Komodo Dragons use venom, not bacteria

[Johonebesus]

Venomous Komodo dragons kill prey with wound-and-poison tactics
Posted on: May 18, 2009 5:00 PM, by Ed Yong

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.

There are some interesting links in the original, and, as usual for Science Blogs, the comments are worth reading too. One claims that there has never been an observed case of the "bite then stalk for days while the pry dies" behavior, and that documentaries claiming to depict this are staged.

This does make me wonder, why did no-one ever dissect the lizard's head and notice the venom glands? I wonder how many other surprises are waiting for us in animals that are supposed to be well understood.

[Shroom Man 777]

This does make me wonder, why did no-one ever dissect the lizard's head and notice the venom glands? I wonder how many other surprises are waiting for us in animals that are supposed to be well understood.

It's an endangered species living in an island that's rather hard to get to, and venomous lizards are very rare - with the commonly known ones being the Gila Monster and that other lizard, both of which are in North America.

Also, because the venom victims die from blood loss due to the hemotoxins, rather than from (more conspicuous) neurotoxin effects on the nervous system, maybe the effects of the venom were also harder to notice.

I think I heard of this news on National Geographic. And while killing animuls with just a very dirty mouth is cool, Komodo Dragons are still totally awesome with their new venom glands. Hemotoxins!

[Tsyroc]

It's an endangered species living in an island that's rather hard to get to, and venomous lizards are very rare - with the commonly known ones being the Gila Monster and that other lizard, both of which are in North America.

The other one is the Beaded Lizard. Most often referred to as the Mexican Beaded Lizard.

It's closely related to the Gila Monster.

[Dark Flame]

Just wondering, what does it mean that venom is "costly" to produce? Does it require a lot of energy? Do they have to look for certain proteins to eat to get the right stuff?

And this is really cool! The question of why they hadn't found it sooner also occurred to me, and I thought that it might have something to do with the structure of the glands. I don't know much on the subject, but the article said that they're different than the venom glands in other lizards. Scientists may have been looking in the wrong place for the wrong structure of gland.

[Ilya Muromets]

Just wondering, what does it mean that venom is "costly" to produce? Does it require a lot of energy? Do they have to look for certain proteins to eat to get the right stuff?

And this is really cool! The question of why they hadn't found it sooner also occurred to me, and I thought that it might have something to do with the structure of the glands. I don't know much on the subject, but the article said that they're different than the venom glands in other lizards. Scientists may have been looking in the wrong place for the wrong structure of gland.

"Costly" has me thinking that maybe the Komodo only produces a relatively minute amount of venom. This means that maybe during the times other people have looked they were "dry" or conserving it. This often happens with poisonous snakes, who often bite dry when their venom levels are low or replenishing to conserve the amount they have left. It's partly the reason why some snake-wranglers get the reputation for being "immune" to snake venom when in all likelihood the times they got bitten and escaped unscathed, without treatment, were due to their snakes dry-biting. Also, the article itself says the bacteria as pre-killer idea isn't to be discounted. Maybe Komodos just inject a really tiny amount of venom to back up the bacteria or something.

Of course, this is just pure speculation on my part.

[Surlethe]

Enough spam. Splitting HamsterViking's, Solauren's, and OKD's posts off to Testing.

[Shroom Man 777]

Unless Komodo Dragons have the Andromeda Strain in their mouths, waiting for days for their prey to die from sepsis is pretty much a worthless strategy if their venom is any good at all. It doesn't make any sense for the venom to "back up" the bacteria.

As for the dry-biting, don't snakes do that just to tell people (who step on them) to fuck off, like the venomous fang version of a first warning shot? Then when the snake realizes that the threat is serious business, or if the threat doesn't back off from the warning, it starts using venom. The snake would rather use its venom on something it could eat, anyway, since that'd be less of a waste.

[Isolder74]

That is really cool. I wonder if other monitor lizards have venom as well?

