This has bearing on how prehistoric reptiles, in particular: quadrupedal dinosaurs, may have stood.
One might rightfully ask if we should expect dinosaurs to have had any lateral movement to their walking cycle at all. Carpenter points out that lateral body bending, though not quite as exaggerated as that of crocs, is present in most tetrapods. Birds seem to be the sole exception, with their extremely stiff thorax. However birds are also obligate bipeds, and the avian thorax is much shorter and stiffer than that of dinosaurs.
So it would seem to be a likely bet that quadrupedal dinosaurs likely exhibited some degree of lateral body bending.
Carpenter’s work rightfully asks us to caution reconstructions of stance based largely off of trackway evidence. A fine case study that the paper brings up, is ceratopians. This group, more than any other, has received considerable attention for how the forelimbs were oriented. Early work on ceratopians, favoured a hefty sprawl to the forelimbs (e.g.? Gilmore 1905, or Lull 1933). This was critically evaluated during the heyday of the dinosaur renaissance. Authors such as Bakker (1986), Paul and Christiansen (2000), instead favoured a fully erect stance. A large portion of the data supporting this assertion, was trackway based. The results of this study call into question that view. However this was not the first paper to have done so. Thompson and Holmes (2007) also questioned the “erect ceratopid” view, using a half scale model of a Chasmosaurus irvinensis forelimb. Their results come closer to the results from this paper. Though Thompson and Holmes felt that there was no real modern analogue to ceratopian forelimb mechanics.
In the end, Dr. Carpenter reminds future researchers of the importance in incorporating the entire animal when analyzing trackways.
The second paper comes from the Journal of Experimental Biology.
Owerkowicz, T., elsey, R.M. and Hicks, J.W. 2009. Atmopsheric Oxygen Level Affects Growth Trajectory, Cardiopulmonary Allometery and Metabolic Rate in the American Alligator (Alligator mississippiensis). J.Exp.Biol. Vol.212:1237-1247. doi:10.1242jeb.023945.
The authors embarked on a study of how previous paleo-atmospheric oxygen levels might have affected the lives of animals that would have been alive through these times. According to Owerkowicz et al, crocodylians were chosen because:
Given their phylogenetic position and highly conserved morphology throughout their evolutionary history, crocodilians are often thought to retain many characteristics of basal archosaurs.
I do take some issue with this, as prior reviews on crocodylomorph diversity (Naish 2001) coupled with many new discoveries ( Buckley et al 2000,? Clark et al 2004, Nobre & Carvalho 2006)? continually cast doubt on the old view that crocodylians have survived “unchanged” for some 200 million years. Nevertheless, the results of the study are both interesting, and relevant to reconstructions of how paleo-life would have adapted to these wildly different paleo-atmospheres.
Owerkowicz et al raised groups of hatchling American alligators (Alligator mississippiensis) under three different atmospheric conditions. A hypoxic (12% O2) condition reminiscent of paleo-atmospheric models for the late Triassic/Early Jurassic periods. Current atmospheric conditions (21% O2), and a hyperoxic (30% O2) condition reminiscent of paleo-atmospheric models for the Carboniferous and Permian periods.
The results were interesting, though not too surprising. As expected, hypoxic alligator hatchlings were smaller than their normal and hyperoxic counterparts. However, the degree of growth stunting is pretty surprising. Hypoxic hatchlings were about 12% shorter and 17% smaller than normal hatchlings.
Surprisingly, hatching time did not change under any conditions. This suggests a degree of “hard wired” embryological development inside the egg. In the case of the hypoxic hatchlings, they came out “almost done.” While all three groups had remnants of a yolk sac upon hatching, the hypoxic hatchlings actually had the yolk sac still protruding (normal and hyperoxic hatchlings just showed distended bellies). In some cases, the yolk sac was larger around than the hind legs, thus making movement clumsy and cumbersome.
