For the longest time, people believed that the world's largest lizard, the Komodo dragon, killed its prey with a dirty mouth. Strands of rotting flesh trapped in its teeth harbour thriving colonies of bacteria and when the dragon bites an animal, these microbes flood into the wound and eventually cause blood poisoning.
But that theory was contested in 2005 when Bryan Fry from the University of Melbourne discovered that a close relative, the lace monitor, has venom glands in its mouth. The discovery made Fry suspect that Komodo dragons also poison their prey and he has just confirmed that in a whirlwind of a paper, which details the dragon's "sophisticated combined-arsenal killing apparatus".
By putting a virtual dragon skull through a digital crash-test, Fry showed that its bite is relatively weak for a predator of its size - instead it's adapted to resist strong pulling forces. This is a hunter built to inflict massive wounds through a "grip and rip" style that involves biting lightly but tearing ferociously.
The wounds provide a large open area for the dragon to inject its venom and Fry unquestionably showed that the dragons poison their prey. By placing the head of a terminally ill dragon in an MRI scanner, he managed to isolate the venom glands, which turn out to be more structurally complex than those of any other snake or lizard. He even managed to analyse a sample of venom, which is loaded with toxins that prevent blood from clotting and induce shock.
And as the icing on the cake, Fry concluded that Varanus prisca, a extinct close relative of the Komodo dragon probably also had venom glands. Also known as Megalania, V.prisca was three times the size of the Komodo dragon, making it (to our knowledge) the largest venomous animal to have ever lived.
Fry used a CT scanner to create a three-dimensional model of a Komodo dragon's skull and he assessed its properties with a technique called finite element analysis. Engineers use the method to crash-test cars; Fry used it to compare the dragon's skull to that of another giant reptile - the saltwater crocodile. Of the two, the dragon had a much weaker bite, exerting just 39N of force with its jaws compared to the 252N chomp of the croc. If a dragon bit with that much strength, its skull would fracture.
The dragon's skull wasn't much better at coping with twisting or shaking movements either - this is not an animal that can bite and hold onto a prey animal for long. However, Fry found that its skull is highly resistant to pulling forces, and that's the key to its method of attack. Biologists watching these animals have noted that when they bite, they often yank their heads back with powerful neck muscles. Their skulls take the brunt and their sharp, serrated teeth open considerable wounds in their prey.
Other studies using finite element analysis have found that other famous predators, like sabre-toothed cats and great white sharks, do similar things. For their size, they have relatively weak bites but they made up for it with strong neck muscles and very sharp teeth. In all cases, prey start losing blood, but those bitten by the Komodo dragon suffer from another weapon - venom.
Fry used a medical MRI scanner to analyse the preserved head of a dead Komodo dragon and found that it has two long venom glands, running down the length of its jaw. They are the most structurally complex venom glands of any reptile. Each consists of six compartments, with ducts leading from each one to openings between the teeth. Other venomous lizards, like the Gila monster, channel venom down grooves that run the length of their teeth but the Komodo dragon doesn't have these - it just drips venom straight into the wounds that it inflicts.
The venom itself consists of over 600 toxins, a chemical arsenal that rivals those of many snakes. Many of these poisons are familiar and they greatly exacerbate the blood loss caused by the dragon's bite. They cause internal haemorrhaging from leaky blood vessels, prevent blood from clotting and cause muscle contractions and paralysis. Fry calculated that a typical adult dragon would need only 4mg of venom proteins to send a 40kg deer into toxic shock from collapsing blood pressure. A full venom gland packs at least eight times this amount.
If the dragon has venom, you can be sure that it uses it. Venom is so costly to produce that the moment it becomes obsolete, natural selection rapidly does away with it. That's happened in other reptiles - members of venomous families that have developed other ways of feeding (like constriction, or egg-eating) quickly lost their venom system. The glands atrophied, the fangs became smaller and the genes that produce toxic proteins built up debilitating mutations. The Komodo dragon, on the other hand, has strong glands that are loaded with poison.