[Shroom Man 777]

Mmm... who knows? Most other monitors aren't that enormous and don't really feed on animals that are as large, if not larger, than themselves. I don't see what use a Water Monitor could have for venom glands. Then again, Gila Monsters aren't so different and they've got venom as well...

[Marcus Aurelius]

As for the dry-biting, don't snakes do that just to tell people (who step on them) to fuck off, like the venomous fang version of a first warning shot? Then when the snake realizes that the threat is serious business, or if the threat doesn't back off from the warning, it starts using venom. The snake would rather use its venom on something it could eat, anyway, since that'd be less of a waste.

Some venomous snakes may use dry bites as warnings, but a much more common reason for dry bites is that many venomous snake species are rear-fanged, which is means that their venom fangs are at the rear of their mouths. Those species actually have to chew their victims in order to inject venom. Single bites are usually dry and may act as warnings, but not deliberately in the sense that the snake would choose not to use venom.

[Ilya Muromets]

Some venomous snakes may use dry bites as warnings, but a much more common reason for dry bites is that many venomous snake species are rear-fanged, which is means that their venom fangs are at the rear of their mouths. Those species actually have to chew their victims in order to inject venom. Single bites are usually dry and may act as warnings, but not deliberately in the sense that the snake would choose not to use venom.

Hm, then maybe a similar case can apply to Komodos after all? Komodo teeth don't look all that specialized for injecting venom like forward-fanged snakes, so maybe it could lead to higher incidences of dry biting?

[Shroom Man 777]

Some venomous snakes may use dry bites as warnings, but a much more common reason for dry bites is that many venomous snake species are rear-fanged, which is means that their venom fangs are at the rear of their mouths. Those species actually have to chew their victims in order to inject venom. Single bites are usually dry and may act as warnings, but not deliberately in the sense that the snake would choose not to use venom.

Rear-fanged venomous snakes that need to chew their food to envenom them are from the family Colubridae, which is mostly composed of typical non-venomous snakes with a few venomous exceptions. There are some Colubrides with venom, and those are the ones with the fangs located at the back of the upper jaw - maybe that indicates that they were earlier in evolutionary development to Elapides (Cobras, taipans, sea snakes) and Vipers (Rattlesnakes, etc.), which have the most species of venomous snakes (hundreds of venomous snakes for both Elapids and Vipers) with fangs and venom glands located at the front of the upper jaw, and with more powerful and more complicated venom than the Colubridaes.

Also, from that episode of National Geographic I saw, tests on dry-biting were not conducted on rear-fanged Colubride pussy-snakes, but on Western Diamondback rattlesnakes. And they definitely do have fangs at the front of their upper jaws. Viper fangs are also more well-developed and flexible than either Elapides or Colubrides, by the way, with the Gaboon viper having enormous inches-long fangs.

The article mentions that the Komodo dragons have venom that causes hemorrhaging and, wait, muscle contractions and paralysis (didn't see the latter before, hmm). Vipers have venom more composed of hemotoxins which affects the blood and tissues, while Elapids (Cobras) have venom more on neurotoxins which affects the nerves - causing paralysis and death. While Vipers have more well-developed fangs, Elapid neurotoxins are actually faster-acting than hemotoxins, with species of sea snakes having venom deadly enough to kill in minutes. But that's understandable, since they have to bite, subdue, kill and eat prey in a very hostile underwater environment. Hemotoxins are slower-acting, breaking down tissue and stuff, but then lots of vipers have heat-sensitive pits to help them track down dying prey.


I really like snakes. I mean, I had a very comprehensive pocketbook for snake species and stuff as a kid, it's probably still there somewhere. I was bloody fascinated with them as a child - watching them on documentaries and handling reticulated pythons whenever our old house was flooded and got filled by them. Mang, those were the days. I love snakes.

[Tasoth]

I can try and field the reason why venom is expensive question.