Other interesting results from this study, included notable changes to the cardiopulmonary system. Hypoxic hatchling lungs were actually smaller than the lungs of normal hatchlings; which appears counterintuitive. The heart, meanwhile, showed distinct hypertrophy in hypoxic animals. The authors believe that lack of lung growth in hatchlings may have been due to the fact that lung function does not start until after hatchlings have hatched.? The heart, on the other hand, is hard at work circulating blood just as soon as it is formed; so it would have experienced the challenges of hypoxia at a very early stage.? Bolstering this hypothesis from the authors was the fact that three months after hatching, hypoxic alligators showed a distinct increase in lung growth rate (the lungs appeared to be “catching up” to the heart).? Hypoxic alligators showed shrunk livers as well. No real explanation for this was given, but it was mentioned that reduced liver mass seems to be a common trait in animals raised in hypoxic conditions. It appears to have some bearing on overall metabolic rate.
Hyperoxic hatchlings exhibited “typical” organ growth rates.? Where hyperoxic animals excelled was in breathing and metabolic rate.
Breathing rates were smaller in this group, while metabolism and growth rate were all larger. The explanation by the authors was that these hyperoxic animals were receiving such high amounts of oxygen in each breath, that they were actually hitting saturation at much shallower breaths; hence the shallow breathing. The higher metabolic rate is believed? due to a lack of right-left shunting in the crocodylian heart. This shunting is usually caused by low oxygen levels (like that experienced in diving), and tends to result in metabolic depression to conserve available oxygen stores.? Since these alligators lungs were constantly saturated with oxygen, right-left shunting never occurred, resulting in an elevated metabolism.
Incidentally, Owerkowicz et el give mention of a cardiac shunt known in embryological birds (via the ductus arteriosis). Though only analogous, one can’t help but wonder what this might have meant for all those dinosaurs that lie between these two groups.
Interestingly, hypoxic alligator hatchlings also showed a higher standard metabolic rate. Though these animals would voluntarily eat less than their normal and hyperoxic counterparts, their metabolism was more like hyperoxic hatchlings than they were normal hatchlings.? Owerkowicz et al believe the reason for the increased metabolism was due to the higher cost of breathing in these animals. Despite taking “normal” breaths, hypoxic hatchlings were taking in a larger tidal volume than their normal and hyperoxic siblings. The heart was also working harder to deliver enough oxygen to tissues.
Finally the authors give mention of growth rates in hyperoxic animals. Basically, it is faster. The authors mention that this might be caused by the persistently elevated metabolic rate, or perhaps from channeling saved energy from breathing (which is one of the main energetic costs in reptiles) into biomass.? It could be a mix of both, but I’m more inclined to think that it comes more from channeling energy reserves into other parts of the body. A high metabolism means nothing, if there is not enough free energy to go around. Just look at the hypoxic gators from this study. Despite their high metabolism, they grew slower than their peers.
The results of this study showed how modern animals can acclimate to different atmospheric conditions. They don’t show how animals would adapt and evolve in these conditions, but they do hint at the general directions, and help give us a clearer picture of what life was like millions of years ago.
Bakker, R. 1986. The Dinosaur Heresies. William Morrow. New York. ISBN: 0821756087, 978-0821756089 pps: 209-212.Buckley, G.A., Brochus, C.A., Krause, D.W., Pol.D. 2000. A Pug-Nosed Crocodyliform from the late Cretaceous of Madagascar. Nature. vol.405:941-944.
Clark.J.M., Xu, X., Forster, C.A., Wang, Y. 2004. A Middle Jurassic ‘Sphenosuchian’ from china and the Origin fo the Crocodylian Skull. Nature. Vol.430:1021-1024.
Gilmore, C.W. 1905. The Mounted Skeleton of Triceratops porosus.? Proceedings United States National Museum. Vol.29:433-435.
Lull, R.S. 1933. A Revision of the Ceratopsia, or Horned Dinosaurs. Memoirs of the Peabody Museum of Natural History. Vol.3:1-175.
Naish, D. 2001. Fossils Explained 34: Crocodilians. Geology Today. Vol.17(2):71-77.