These results don't discredit the salivary bacteria idea, but Fry has little time for it. For a start, he says that since the dragon was first 'discovered' by Western scientists in 1912, no one has actually documented a case of a dragon victim falling foul of blood poisoning. While dangerous bacteria have been isolated from the mouths of Komodo dragons, no single species has been consistently identified in all individuals. This variability makes it very unlikely that dragons could rely on the presence of toxic bacteria as a reliably strategy to hinge their evolutionary success upon.
One study suggested that the bacterium, Pasteurella multocida, accounted for much of the saliva's killing power, but the researchers didn't find this lethal bug in all the dragons they looked at. P.multocida is rare in reptiles but common in mammals, especially those that are sick or old - exactly the demographic that dragons prefer to kill. As such, Fry believes that the bacteria isolated from the mouths of dragons actually came from the animals they fed on. To him, Komodo dragon victims die not from bacterial sepsis, but from heavy, bleeding wounds that are exacerbated by the toxic effects of the giant lizard's venom.
The dragon's extinct and even bigger relative, V.prisca or Megalania, may have done the same. This giant lizard also had a strong skull but relatively slender jawbones. It was very closely related to the Komodo dragon and the lace monitor, both of which are venomous. And Fry has previously shown that the capacity to produce venom evolved once in the common ancestor of snakes and lizard groups like the iguanas and monitors. There's every reason to think that V.prisca used venom too, which would make it the largest venomous animal to have ever lived.
For more venomous animals, and some truly amazing stories, be sure to check out Bryan Fry's blog Sex, Drugs and Rockin' Venom: Confessions of an Extreme Scientist.
Reference: Fry, B., Wroe, S., Teeuwisse, W., van Osch, M., Moreno, K., Ingle, J., McHenry, C., Ferrara, T., Clausen, P., Scheib, H., Winter, K., Greisman, L., Roelants, K., van der Weerd, L., Clemente, C., Giannakis, E., Hodgson, W., Luz, S., Martelli, P., Krishnasamy, K., Kochva, E., Kwok, H., Scanlon, D., Karas, J., Citron, D., Goldstein, E., Mcnaughtan, J., & Norman, J. (2009). A central role for venom in predation by Varanus komodoensis (Komodo Dragon) and the extinct giant Varanus (Megalania) priscus Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0810883106; Dragon photos by Chris Kegelman; skull image from paper.
More on komodo dragons and other lizards:
- Virgin birth by Komodo dragons
- The pink Galapagos iguana that Darwin never saw
- When the heat is on, male dragons become females
- Mayfly-like chameleon lives mostly as an egg
- Running dragon lizards do wheelies
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Seems strange that something so large needs venom, but there you go. Terrific stuff.
Stupidly, I never really thought hard about that whole septic bite thing - I mean, wouldn't that take AGES to kill the prey?
Frank, it might not take ages to kill the prey if the bacteria that infected the wound were toxic enough. The theory I'd heard is that the deer get sick (and blind!) over the course of a few days, and then when it dies the dragons sniff out the carcass nearby and gather and feed. So it is a "bite and wait" strategy. I have no idea if this is true or how this new venom evidence affects the time for the deer to drop or drop dead.
Is there any documentated cases the whole way from bite to devour with these dragons? That would give some good answers. I have also seen a program in which a local man shows a leg bite he got from a dragon, and how these bites take a long time to heal and even then they don't heal well.
@frank
it might take a day or two, but it would be low energy cost.
Fascinatingly enough, no one has ever seen a dragon track a deer for a few day, wait for it to die of infection and then eat it. Every documentary purporting to show this has staged the scenes. In attempt to recreate... something that doesn't actually exist!
What we have seen, however, are sustained frenzied attacks persisting for several minutes until the large prey item is dead from blood loss. The venom supplements the mechanical damage by keeping the bleeding going through anticoagulation and also helping induce shock.
Cheers
B
In the 1990s, I got to visit the island of Komodo with a CA Academy of Sciences group (mostly ichthyologists & their diver friends, looking for various fish species-- no herpetologists on the trip.