Assuming it isn't based off cellular waste, the process of making the protein for the toxin is going to be regulated by ATP and a metric fuckload of steps. ATP is your bodies main way of storing short term energy for doing work in the cell, and if you're using it to switch enzymes for venom production on and off, it means you're not using it for cellular growth and maintenance, or production of important chemicals needed for survival. Hence why it's expensive and why organisms that don't need venom anymore quickly use it. More energy for the organism to use.

Additionally, alot of organisms that are toxic to eat don't metabolize the toxins, but tend to concentrate it from food sources, such as red spot newts, or poison arrow frogs. Some sea slugs even go as far as to steal the cnidacytes from devoured sea anemones and store them in their tentacles.

[Isolder74]

Alot of venoms are made of proteins that require amino acids to construct and thus food. The amino acids help make that a double whammy. You need the raw materials from the food and the energy in ATP from the food to produce the proteins. If the Venom doesn't give a net positive effect they'll stop using it.

[Alyrium Denryle]

This does make me wonder, why did no-one ever dissect the lizard's head and notice the venom glands? I wonder how many other surprises are waiting for us in animals that are supposed to be well understood.

They are really really small. Small enough that a conventional dissection would probably not work.

Also, because the venom victims die from blood loss due to the hemotoxins, rather than from (more conspicuous) neurotoxin effects on the nervous system, maybe the effects of the venom were also harder to notice.

Hemotoxins dont just do that. They affect the entire blood clotting cascade, in one part of the body the blood will clot, while at other parts of the body the blood wont clot at all. Massive blood loss, multiple organ failure from infarction, hemorrhaging, thrombosis (throwing clots) are the causes of death. Considering that the prey animals are consumed, evidence of this would be difficult to find, and external symptoms (necrossis, etc) are going to be similar to that of sepsis.

Just wondering, what does it mean that venom is "costly" to produce? Does it require a lot of energy? Do they have to look for certain proteins to eat to get the right stuff?

Protein based toxins are energetically costly to produce, and if done in small quantities, with little storage, can be even more costly, and the dragon cannot afford to waste even a micoliter.

Unless Komodo Dragons have the Andromeda Strain in their mouths, waiting for days for their prey to die from sepsis is pretty much a worthless strategy if their venom is any good at all. It doesn't make any sense for the venom to "back up" the bacteria.

Sure it does. A venom like that can exacerbate the bacteria, and vice versa. Especially a hemotoxin

As for the dry-biting, don't snakes do that just to tell people (who step on them) to fuck off, like the venomous fang version of a first warning shot? Then when the snake realizes that the threat is serious business, or if the threat doesn't back off from the warning, it starts using venom. The snake would rather use its venom on something it could eat, anyway, since that'd be less of a waste.

Yes. A lot of bites are dry bites. They are the sole reason many snake charmers and snake handlers in WV are still alive.

That is really cool. I wonder if other monitor lizards have venom as well?

Yes.

There is actually a venom clade that encompasses not only many snakes, but also varanid lizards and Heloderma (beaded lizards). The proteins involved are homolagous to structural proteins and in many cases have no toxic function, or that function has been secondarily lost due to diet shift. If Dragons and Lace Monitors have venom, chances are other varanids have some sort of venom compound. Hell, even "non-venomous" snakes have toxic saliva compounds. My friend (Read: Undegrad research slave) Garrett, is personally acquainted with this, as he was envenomated by a garter snake several weeks ago. The wounds bled MUCH more than a bite of equivalent size and seriousness (anti-coagulant) and heal much slower than a water snake bite of the same size, and in same location.

Some venomous snakes may use dry bites as warnings, but a much more common reason for dry bites is that many venomous snake species are rear-fanged, which is means that their venom fangs are at the rear of their mouths. Those species actually have to chew their victims in order to inject venom. Single bites are usually dry and may act as warnings, but not deliberately in the sense that the snake would choose not to use venom.

