PZ Myers. 2005 Apr 19. Hot-blooded crocodiles?. <http://pharyngula.org/index/weblog/hot_blooded_crocodiles/>. Accessed 2008 Dec 01.
Posted on M00o93H7pQ09L8X1t49cHY01Z5j4TT91fGfr on Tuesday, April 19, 2005
Hot-blooded crocodiles?
Crocodiles are beasts with an odd mix of features: they are ectothermic (meaning that they derive their body heat from their environment) reptiles, like lizards and snakes, but unlike those smaller critters, they have a fairly sophisticated, high performance cardiovascular system: they have a true four-chambered heart, just like us mammals and birds, and they also have a diaphragmaticus, a muscle analogous to our diaphragm that is used to inflate the lungs. At the same time, their hearts are relatively small—heart mass is roughly 0.15% of body mass, compared to 0.4%-0.7% of body mass for mammals—and generates relatively low systemic blood pressure.
It's weird. It's like they have this fancy, sophisticated engine in low-tech chassis, that the animal never revs up to its full potential. How did it get in there, and why do crocodiles have such fancy hearts?
The answer may be that they inherited it from more active, endothermic ancestors.
Here's what a crocodile heart looks like. Look at all that plumbing! It has a few features that we don't have, that I'll get to in a moment, and that are special adaptations for the life of an ectothermic, diving ambush predator.
The key feature, and one that I recall being brought up in comparative physiology courses, is that unlike other reptiles, they have complete four-chambered hearts. What this means is that their heart, like ours, is a double-circuit pump, essentially two pumps in one. Each side has an atrium that receives venous, blood under low pressure, and pumps it into a ventricle, which drives blood under higher pressure to the periphery. In a double-circuit pump, the right side is specialized to handle just the pulmonary circulation: it pumps deoxygenated blood to the lungs. The left side is the more powerful side, which has the job of pumping blood to all the rest of the body.
Lizards and snakes have a three-chambered heart. One atrium receives venous, oxygenated blood from the lungs, one receives venous, deoxygenated blood from the body, and both pump them into the single ventricle. When the ventricle contracts, it drives blood simultaneously to both the lungs and the body. This is inefficient in some ways, because there is the potential for mixing oxygenated and deoxygenated blood, and pumping some blood that is already carrying its oxygen load to the lungs, and some blood that hasn't been oxygenated yet to the tissues—although the animals may also have other features in the design of their hearts to minimize mixing.
A common kind of human birth defect is the "hole in the heart", such as the Tetralogy of Fallot, in which there is leakage between the pulmonary and systemic circulation. This reduces the efficiency of the heart. It's not quite kosher to compare a congenital defect to the well-adapted arrangement of another organism's heart, but that'll give you a feel for the nature of the problem. The animals with three-chambered hearts tend to have much lower metabolic rates, so the loss of efficiency is tolerable.
I was always taught that the most important function of the separate pulmonary and systemic circulation was to prevent mixing of oxygenated and deoxygenated blood. Seymour et al. mention another very important reason, though: it also allows the two pumps to generate different blood pressures. In us mammals, our systemic pressure is relatively high (120/80 mm Hg, on average), but our pulmonary pressure is much, much lower—more like 20-40 mm Hg. Pumping blood at systemic pressures through the thin, delicate membranes of the lung would not be a good thing. More plasma would be forced out into the spaces of the lung, drowning us.
(Note: in modern crocodilians, but the systemic and pulmonary pressures are in the region of the pulmonary range for mammals and birds)
So here's the idea: a major force driving the evolution of the four-chambered heart was selection for a high-pressure systemic circulation that could support an active lifestyle and endothermy, while isolating the pulmonary circulation from that damaging high pressure. Crocodilians don't need this now—their systemic and pulmonary pressures are roughly in the same range—but maybe their ancestors did.
