• Tag Archives alligator mississippiensis
  • The 3D alligator

    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:

     

    A stillborn hatchling rests inside the left nostril of a large 3.7m (12ft) adult which is some 5000 times larger!
    A stillborn hatchling rests inside the left nostril of a large 3.7m (12ft) adult which is some 5000 times larger!

    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. 🙂

    ~Jura


  • Study shows shunting in crocs is all about the acid

    Baby _C.palustris_ says:

    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.

    </exposition>

    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.


    Xenomorph
    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.

    ~ Jura