Nobre, P.N. and Carvalho, I.S. 2006. Adamantinasuchus navae: A New Gondwanan Crocodylomorpha (Mesoeucrocodylia) from the Late cretaceous of Brazil. Gondwana Research. Vol.10:370-378.
Paul, G.S., and Christiansen, P. 2000. Forelimb Posture in Neoceratopsian Dinosaurs: Implications for Gait and Locomotion. Paleobiology, 26(3):450-465.
We used electromyography on juvenile American alligators to test the hypothesis that the following muscles, which are known to play a role in respiration, are recruited for aquatic locomotion: M. diaphragmaticus, M. ischiopubis, M. rectus abdominis, M. intercostalis internus, and the M. transversus abdominis. We found no activity with locomotion in the transversus. The diaphragmaticus, ischiopubis, rectus abdominis and internal intercostals were active when the animals executed a head-down dive from a horizontal posture. Weights attached to the base of the tail resulted in greater electrical activity of diaphragmaticus, ischiopubis and rectus muscles than when weights were attached to the head, supporting a role of this musculature in locomotion. The diaphragmaticus and rectus abdominis were active unilaterally with rolling maneuvers. Although the function of these muscles in locomotion has previously been unrecognized, these data raise the possibility that the locomotor function arose when Crocodylomorpha assumed a semi-aquatic existence and that the musculoskeletal complex was secondarily recruited to supplement ventilation.
Scientists at the University of Utah have discovered the unique internal subtleties that allow crocodylians to sink, rise, pitch and roll; all without disturbing the water (much). It turns out that the main muscles used for breathing, are also used to actually shift the lungs within the body!
That’s just crazy awesome. Uriona & Farmer’s work raises the question of how prevalent this ability is in other semi-aquatic animals (e.g. seals, terrapins, manatees). By shifting the lungs further back in the body, crocodylians are able to change their local density. This allows the front, or back of the animal to rise and sink separately from the rest of the body. So too does moving the lungs from side to side allow for rolling in the water. All of this can occur without the need to move any external body parts. This means no extra turbulence gets created in the water, thus allowing crocodylians to better sneak up on their fishy, or fleshy prey.
Baby crocodiles exhibiting their unique pulmonary powers.
If anything, it sure speaks to why crocodyliformes have held dominion over the semi-aquatic niche for over 200 million years. Uriona and Farmer do suggest that the ability of these respiratory muscles to do this might not be an exaptation. Rather, this might have been the initial impetus behind the evolution of these muscles. Only later would they have been exapted to help with breathing on land. Though the authours provide some good parsimonious reasons for why this may be (basically it would take less evolutionary steps to accomplish than the other way around), it doesn’t really jive with the fossil evidence. Part of the reason why the crocodylian diaphragm works, is because the pubis (the forepart of the hip bone in most animals, and the part that juts out so prominently in theropod dinosaurs), is mobile. This mobility occurred early on in crocodyliforme evolution, with the crocodylomorph Protosuchus having a pubis that was almost mobile. The problem arises when one looks at this early crocodylomorph. Protosuchus was obviously terrestrial. If Protosuchus was evolving a mobile pubis already, then it was doubtful that it was being used to allow lung shifting in the body (an ability that is helpful when underwater, but pretty pointless on land). Furthermore, Crocodylia proper is the umpteenth time that crocodyliformes have returned to a semi-aquatic existence. It is doubtful that all the numerous land outings that occurred during crocodyliforme evolution, would have retained the ability to move the lungs to and fro. It seems far more likely that this was an ability that evolved in Crocodylia, or somewhere close by on the evolutionary tree, in some taxa that was still semi-aquatic.
Protosuchus richardsoni. An example of an early crocodylomorph.
Of course it is also possible that crocodyliforme phylogeny is just all f-ed up. With the amount of convergence rampant in that lot, this remains a distinct possibility.