In addition to the completely divided heart, the authors note other curiously sophisticated properties of the crocodile. They have complex, bird-like lung structure, and birds are the pulmonary champions among the vertebrates, with amazingly efficient respiratory surfaces. They have muscular specializations for lung inflation during active locomotion which seem superfluous in a sit-and-wait ambush predator. Their bones have the characteristic richly vascularized structure of fibrolamellar bone, one of the hallmarks of endothermy and one of the pieces of evidence that dinosaurs were warm blooded. Interestingly, one bit of counter evidence used against the hot-blooded dino hypothesis was the fact that crocodilians have the same structure…maybe there's another reason for the similarity, that crocs are also descended from hot-blooded ancestors.
One particularly interesting piece of evidence to me was the analysis of development of the crocodile heart. Despite being four-chambered, the crocodile heart also has a couple of specializations that reduce its efficiency, although they seem to be important for endurance in diving. There is a hole in the heart called the foramen of Panizza, which allows blood to be shunted from the right side of the heart to the systemic circulation, introducing deoxygenated blood. There is also a special valve called the cog-tooth valve in the right ventricle; when constricted, it increases the pressure in the right chamber of the heart and promotes circulatory shunting into the left aorta and foramen of Panizza. The animal can regulate shunting, increasing it during dives to send more hypoxic blood to the tissues, inducing a hypometabolic state—just the thing when you want to lurk underwater for a good long while.
The result of the developmental analysis was to find that these specialized diving features are all late add-ons. The heart initially develops a complete separation, and then secondarily punches the foramen of Panizza through. All this suggests that the crocodile heart evolved to support an active, endothermic lifestyle, and only in the modern lineages did they secondarily acquire features to adapt themselves to a more sluggish, conservative metabolic pattern.
The authors do concede that none of the evidence is definitive, and even that there is some counter-evidence: the absence of evidence for any kind of thermal insulation, and the absence of nasal turbinates, which form the convoluted outflow surfaces of the respiratory system and are important in conserving temperature and water.
Still, it's cool to imagine that 10-ton Sarcosuchus, the super-crocodile of the Cretaceous, might also have been an endotherm…which would have meant that it was hungry all the time, and would have had the high-powered, fast-moving metabolism of a mammal, a bird, or a dinosaur.
Alternatively, it could have already made the transition to a cold-blooded, slower-moving killer, and it was its smaller ancestral stem archosaurs that were endothermic. That's interesting, too, because it makes endothermy a primitive condition in this large and successful clade of reptiles.
Seymour RS, Bennet-Stamper CL, Johnston SD, Carrier DR, Grigg GC (2004) Evidence for endothermic ancestors of crocodiles at the stem of archosaur evolution. Physiological and Biochemical Zoology 77(6):1051-1067.
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- Good stuff! Hunger would have been the driving force to ectothermy... Could be.
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Neat article, PZ. A mildly OT question: why is called the Tetralogy of Fallot if it is basically a leak between the systems? I'm assuming that Fallot was the guy who described it originally.
#: Posted by on 04/19 at 08:49 AM
- Yes, and it refers to four associated heart problems: the septal leak, stenosis of the pulmonary arteries, an aortal override, and thickening of the right ventricle wall.
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I would like to see lots of articles on such evolutionary changes in direction.
#: Posted by on 04/19 at 09:17 AM
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In med school, our lecturer taught us to focus on the overriding aorta, that the three other features essentially develop as a result of it.
Interesting article, Dr. M. There's some critical care science in there somewhere, I think. Lemme think about this some more.
BCH#: Posted by Burt Humburg on 04/19 at 10:18 AM -
Very interesting - thank you PZ.
#: Posted by on 04/19 at 10:23 AM
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How much do we really know about cocodiles' circulation? Does their blood pressure change at all during deep diving or when they are attacking prey?
#: Posted by Reed A. Cartwright on 04/19 at 11:59 AM
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Fascinating, PZ.
I wonder if nasal turbinates had the primary purpose of conserving water, and temperature conservation was just a secondary benefit for some. Animals on the African plains sure would benefit if they made less trips to the watering holes during the dry season where the lions might be waiting. Crocodiles and their precedents live in a watery environment, so water conservation by this means is unnecessary.
