• Category Archives Extant Reptiles
  • Articles about extant, or living reptiles

  • 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


  • Life in Cold Blood

    Life in Cold Blood

    I just found out that David Attenborough’s latest (and possibly greatest) documentary series: Life in Cold Blood is now out on BBC One.

    I remember talking to a fellow who was working on this series, about 2 years back. At the time, he mentioned that the goal was to portray reptiles doing things no one had ever seen before. It sounded great, and now I’m looking forward to seeing how successful it was in its portrayals.

    Unfortunately, as a citizen of the U.S., I’m going to have to wait until it gets released on DVD here, much like Planet Earth, or Life in the Undergrowth…or Blue Planet: Seas of Life…or…you get the point.

    Damn you BBC. Why do you have to make such enticing programming. >:)

    Anyway, for those in the same boat as me, make sure to visit the official site. It features clips and interesting behind the scenes shots. We might not be able to watch it yet, but we can at least whet our appetites.

    ~ Jura


  • Gharials dying

    _Gavialis gangeticus_

    Gharials are dying from a mysterious disease.

    I have been meaning to talk a bit about this one for a few days now. Getting around to doing this blog has been a bit of a hassle for me. I’m still not quite used to the whole “update often” format.Anywho, it appears that a group of gharials in a river sanctuary in India were dying from some enigmatic disease. Whatever the pathogen was/is, it appears to affect the liver and kidneys. So, the pathogen seems blood based then. Current thoughts seem to focus on pollution from the nearby Yamuna river. The animals may have eaten some badly contaminated fish.The end result has been the unfortunate demise of 50 animals so far. Given the critically endangered status of Gavialis gangeticus (they only live in India), setbacks like these are always a big deal. Hopefully this will turn out to be a localized event, and not a contagion.

    ~ Jura


  • Crocodile tears are real. Who knew?

    _Crocodylus niloticus_ eating an impala. Do you see tears?

    We’ve all heard the old wives tale about crocodiles crying during feeding. Many of us have probably also run across numerous references that discount this statement. That said, there has never been much scientific interest in determining whether, or not crocodiles actually cry.That has just changed:

    Shaner, D.M., and Vliet, K.A. 2007. Crocodile Tears: And thei eten hem wepynge. Bioscience. Vol. 57. No. 7: 615-617. doi: 10.1641/B570711

    The subtitle there translates to: “And they eat them weeping.”

    The aforementioned authours were studying an apparent phenomenon in humans called: parareflexes. The hypothesis for this goes that a screwup in our normal genetic makeup, may result in the expression of traits/behaviours from an older phylogenetic time. Interestingly, the hypothesis came about from the supposed phenomenon of crocodile tears. In this case, the hypothesis is assuming that the trait of crying while eating, is one that was found in the last common ancestor between humans and crocodiles (over 300 mya). Er, yeah.

    Coming back to the story, the authours tested Alligatorids (2 caiman species and the American alligator) from the St. Augustine Alligator Farm in St. Augustine Florida, USA (highly recommended for folks who like crocs). The animals were trained to walk up on the bank, out of the water, and accept food from there. All the animals were dry when they came up to eat.

    The results showed that crocodylians do cry while eating. At least, they seem to when eating away from water (rather hard to tell if they are crying while in the water). They also show a large amount of bubbling around their eyes. Stranger still, the eyes cry at different rates (i.e. one eye has more bubbles and water than the other).

    So what does it all mean?

    That, the authours didn’t delve too much into. They mentioned that the phenomenon they witnessed was different from the one mentioned on Adam Britton’s site. The authours suggest that this might be a byproduct of crocodylian anatomy. Due to the extensive sinuses found in crocodylian skulls (see: here), there is a connection between the nasolachrymal duct (the nose/tear ducts) and the nasopharynx (where the internal nostrils meet the throat). The authours speculate that during bouts that involve pushing air back and forth through this duct (e.g. feeding, or extensive fighting), the lachrymal glands are stimulated to release tears. This would also explain why there was bubbling found in nearly all the animals studied.

    So crocodylians cry; but they still don’t seem to do it out of remorse. I guess this makes the old wives tale only partially true.

    ~Jura


  • The fibrolamellar smoking gun.

     

    Three different types of bone growth scene in vertebrates. A. Low vascular, lamellar bone. B, highly vascular, woven bone. C. Fibrolamellar bone. Arrows indicate Lines of Arrested Growth (LAGs). Image from http://ltc.smm.org/histology/
    Three different types of bone growth seen in vertebrates. A. Low vascular, lamellar bone. B, highly vascular, woven bone. C. Fibrolamellar bone. Arrows indicate Lines of Arrested Growth (LAGs). Image from http://ltc.smm.org/histology/

     

    For over twenty years now it has been assumed that there is a black and white divide between bone histology and thermophysiology. Automatic endothermic “warm blooded” animals tend to show a haphazard composition of bone deposition, in which bone is laid down around surrounding blood vessels very quickly, with interspersals of more organized bone deposition (for strength). The term, coined by histologist Armand de Ricqles (1980), is fibrolamellar bone.