Foraging mode has molded the evolution of many aspects of lizard biology. From a basic sit-and-wait sprinting feeding strategy, several lizard groups have evolved a wide foraging strategy, slowly moving through the environment using their highly developed chemosensory systems to locate prey. We studied locomotor performance, whole-body mechanics and gaits in a phylogenetic array of lizards that use sit-and-wait and wide-foraging strategies to contrast the functional differences associated with the need for speed vs slow continuous movement during foraging. Using multivariate and phylogenetic comparative analyses we tested for patterns of covariation in gaits and locomotor mechanics in relation to foraging mode. Sit-and-wait species used only fast speeds and trotting gaits coupled with running (bouncing) mechanics. Different wide-foraging species independently evolved slower locomotion with walking (vaulting) mechanics coupled with several different walking gaits, some of which have evolved several times. Most wide foragers retain the running mechanics with trotting gaits observed in sit-and-wait lizards, but some wide foragers have evolved very slow (high duty factor) running mechanics. In addition, three evolutionary reversals back to sit-and-wait foraging are coupled with the loss of walking mechanics. These findings provide strong evidence that foraging mode drives the evolution of biomechanics and gaits in lizards and that there are several ways to evolve slower locomotion. In addition, the different gaits used to walk slowly appear to match the ecological and behavioral challenges of the species that use them. Trotting appears to be a functionally stable strategy in lizards not necessarily related to whole-body mechanics or speed.
I haven’t had a chance to read much more than what was written already. I do take a bit of offense to the authours referring to scleroglossan foraging technique as “slow,” but what are you going to do?
I do find it interesting that lizards seem to have lost the ability to “walk” numerous times. That almost seems bizarre. The study points out that ecology produces heavy pressures on lizards in terms of their locomotion type. This is extremely pertinent given how often one hears the old (and wrong!) adage about “reptiles” being incapable of intense aerobic activity.
According to the above study (among others), it all depends on the animals being tested.
There we go. Two really cool papers on reptiles, being released in one day.
Yesterday a new study was released in the journal of Physiological and Biochemical Zoology. Researchers from the University of Utah, studied the effects of the well documented right-to-left shunt in crocodylians.Okay, let’s get the exposition out of the way first.
Mammals and birds are both characterized by a 4 chambered heart. This heart allows the complete separation of oxygenated and deoxygenated blood streams. Less publicized, but equally as important, this separation also allows for a pressure differential to exist between the two ventricular chambers. That way the right – pulmonary side – of the heart can pump deoxygenated blood at low pressure to the delicate walls of the alveoli in the lungs, while the left – systemic side – of the heart, can pump oxygenated blood at much higher pressure (~7 times higher) to the entire body.
Reptiles and amphibians differ from mammals and birds, in that they have a heart divided into 3 chambers (two atria, one ventricle). This allows for mixing of oxygenated and deoxygenated blood, which reduces aerobic efficiency.
Please note the qualifier: aerobic.
Now, as is often the case with herps, this is a rather broad generalization. The hearts of all reptiles, show various degrees of ventricular separation. Also, for all extant reptiles, there are physiological/haemodynamic mechanisms in place that reduce the amount of blood mixing. Meanwhile, some lizards (e.g. varanids), and snakes (e.g. pythons) have such a large muscular septum near the middle of their ventricle, that it actually completely separates the ventricle during the contractile phase (ventricular systole). Thus making varanids and various snakes, functionally four chambered. These reptiles are capable of producing pressures on their systemic side, that are 7 times higher than the pressures in their pulmonary side. In other words, their functional four chambered hearts allow for pressure differentials that are on par with mammals.
Then there are the crocodylians. Crocs have the most complicated heart of any vertebrate. They are the only reptiles that have evolved a complete seperation of their ventricles. They are anatomically four chambered. Yet, they also retain the ability to mix their oxygenated and deoxygenated blood supplies. This is accomplished through a small connection between the right and left aortic arches (which come out of each respective ventricle). This connection is referred to as the foramen of Panizza. Making things more interesting still, croc hearts also feature a cog toothed valve that can completely block the flow of blood to the lungs, thus turning their hearts into a double pump systemic circuit.