Are the hearts of sea going mammals the same as land mammals, or have they developed any changes like those in the crocodile that would allow them to stay under water longer? -
I mentioned this turbinal business to one of our local palaeos, and he said that the correlation between the presence of turbinals and endothermy will be much weaker for a uricotelic animal. Which makes sense. On the other hand, crocs apparently produce a hyposmotic urine - but as Bill points out, life in a watery environment may have something to do with this.
#: Posted by on 04/19 at 02:13 PM
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Good point. And for those following along, uricotely is way to get rid of nitrogenous wastes as a precipitate, minimizing the amount of water needed to get rid of it -- it's that white stuff in bird poop.
Reed: the paper mentions that there is no data on metabolic rates and shunting in submerged crocodiles, unfortunately. -
What does fibrolamellar mean? All I can get from Google is that it's bone that grows quickly.
#: Posted by Philip Brooks on 04/19 at 03:01 PM
- It's bone that was deposited quickly in rapid growth, with a complex structure of layers and fibers. Its presence is an indicator of a high metabolic rate.
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I have in front of me a reference (Crocodiles: Inside Out. Richardson, KC, GJW Webb and SC Manolis. 2002. Surrey Beatty & Sons, NSW) which I don't have time to summarize right at the moment but which does go into blood shunts and metabolic rate while diving in crocs. Will try to find the time later if anyone's interested. But basically, metabolic rate drops while diving, and you have peripheral vasoconstriction (as would be expected); gradual acidosis helps unload oxygen from haemoglobin, and there's a great eventual dependence on anaerobiosis, but without the sort of oxygen debt that you see in diving mammals.
#: Posted by on 04/19 at 05:48 PM
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For what its worth, Sarcosuchus probably would have had the same derived cardiovascular condition as do modern crocs, not that they are especially close related though. I would be willing to bet that this system, or a least a functionally similar one, was arived at, and even reversed, several times during the course of crocodilian evolution given that there isn't allways a cut and dry seperation of sit an wait lifestyles and those which are presumed to have been more active.
#: Posted by on 04/20 at 12:02 AM
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I was taught that the regulation of body temperature was such a great biological advance that only a few marginal terrestrial coldblooded animals survived. Now it seems that crocs regressed to their original coldblooded state. Apart of possible energy savings, what is the advantage of being coldblooded?
#: Posted by on 04/20 at 03:39 AM
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Lars, I would be very much interested in finding out more about the physiology during crocodile dives.
My understanding is that diving mammals have oxygen reserves stored in blubber and other tissues which returns to the blood during dives to allow these mammals to to stay under water longer. When the mammal returns to the surface, it must remain there for some time to replenish these oxygen stores in the tissues before diving again. Is this the "oxygen deficit" you mentioned?
In crocodiles, the shunting reduces the flow of blood in the pulmonary system. As blood oxygen levels are reduced in the rest of the body, metabolism slows and there develops a greater reliance on anaerobiosis. It seems like the air in the lungs provides an oxygen reserve analogous to that in the blubber in mammals. The slow flow of blood through the lungs due to shunting results in this oxygen being retrieved for use over an extended period of time allowing the animal to stay under water longer. When the animal surfaces, a few breaths renews the oxygenated air in the lungs, so there is no prolonged oxygen deficit as in mammals.
The products of metabolism have to be dealt with, the build up of carbon dioxide and the lactic acid produced by anaerobiosis. Is there anything in the literature about that?
Another thought: If the crocodile line was once endothermic, is there evidence it has, or once had, a digestive system which was efficient enough to support a high metabolic rate? It seems that reptiles today gobble down some food which they may take several weeks to digest. Unless they could process food much faster than that, the idea that they were once endothermic appears unsupportable. BW -
Digestion rate in vertebrates mostly depends on temperature. A warm ectotherm digests and translocates food faster than a cool one. I think that a transition to endothermy/ectothermy would require no special adaptation.
Another reason for… “It seems that reptiles today gobble down some food which they may take several weeks to digest.”…is that, in these circumstances, these are large reptiles ingesting large-whole animal foods. Unlike mammals, they cannot chew, an important first step in our digestion.#: Posted by on 04/20 at 01:44 PM -
Bill -
Not an awful lot that I can add to your account of crocodile diving - looks as though you have most of the points that I would have made in your last posting
...it must remain there for some time to replenish these oxygen stores in the tissues before diving again. Is this the "oxygen deficit" you mentioned?