The dragons are so amazing. I believed the "dirty mouth" hypothesis at the time.
The venomous mouth theory is so much more parsimonious.
It is an amazing thing to see an old, unproven, but widely held belief be replaced overnight.
> toxic shock from collapsing blood pressure
@Ed
You're describing cardiogenic shock here, not necessarily toxic shock. Failure of circulation secondary to hemorrhage is cardiogenic.
The article however is nice. I always (wrongly) believed the "toxic" saliva theory.
Bryan - Thanks for commenting. Awesome work.
Frank - I think the point is that their arsenal allows them to kill things that are even larger.
J - I concur. All of this reminds me of a story I covered two years ago, where moray eels were found to have a second set of ballistic jaws in their throat, rather like Giger's Alien. And they're animals found in most large aquariums. It just goes to show that even "familiar" species are often nothing but.
Wow, those things are freaking scary dude!
RT
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Fascinating! I'll have to have a look at this paper and see if there are any unusual peptides in the venom.
A quick read of the article leads me to disagree here. Admittedly, I'm a med student, not a physiologist.
Table 1 from the article lists the following in toxins
PLA2(T-III) - thats a platelet inhibitor, which is anticoagulation and would lead to hemorrhage
Kallikrein - increases vascular permeability
Natriuretic - I believe this one is both antiplatelet, AND vasodilator (someone correct me if I'm wrong here).
So, Kallikrein and natriuretic would be vasodilation related shock that decrease blood pressure, rather than hemorrhage leading to decreased blood pressure, so I think that counts as toxic shock.
Reading this makes me wonder what we can really tell about other reptiles - like dinosaurs for example - that we only have palentological data for. Is it possible that those large reptiles also may have had venom? Would we be able to tell by looking at bones if venom glands had existed to help paralyze particularly large prey?
Heather, sadly, venom sacs don't have osteological correlates. The only way you could really tell is if a theropod tooth (I assume it would be a theropod) had a groove or hollow channel in it, which would indicate the presence of venom. So far, that hasn't been found.
Ed, wonderful article. Makes me all the more reluctant to visit the Komodo islands. :-)
And as this story shows, the grooved tooth isn't necessary for venom.
Re-read bits of the post. You're right! That's terrifying. So in a way, it's a simplified version of the gila monster system: the venom is almost mixing in with the saliva, which sops into the wound instantly.
So, damn. I guess there's no way to tell for sure whether theropods had venom systems or not. It would certainly make life easier for the smaller carnivores, like compsognathids and microraptorines. But as we've seen with giant varanids, it's clear that enormosity does not preclude the evolution of venom glands.
I suppose we'd have to use phylogenetic bracketing. No living crocs have venom, and as far as I know, no bird has a venom delivery system (in its mouth). So until we find some osteological evidence for venom in a dinosaur, the only thing we can say right now is "probably not."
So regarding the phylogeny, one of Fry's earlier papers suggested a clade called Toxicofera, which includes all snakes, and some lizards including monitors, iguanas, agamids, chameleons and the two species we've known were venomous for the longest time. I think the idea is that reptile venom evolved once in the origin of this group and has been subsequently lost in some.
Right. Secondarily lost in many groups, and is BEING lost in some right now. Also, chameleons? Whoa!
Ah, varanids, how I love thee.
I did take an accidental bite from an exanthematicus once and didn't notice any ill effects beyond the actual, physical trauma and the amount of bleeding seemed normal.
That was also the last time I fed a varanid by hand (hey, I was young....ish).
Why 600 toxins? That seems overkill. I'm struggling to see how a creature with 599 toxins in its bite might need to evolve the 600th...
Wouldn't 10 or 20 be enough?
There are no toxins listed in this article.
@zutroi67
To be precise, all shock is defined by falling perfusion and, frequently, falling blood pressure...be that septic, hypovolemic, cardiogenic or obstructive shock. Blood pressure falls in septic shock just as it would in cardiogenic shock. And, shock due to blood loss is hypovolemic, not cardiogenic. Cardiogenic would be some defect of the function of the heart itself (like myocarditis).