Here there be dragon drool!!!

Sci was going to save this one for a Friday Weird Science, but it’s just so awesome that she couldn’t bring herself to save it. She had to blog it NOW! It’s not neuroscience, but it’s awesome. Also, there’s dragons.
Not this kind:
dragonage origins.jpg
(Anyone else think Dragon Age Origins is really awesome?! Well, Sci spends a lot of her time wondering why the ladies are so dang naked. You’re climbing a high mountain pass in the winter! Your cleavage will suffer frostbite!!!)
It’s this kind:
I’m sure you all know that dragons have TERRIBLE breath, but what about that whole “poison” thing? Bull et al.”Deathly Drool: Evolutionary and Ecological Basis of Septic Bacteria in Komodo Dragon Mouths” PLoS ONE, 2010.

Komodo dragons hold the current title for largest lizard, up to 200 lbs and almost 10 feet long (long and skinny, it seems). But what the komodo dragons have become known for is not their size, or their enjoyment of such delicacies as dead water buffalo. No, it’s their MOUTHS. The komodo dragon obviously kills by biting, but they sometimes don’t kill DIRECTLY by biting. They chomp down, the animal gets away, and then…the animal gets sick, or is weakened enough to be dispatched by the same dragon or another dragon some time later. The effect was so pronounced that for some time scientists have speculated about the poison that komodo dragons use.
But it may not be a poison after all. Instead, it’s currently thought that the death in the dragon drool is in their TERRIBLE oral hygiene.
komodo dragon breath1.jpg
(Love that fresh-brushed feeling).
In a previous study involving multiple dragons (hehehe, Sci admits that she is writing up this study in large part because she can say things like “percentage of dragons” and “studies involving dragons”. Makes life so much more exciting!) scientists identified more than 58 species of nasty bacteria hiding in the dragon molars, 93% of which could potentially infect a prey animal. So the idea has recently been that dragons chomp down, the prey escapes, and the bacteria the prey receives act as a slow-acting “venom” that infects the bite. The combination of bite and infection slows the animal down enough to make it an easy target for the same or another dragon sometime later. Since a large animal can easily feed multiple dragons, it’s not a bad thing if one dragon does the first bite, and another goes in for a kill, as both dragons may end up dining on the same carcass served with fava beans and a nice Chianti.
But the question now becomes, HOW did the dragons get these bacteria in their mouths (aside from not going to the dentist)? The bacteria don’t appear to infect the dragons, so why are they along for the ride?
Several theories have been proposed as to why.
1) The bacteria sidekick theory: this theory proposes that the bacteria are helpful to the lizards in taking down prey (duh), but this doesn’t really provide any explanation for why they are there in the first place.
2) The prey infection theory: This theory states that the bacteria in the dragon’s mouth are useful for prey, and that the bacteria are acquired via the dragon feeding on infected carcasses and picking up some bugs in the process.
But the scientists writing this article questions these theories, and have a further one: the lizard-lizard hypothesis.

The lizard-lizard hypothesis is this: the lizards get the bacteria from other lizards. Now, in social species, this would be no question at all. Social species interact and share germs like it’s their job. But dragons are solitary from their time of birth pretty much throughout their lives, with a brief interim for mating. And no kissing.
Rather, their hypothesis is that, when a dragon bites prey and the prey gets away, the next dragon that takes it down and chomps into it is going to get a big mouthful of the PREVIOUS dragon’s bacteria. If you do enough of this (and multiple dragons will be feeding on the same animal if its large), you can get a good spread of bacteria from one lizard to another. So a lizard infects its prey, and the prey in turn infect the lizards.
Of course, this model has some caveats. The lizard has to infect the animal. That’s not too hard, dragons have sharp teeth that cause some damage, infection can set in easily. No, the problem is the time lag. The prey gets away, and then develops infection, but this can take hours or even days, during which time the animal is probably moving around. So it’ll probably die eventually, but who’s to say that a lizard is going to be on the receiving end of the meat donation?
Not only that, but it also assumes that all the dragons are going to be eating similar carcasses. This may not be the case. The largest prey often goes to the largest lizard, so it might be only the big lizards that are infected this way.
So to investigate this theory, the authors put together a series of requirements:

1.Prey escape lizard attacks after being bitten
2.Bacteria acquired from the lizard mouth cause a systemic infection in prey, achieving high densities in tissues that are normally consumed by lizards.
3.Infected prey are later eaten by lizards, enhanced by
a.The bacteria weaken or kill large prey, facilitating subsequent capture by lizards
b.Infected, large prey do not escape to habitats not frequented by dragons (e.g., savanna grassland, the majority of island habitat).
c.Prey are sufficiently large to enable multiple (unrelated) lizards to consume a single carcass
4.Bacteria survive in a dead carcass, colonize and reproduce in a lizard mouth.