Yes, basically, though I think that I said "oxygen debt". As I recall, at least the cetaceans (not sure about pinnipeds) can also rely upon the oxygen-storing capacity of myoglobin - whale muscle is exceedingly dark with it, apparently. I don't believe aquatic reptiles have this high level of myoglobin.
Interestingly, all of the physiological strategies that you mentioned in your last post are not followed by a startled diving crocodile - it can't stay down for very long and gasps when it comes to the surface.
Sorry, did not have time today to go looking for croc diving physiology primary literature. Exam week - I am very popular with my students right now. If you can locate a copy of the book I cited above, it would be a good place to start from.
Should also have mentioned that vasodilation and vasoconstriction are important thermoregulatory responses, and also feature in flight/fight reactions, so it is difficult to tease out the contribution of these responses to diving physiology.
Birds choke down their food as well - it doesn't stop them being endotherms. Crocs frequently have stones in the stomach - these may help macerate large chunks of food, although it is also possible that they are swallwed by accident. Other than this, the gut isn't particularly elaborate, but it wouldn't really have to be - ceibatree has the right of it.#: Posted by on 04/20 at 07:02 PM -
I tried to follow the workings of the croc heart (I am a professional water engineer) and here are some comments:
* The croc has two pumps working at different regimes, and a membranous septum, presumably flexible, separating the two pumping chambers. This membrane would allow the transmission of pressure (but not of liquid). This flexible wall reduces the efficiency of both pumps and makes little sense. On the other hand, it could work as a fail safe mechanism, allowing that in the event of a total breakdown of one of the pumps, the good one could actuate the other and pump some liquid in the other circuit.
* The cog tooth valve seems to me to act like the throttle valve in carburators, that controls air intake in gasoline engines (it is operated by the accelerator). Reducing Q inflow the P pressure will increase. This animal seems able to delicately control its blood pressure.
* The position of the unidirectional valves in the figure above seems wrong from the hydraulic point of view. During overpressurization you need tight sealing valves, and some of the gate valves in the figure would tend to leak.
* The Panizza foramen seems to link downstream the pumps. There should be equipped with a control valve to close that hole, because it existence makes no sense to me. It is like the birth defect mentioned by PZ that communicates two systems carefully designed to work more or less independently. On a second thought, taking into account how evolution works, we could have had originally an animal with a perfectly good two-pump system. A birth defect opened up a hole that communicated the two circuits reducing overall effectivity but providing some other advantage, so it became standard in crocs. Successive mutations and adaptations produced the complex and suboptimal croc heart we find today. Evidently this animal started as a completely different thing and has been trying desperately to adapt itself to a new environment. I could imagine it even hates the taste of antilopes.#: Posted by on 04/21 at 12:23 AM -
ceibatree, now I recall film of baby birds being fed. They gulp down a chunk and are ready for the next. Looks like the digestive system is flexible enough to adjust to a large range of volumes to accommodate the needs of the animal.
Lars, by all means, keep your students happy. I can see why a startled crocodile would want to stay alert, rather than go into a lower metabolic state, so your observation makes sense. It's not too handy if the crocodile would like to hide on the bottom, though. Crocodile meat is nearly as white as chicken, so I would guess it lacks the myoglobin like in whales.
jaimito, Perhaps the flexible septum allows for the different flow volumes on the two sides of the pump during shunting. Then there would be less return flow from the pulmonary system and more from the rest of the body, requiring the pulmonary chamber to be larger, the other smaller preceding each beat. I don't favor the taste of antelope, either, too gamey. -
Regarding antelopes, I meant that we could probably find also the same patchwork adaptations in the crocodile's digestive system. Presumably, as shown in the illustration, crocs originally fed on dinosaurs, and now they are forced (by a lack of dino prey) to feed on gamey mammals. In their croc mind, they may be dreaming of a good rare dino steak as of old times.