So now we know the how it works, the question we want answered next is: why did it evolve in the first place? The classic “orthodox” explanation has been that all of these traits evolved to allow formerly land dwelling crocodyliformes stay underwater for long periods of time. A four chambered heart is great for aerobic endurance, but pretty darn useless for an animal that spends most of its time holding its breath. In that arena, a three chambered heart is a more efficient system. By mixing oxygenated and deoxygenated blood together, crocodylians and other reptiles are able to siphon as much oxygen as possible from their blood, and thus stay underwater longer.
As I said, that was the old explanation. Now there is a new one:
Farmer, C.G., Uriona, T.J., Olsen, D.B., Steenblick, M., Sanders, K. The Right-to-left Shunt of Crocodilians Serves Digestion. Physiological and Biochemical Zoology. Vol. 81(2): 125-137. doi: 10.1086/524150
Farmer et al studied several groups of juvenile American alligators (Alligator mississippiensis). Each group underwent surgeries of various sorts to measure, and/or block the right to left shunt. The working hypothesis was that crocodylians use their right to left shunt, to serve digestion, by providing a greater reservoir of hydrogen ions (left over from the retention of CO2) for stomach acid secretion. It was suspected that if this was true, then one should see a greater degree of right to left shunting in animals that have just eaten.
So what did they find?
Well, for one, they found that juvenile alligators have a preferred postprandial body temperature of ~30?C, and will maintain that temperature to within .03?C. That’s a degree of temperature control worthy of any mammal, or bird.
Another thing they learned was that alligators that were allowed to stay at that temperature, were a real bugger to keep under control. So they had to drop the temp down 3 degrees, to 27?C instead.
Farmer et al learned that gastric acid secretion is temperature sensitive. Alligators produced greater quantities of gastric acid at 27?C, than at 19?C.
Oh yeah, they also learned that crocodylians produced a tonne of acid. At maximum secretion, acid production was an order of magnitude greater than that measured in any mammal, or bird. For those keeping tally at home; that’s 10 times greater.
The authours final observations warrant some thoughts.
That the left aorta, which arises from the right – pulmonary – ventricle, is the main blood delivery route for the digestive system. During right to left shunting, oxygenated blood from the left ventricle, gets shoved to the left aorta, and down to the digestive system. That this coincides with increased gastric acid secretion is telling, and strongly suggestive as to the role of the R-L shunt.
Yet R-L shunting also occurs during dives, and this is still the best explanation for the cog toothed valve. If the crocodylian heart really was specifically developed to increase digestion, then why block the path to the lungs at all? This study shows that the gastrointestinal system benefits from increased oxygen to these tissues. So why block the lungs, if one is trying to keep them oxygenated. Unfortunately the paper doesn’t really mention whether, or not the cog toothed valve was activated during this process. Personally, I don’t remember reading any case of the R-L shunt being used in crocs, without incorporating the cog tooth valve, so…
I felt that the authours put too much emphasis on endothermy vs. ectothermy. Their final observations involved a blanket statement regarding the R-L shunt in all reptiles. As I mentioned above, crocodylians are unique in their cardiovascular anatomy and physiology. They are also renowned for their very acidic stomach acid. It would seem more parsimonious to say that the R-L shunt in crocodylians, plays a large role in gastric acid secretion for these animals only; and wait for subsequent studies in other reptiles before saying this is true for the whole class.
Okay, so maybe their acid isn’t quite this strong, but you get the point.
Lastly (I know, I know, this just keeps going), I found it interesting that they studied the effects of gastric acid secretion on the vertebra of a cow. This vert took over 2 weeks to digest! While I can accept that this was partly due to the size of the object, and it’s material (bone is tough, after all.), but 2 weeks! Even at the lower temperature that the experimental group was kept at, it seems hard to believe. The authours gave no mention of gizzard usage in these animals, which suggests that the animals were never given access to gastroliths, which should have sped up the digestive process considerably.
Either way, the study was interesting. I just think that the authours took their final results a little too far.