    In contrast, bradymetabolic “cold-blooded” animals tend to show a regular deposition of layered, or lamellar zonal bone. This bone is not as well vascularized as fibrolamellar bone, and is often deposited at a much slower rate.

    Back in 1980, this evidence was used along with a chain of other circumstantial evidence to show that dinosaurs were actually “warm-blooded” animals (Bakker, 1980). This challenge did not go unanswered, and even back then there were people questioning the evidence being proposed in favour of dinosaurian automatic endothermy. As far back as 1982, there were authours claiming to have histological evidence of fibrolamellar, “warm-blooded” bone growth in crocodylians (Ferguson et al, 1982). This evidence has often been scoffed at as being questionable at best (Horner & Padian, 2004). Skeptics have pointed out that the fibrolamellar crocodylians mentioned have all been captives. Being kept in a stable environment with easy access to food has resulted in these skewed results. Wild individuals would doubtfully show these traits, as access to scenarios like those provided in captivity, are unlikely.

    For awhile this seemed to keep the argument of fibrolamellar bone, strictly in the pro-automatic endotherm camp. Well, not anymore.

    Tumarkin-Deratzian, A.R. 2007. Fibrolamellar bone in adult Alligator mississippiensis. Journal of Herpetology. Vol. 41. No.2:341-345.

    This paper reports the observation of long bone histology in alligators from Lake Griffin in Lake County, Florida. The findings are most interesting. Seven specimens were studied. Of these, three had extensive fibrolamellar growth in their long bones. In fact, one could put a fibrolamellar individual next to a lamellar zone individual and it would look like one was comparing a “classic mammal” to a “classic reptile.” The difference is incredibly dramatic; even moreso than comparing frame A with frame C in the above picture.
    That’s not the best part though. You see, these lake Griffin alligators were not only wild animals, but they were stressed animals too. Currently the Lake Griffin alligator population is suffering from an intense die off. The reasons behind the high mortality at Lake Griffin remain uncertain, but there seems to be a link to thiamine deficiency in the animals dying.

    This means that, not only are we seeing different bone deposition patterns in animals from the same population, but we are also seeing them from animals that were living under stressed conditions. This throws the whole “crocodylians can only show automatic endothermic growth rates under perfect conditions” argument right out the window.

    So what does fibrolamellar deposition really show? Currently it remains unknown. It might still indicate faster growth. What it doesn’t indicate, though, is the thermophysiological preference of the animal in question.

    Id est: it doesn’t seperate the “warm-bloods” from the “cold-bloods.”

    More to come. Stay tuned.

    ~Jura

    References
    Bakker, R. 1980. “The Need for Endothermic Archosaurs.” In: Thomas, R. D. K., and Olson, F. C. (eds.). A Cold Look at the Warm-Blooded Dinosaurs. Westview Press, Boulder.
    de Ricqles, A. J. 1980. “Tissue structures of dinosaur bone: Functional significance and possible relation to dinosaur physiology.” In: Thomas, R. D. K., and Olson, F. C. (eds.). A Cold Look at the Warm-Blooded Dinosaurs. Westview Press, Boulder. Pp. 103-139.
    Ferguson, M.W.J., Honig, L.S., Bingas Jr, P., Slavkin, H.C. 1982. In vivo and in vitro development of first branchial arch derivatives in Alligator mississippiensis. Progress in Clinical nad Biological Research. Vol. 101: 275-286.
    Padian, K. and Horner, J.R. 2004. “Dinosaur Physiology.” In: Weishampel, D.B., Dodson, P. and Osmolska, H. (eds.), The Dinosauria 2nd edition. Univ. California Press., Berkeley. pp. 660-671.

  • American crocodile bounces back.

    A little over 30 years from when it was originally put on the endangered species list, the American crocodile (Crocodylus acutus), has been officially moved from “endangered” to “threatened.”

    American crocodile pick from: stockpix.com

    Crocodylus acutus

    Though the animal remains endangered in South America, in the states things seem rosier.
    In Florida the animals have gone from a scant 300 wild animals, to 2,000. Though this pales in comparison to the amazing comback that the American alligator (Alligator mississippiensis) made (over 1 million individuals live in the Southern U.S.), it is still an impressive bounceback.

    Kudos to the American croc and the conservationists who worked tirelessly to bring it back from the brink.


  • Supersize crocs on PBS

    Last week the long running PBS series, Nature, showed an episode entitled: “Supersize Crocs”. The premise was to follow croc conservationist, Rom Whitaker as he attempted to see if any 20+ foot crocodiles survive today.