Um... No. Many rear-fanged snakes dont need to chew, they just need a good bite with their full jaw (try telling what you just said to a boomslang)

Rear-fanged venomous snakes that need to chew their food to envenom them are from the family Colubridae, which is mostly composed of typical non-venomous snakes with a few venomous exceptions.

See above.

Most of them are actually venomous, just not dangerous to people. The subfamily natricinae (to which garters belong) actually has a few members who can kill people.

maybe that indicates that they were earlier in evolutionary development to Elapides (Cobras, taipans, sea snakes) and Vipers (Rattlesnakes, etc.), which have the most species of venomous snakes (hundreds of venomous snakes for both Elapids and Vipers) with fangs and venom glands located at the front of the upper jaw, and with more powerful and more complicated venom than the Colubridaes.

No. Viperidae is actually basal to the colubrid clade, from which elapids are derived. In fact colubridae is paraphyletic because of the existence of elapidae.

[Shroom Man 777]

Alyrium you are awesome in that you're a herpatologist. Just like that old man in that B-Movie King Cobra who used oversized snake handling equipment to try and wrangle a fifty-foot long monster King Cobra-rattlesnake hybrid.

The giant King Cobra killed him, but it took several bites from the snake's foot-long fangs to put the guy down.

See above.

I'm sorry, it was just a response to Marcus' post about some rear-fanged venomous snakes and dry-biting. I guess his misconception contaminated me.

But yes, rear-fanged venomous snake bites are more counter-intuitive and unergonomic than front-fanged venomous snakes. Maybe that's what he meant, by having the snake "chew" prey.

Most of them are actually venomous, just not dangerous to people. The subfamily natricinae (to which garters belong) actually has a few members who can kill people.

There is actually a venom clade that encompasses not only many snakes, but also varanid lizards and Heloderma (beaded lizards). The proteins involved are homolagous to structural proteins and in many cases have no toxic function, or that function has been secondarily lost due to diet shift. If Dragons and Lace Monitors have venom, chances are other varanids have some sort of venom compound. Hell, even "non-venomous" snakes have toxic saliva compounds. My friend (Read: Undegrad research slave) Garrett, is personally acquainted with this, as he was envenomated by a garter snake several weeks ago. The wounds bled MUCH more than a bite of equivalent size and seriousness (anti-coagulant) and heal much slower than a water snake bite of the same size, and in same location.

Intriguing. It is because venom is basically an enhanced and "weaponized" salivary enzyme, right? Human saliva is good for breaking down carbohydrates and sugars, while I guess snake saliva has more potent breaking-down effects. Aren't venom glands super-specialized salivary glands?

Could it be said that some snakes have nasty bites because of those enzymes, and because of the evolutionary path that leads to specialized salivary glands developing into venom glands, or the venom glands atrophying but still being sorta there?

Hemotoxins don't just do that. They affect the entire blood clotting cascade, in one part of the body the blood will clot, while at other parts of the body the blood wont clot at all. Massive blood loss, multiple organ failure from infarction, hemorrhaging, thrombosis (throwing clots) are the causes of death. Considering that the prey animals are consumed, evidence of this would be difficult to find, and external symptoms (necrossis, etc) are going to be similar to that of sepsis.

We studied about stuff like disseminated intravascular coagulation in NUERSING school, and the systemic effects of snake venom you detailed sounds very fascinating. Of course, it's evolutionary biochemical warfare - goddamn that's wonderful. It's more elegant than constricting prey and stopping them from breathing, or other basic things like that.

No. Viperidae is actually basal to the colubrid clade, from which elapids are derived. In fact colubridae is paraphyletic because of the existence of elapidae.

Ah, pardon me. It's just the stuff that I gleaned off a pocketbook as a kid fascinated by snakes. Since colubridae have non-venomous snakes and then elapids and vipers are venomous, and since colubridae came before elapids and vipers in the pocketbook, I made that error.

When were these snake taxonomies established, anyway?