They then examined whether the dragons fit any, or all of these requirements.
1) injured prey sometimes escapes: a previous study showed that about 30% of large animals escaped from a dragon. Of those, a small percentage appeared to be able to get away long enough to be infected by the bacteria
komodo dragon breath2.png
(Swim, little guy! SWIM!!! This deer escaped the first dragon only to fall prey to the second dragon, who is here approaching his dinner. The dragon’s backpack, btw, is a GPS tracking device.)
The prey that escapes is often pretty large, which in this case can be good, as it allows infection to develop and increases the likelihood that the animal will be fed on by more than one lizard, leading to…
2) Prey is eaten by multiple lizards. Dragons will group around a fresh kill, and a big one can feed more than ten dragons! Other studies have shown that 70% of large kills will be eaten by more than one dragon.
komodo dragon breath3.png
So group feeding means that this part of the model is correct, the bacteria from one dragon has the opportunity to spread to other dragons via communal feeding.
3) Dragon’s bites are infectious. Here the data is more sparse. Some studies have shown infection resulting from bites from large dragons, while bits from smaller dragons appear to end up just fine. This could work into the scientists’ hypothesis, that large kills are eaten by large dragons, and therefore the large ones are going to have deadly drool, while the small ones will have to rely more on bite.
But the real question is this: if the lizard-lizard infection model works, the lizards have to have similar bacteria in their mouths, at least within a travel range where the lizards are likely to encounter each other.
A study has been done on bacteria in dragon mouths, and the most common one was P. multocida, which is actually a bacteria found in the mouths of mammals. You know, like deer. Deer dragons like to eat. Some strains of this can be pretty infectious. But not all of them are. If the bacteria in the komodo dragon’s mouth CANNOT infect their prey adequately, this whole model’s going to come crashing down, but the scientists believe that the original study looking at bacteria in the dragon’s mouths was flawed.
4. Can the bacteria survive on a carcass? This is a relatively easy one, many bacteria survive very well on carasses, in particular P. multocida. So it could be that, if the bacteria in the lizards mouths DO infect the prey, it would then be quite easy to spread the infection to other dragons feeding on the same carcass.
So is this model better than the other two listed above? Well it’s certainly better than the first one, especially because it actually proposes a cause. As for the second, it’s hard to say. The good thing about the second model, the prey infection theory (they call it “passive acquisition”) there are many different ways for the lizards to acquire the bacteria in their mouths.
But the good thing about this current model is that it’s very easily testable. Take some dragons, small and large. Take some bacterial swabs of their mouths. Check the bacteria. Grab some freshly killed dragon victims and check THEM for bacteria. Check the dragon’s bacteria before and after feeding on a carcass, and see if they bacteria change. Hopefully the scientists are planning on doing this next. In the meantime, you may want to disinfect any dragon bites you may acquire. Just in case.
So now, Scientists! Go forth and hunt ye some DRAGONS.
komodo dragon breath4.jpg
Bull JJ, Jessop TS, Whiteley M (2010). Deathly Drool: Evolutionary and Ecological Basis of Septic Bacteria in Komodo Dragon Mouths PLoS ONE, 5 (6) : 10.1371/journal.pone.0011097

5 Responses

  1. I was under the impression that recent tests determined that Komodo Dragons were, in fact, venomous.
    I wonder if this is taking that into account.

  2. I propose another model. One where the bacteria are naturally occurring in the soil. The animal dies, injured or old age, and the soil bacteria colonize the carcass. This is seen a lot in humans with gangrene and other infections. And is even more common in swamps with stagnant water and high temperatures. Lots of bacteria out there that lives off dead animals and devitalized flesh and the corpse provides nutrient rich broth that selectively favors bacteria that live on it.
    Of course if you have a nice stinky corpse your going to attract scavengers. Enter the lizards. Lizards which are not dead or devitalized, or warm blooded, so they aren’t fed upon by the bacteria.
    The lizards are not very fastidious, seldom use a toothbrush and never floss. Their mouths are not very good at self-cleaning so bits of rotting meat ride between the teeth and in the tooth pockets for a long time. With regular feedings, ever present bits of meat on hand, and kept moist by lizard saliva the lizards are walking petri dishes of bacteria that favor life on weakened and dead animal flesh.
    Over time this becomes a symbiotic relationship between bacteria and lizard.
    One way of checking this is to ignore the lizards and to kill a deer and lay it out in a cage or enclosure so the lizards can’t infect it. After festering in the sun for a while test the carcass for bacteria. I’ll bet that your going to find the bacteria your looking for in a good proportion of corpses independent of the lizards.

  3. (Anyone else think Dragon Age Origins is really awesome?! Well, Sci spends a lot of her time wondering why the ladies are so dang naked. You’re climbing a high mountain pass in the winter! Your cleavage will suffer frostbite!!!)

    There’s a pun on “hot” in here somewhere …
    Re: dragons, I too was of the impression that poisonousness had been reestablished. However, there’s no obvious reason that they shouldn’t be simultaneously employing (slow-acting) poison and poor hygiene.

  4. Left_Wing_Fox: Komodo dragons are venomous, but you still have a lot of bacteria in their mouths to account for. This is one reason why the disease hypothesis is interesting: it says the bug is a bug, not a feature.
    Also, another take here:

  5. Yarı akülü istif makinası

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