    Unfortunately, much like the Discovery Channel’s “In Search of the Giant Squid” documentary, the results garnered from this doc were inconclusive at best. By the end of the show, the largest croc actually found, was 18ft long. Compared to the late, great Steve Irwin’s attempts at finding giant crocs, it would appear that Whitaker was short by 1 foot. There was some allusion to a 20 foot beast that was seen briefly before it ran into the water. Unfortunately Whitaker could only give a guestimate of its size based of its slide print (which was not all that clear).

    Overall, the documentary made for a nice hour long diversion. There was a lot of crocodile measuring, and some unecessary CGI used to explain crocodylian anatomy. It also featured Croc biologist, Adam Britton, though only for about 10 seconds.

    There were, however, some problems with the program that bugged me.

    First, was the purported maximum size. Whitaker wanted to find a 20 ft croc. During the program he ran into a person who said that he had seen a 22ft individual. He said that this was 2ft longer than the longest individual ever recorded. The problem with this is that there have been reports of saltwater crocodiles (Crocodylus porosus) reaching sizes of 23 ft. While not all these reports may be valid, there are enough credible ones to suggest that they once could reach this size (Ross, and Magnusson, 1989).

    My second qualm comes from Whitaker’s statement that crocodiles grow slowly. In the documentary, Whitaker states that the largest crocodiles (the 20+ footers) would have taken 80 years to reach that size. That is a completely unrealistic statement. Most crocodylians studied to date, tend to take between 10-15 years to reach sexual maturity. At this point they are often very close to their maximum size. From this point on, growth slows substantially (though never completely stops). Large crocodiles might live to 80 years old (some may be centennial), but they don’t take 80 years to get there.
    My final problem with the program was that it continues to promote the myth that crocodylians have remained unchanged for over 200 million years. Crocodylians (i.e. Eusuchia) weren’t even around 200 million years ago. In fact, true crocodiles are a fairly recent group, having evolved around 80 million years ago (something that the Nature website gets correct, but the actual documentary does not). They are but one branch of a highly successful group of animals called crocodyliformes; which in turn are a branch of the highly successful crocodylomorphs. Finally, all are members of the Dinosaurian sister group: Crurotarsi, or Pseudosuchia (for those who would like to continue the croc naming trend).

    The only reason why crocodiles always get lumped into the “living fossil” category, is because the bodyplan that they do have, happens to have been a popular bodyplan for the past 200 + million years. Crocodylians are just the latest group to use it. Before them, there were pholidosaurs, and way before all that, we had phytosaurs.

    Calling crocs living fossils, is doing a disservice to their lineage. Just among the Crocodylia, we had such out there animals as the land dwelling, panzercroc Pristichampsus, and the weird Australian mekosuchines (e.g. Quinkana, Mekosuchus, Trilophosuchus, to name a few).

    Not to mention strange behemoths such as the “duck billed” Purussaurus.

    Regardless, the point is that crocs are way more diverse than they are ever given credit for.

    Overall, I’d say the best part of the entire documentary would be the scenes of freshwater crocs (C.johnstoni) galloping into the water.

    Oh, and the only reason I’m bringing this up now is because I just saw it last night.

    ~Jura


    Refs:Ross, C.A. and Magnusson, W.E. 1989. “Living Crocodilians” in Crocodiles and Alligators. Ross, C.A. ed. Facts on File pg: 68


  • Virgin Dragons and Fossil Sharks

    This is more of a catch-up post than anything else. A few months back, it was announced that Flora, a Komodo dragon had laid fertile eggs even though she had never been with a male dragon. Now, just last week, it was announced that the eggs hatched.

    The overall story is interesting, for it shows that parthenogenesis is more common in reptiles than previously thought (the original suggestion for this came from a timber rattlesnake [Crotalus horridus] that also gave birth to young without the aid of a male). Surprisingly the story doesn’t mention the sex of the babies. Reptiles that have genetic sex determination, rely on ZW and ZZ chromosomes. Unlike mammals, though, the heterogametic sex is female (i.e. they have the ZW chromosomes). This means that in a parthenogenic clutch of eggs, the choice of chromosomes is either ZZ (male), or WW (infertile), but never ZW. As such, all parthenogenic hatchling reptiles and birds, are male (there are exceptions for certain all parthenogenic species, but they create all females by doubling [tripling] their chromosomes). This has implications for colonization. A parthenogenic female can give birth to male offspring, which can then mate back to the mother (nasty, I know), and produce a more even sex ratio and a more stable blood line. Perhaps it’s because males are the heterogametic sex in mammals, that parthenogenesis is not found in this group. A female that can only give birth to other females, is far less likely to make a lasting line in new environments.

    The second bit of interesting news, comes from Japan, where scientists report the discovery of a rare ancient shark. The frilled shark (Chlamydoselachus anguineus) is a rare site, as it usually lives hundreds of meters below the surface. The video that Reuters (and other news outlets) has, is stunning to watch. The shark almost looks like an animatronic piece of Hollywood fiction. It was unfortunate that this female didn’t survive. Hopefully we will be able to capture footage of healthier individuals in the future.

    That’s it for now.

    ~Jura