#: Posted by on 04/21 at 11:16 PM
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It is amazing to think of an entire bodily system evolving (scrambling?) to keep up with whatever the environment provides. Or more accurately, to keep up with whatever stuff just happens to allow it's host body to flourish in said environment.
Of course, the system is not alone. The systems around it must not only adapt to whatever allows it's host body to flourish in each new environment, but also (simultaneously?) to the adaptations of it's fellow systems.
Alright, that's it... I gotta come clean:
My diluted pseudo-epiphany just convinced me that humans were (and still are) intelligently Designed - by Pomegranates!
And for the record, so were crocodiles. I mean, come on... 4 chambers? That's brilliant!#: Posted by on 04/22 at 12:09 AM -
Presumably, as shown in the illustration, crocs originally fed on dinosaurs, and now they are forced (by a lack of dino prey) to feed on gamey mammals. In their croc mind, they may be dreaming of a good rare dino steak as of old times.
Crocodiles don't eat fresh meat, their teeth are all wrong for that - they drag their victims down and log them underneath something in the water, letting them rot. Only when they are soft, will they tear off chunks of meat, and eat.#: Posted by on 04/22 at 12:16 AM -
Yes, but they didn't always depend on the water, as they do today. They may have had different eating rituals.
#: Posted by on 04/22 at 12:43 AM
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Oops. That's what you (obviously) meant. Sorry!
I'm so, so tired...#: Posted by on 04/22 at 12:49 AM -
No problem Jeebus, it could have been clearer.
#: Posted by on 04/22 at 12:57 AM
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I can't help wondering if there is any difference in a digestion system targeted towards fresh meat and one targeted towards rotten meat - intuatively it would seem so, but some times intuition is wrong. Also, how about the immune system?
#: Posted by on 04/22 at 01:00 AM
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The result of the developmental analysis was to find that these specialized diving features are all late add-ons.
That would be in sync with the research on fossil skulls as explained in this National Geography article:
In Crocodile Evolution, the Bite Came Before the Body
Of a different tangent, I was searching for pictures of a crocodile running on its hind legs (there are such pictures out there), and in that process I found out that everything I've heard about crocodiles' speed is wrong. It is in fact possible for a human to outrun a crocodile on land, contrary to everything I've heard or even read.#: Posted by on 04/22 at 01:27 AM -
Correction: "Originally, crocs lunched on fresh dinosaur meat but in their current reduced circumstances, they feed on putrid mammal soft chunks. In their croc minds, they may be salivating with dreams of a good rare dino steak as of old times."
#: Posted by on 04/22 at 04:16 AM
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Kristjan, Have you heard about the famous Australian jumping crocs?
www-atm.physics.ox.ac.uk/ user/bottger/CROC.JPG#: Posted by on 04/24 at 11:35 PM -
jaimito, I can't say I have, but I know that when I was in croc-infested areas in Australia, you were laways given the advice not to climb into trees over water to get away from the crocodiles. According to the locals, a croc can jump up so only its tailtip is touching the water, and given their length that's pretty far.
At the crocodile farms in Singapore, and other Asian countries, the crocodiles are in fairly deep holes (3-4 m ~ approx. 3½-5 yards), and there was fence around those. Yet, there were big signs warning not to lean over the fence, or even stick your arm out too far, as the crocodiles might be able to jump up and grab you.
Given the fact that the running speed of a crocodile apparently is much less than commonly known, I guess one would have to take this with a grain of salt, yet your picture shows that it might be somewhat plausible.#: Posted by on 04/25 at 12:12 AM -
Crocs may be slow compared to us, but they are equipped with high tech articulations and muscles of a running terrestrial predator. The question is: Why are crocs so ugly? Most predators - eagles, falcons, sharks, leopards, jaguars, lions, etc. - are beautiful. Maybe the females have too poor a vision to perform some sexual selection.
#: Posted by on 04/25 at 11:08 AM
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It's not the outer beauty that counts. They might be really sensitve family fathers (no, not really, given their tendencies to eat their young).
#: Posted by on 04/25 at 11:31 AM