A Dirty, Deadly Bite

Dragons aren’t real. At least, the fire-breathing wyverns and coiling wyrms of medieval lore aren’t. Those reptilian menaces were products of superstition and pre-scientific ideas about prehistoric creatures. They were ugly amalgamations inspired by our fears and actual fossil remains of long-extinct mammals and dinosaurs. But in the early 20th century, reporters excitedly relayed the discovery of what quickly became known as the Komodo dragon – ten foot long lizards that had coexisted with humans on South Pacific islands for thousands of years, but had only just been recognized by western science.

In 1934, artist Charles R. Knight – arguably the most important illustrator of prehistoric life to ever wield a brush – wrote a cranky editorial to the New York Times objecting to the newspaper’s coverage on a pair of the reptiles that had recently arrived at the Bronx Zoo. “The Komodo lizard is no ‘dragon’”, Knight chastised, and rightly reminded reporters that the squamate “has no relationship to Tyrannosaurus Rex, the great Cretaceous Dinosaur.” The imported oras were giant lizards, not mythological beasts or prehistoric holdovers like the island-dwelling dinosaurs in King Kong that the captive lizards helped inspire.

Knight’s proposal never took hold. For the past century, Varanus komodoensis has been known to specialists and the public alike as the Komodo dragon. And despite my respect for Knight’s gorgeous artistic work and his efforts to accurately reflect the science of his time, I have to agree with public opinion on this one. The size and ferocious feeding habits of the Komodo lizards truly do recall something monstrous and dragon-like.

Of course, Komodo dragons do not breathe fire like their fictional counterparts. But over the past few years scientists have discovered that these lizards hold even more frightening secrets in their scabrous mouths. The jaws of these great lizards seem to be evolutionary overkill, combining slicing teeth, an ecosystem of microorganisms, and possibly even venom. Blondie’s “One Way or Another” is a fitting song for the way these lizards kill. If it’s not the shock and trauma of losing large swaths of flesh, envenomation or bacterial infection may do you in. The question is which part of the Komodo dragon arsenal is the most deadly.

Naturalists have known about the vicious, dirty bites of Komodo dragons for decades. The standard story, as I heard it as a child, was that these lizards applied an extended bite-and-wait strategy while on the hunt. All they had to do was briefly sink their teeth into a deer or pig, and the bacteria they slobbered into the wound would rapidly infect the victim. Thanks to their slow reptilian metabolisms, the Komodo dragons could easily wait days or weeks while the bacteria did the dirty work.

Only, most of the time, the lizards quickly kill and consume their victims. Komodo dragons can move with baleful swiftness when motivated, and the slow stalking strategy has never been scientifically documented. It was an idea that fit our belief in sluggish, malicious dragons that watch their prey’s gradual, agonizing demise, but has turned out to be little more than a popular story. Komodo dragons wait in ambush for pigs, deer, dogs, and other hapless creatures, and, during the attack itself, the lizards rely on their powerful musculature to boost the power of their bites, pulling their recurved, serrated teeth through the flesh of their prey. Most often, Komodo dragon victims die quickly and horribly.

But this brings us to another puzzle. Komodo dragons undoubtedly host a wide variety of pathogenic bacteria in their mouths, but it also appears that these lizards have a complex venom-delivery system. The teeth of these reptilian carnivores are the most formidable part of their arsenal, but do bacteria or venom give the lizards more of a predatory edge?

In 2009, a group of 28 scientists published a study in PNAS describing venom glands and ducts in the lower jaws of Komodo dragons. Based on an MRI of a preserved Komodo dragon head, the researchers identified a venom gland in the lower jaw which fed a series of ducts which opened between the lizard’s teeth. The venom seeped out of these openings – there was no sign of ducts or tubes in the reptile’s teeth that would have helped inject venom directly. Additionally, the biochemical signature of the venom itself was similar to what previous studies have found in the venom of other lizards and snakes, including toxins that induce shock, cause hemorrhaging, and subdue the blood’s ability to coagulate. There would be no reason for the Komodo dragons to have such deadly biological weapons if they were not in use – snakes and lizards from venomous lineages that switch to diets that don’t require venom, such as egg-eating, have lost their toxic potency through anatomical and genetic changes – and so it seems likely that venom contributes to the trauma inflicted by the dragons. There is some disagreement and debate, but, for now, the evidence for venom in Komodo dragons looks strong.

The same paper downplayed the role of bacteria in Komodo dragon kills. The supposedly lethal community of sepsis-inducing bacteria was a red herring that had obscured the way the lizards actually killed their victims. While it could conceivably cause debilitating infection in the long term, the flora in the mouths of the Komodo dragons seem to be a by-product of the way they feed. Indeed, not all Komodo dragons have the same amount of bacteria in their mouths, and individuals raised in captivity lack the immense bacterial load seen in their wild relatives. This indicates that the bacteria are taking up in the mouths of the lizards due to environmental circumstances, such as feeding on diseased mammals and dragons spreading bacteria to one another when groups of individuals, all acting in their own interest, rip apart hefty carcasses. There’s no sign that the dragons rely on bacteria to bring down prey, and the diverse bacterial communities in their mouths are more likely a byproduct of being messy eaters. Catastrophic bite wounds and venom are more important to how these lizards feed.

The Komodo dragon is not the first giant venomous lizard, though. As pointed out by the authors of the 2009 study, the history of venom-bearing titans probably extends back at least 1.8 million years to an even larger lizard that scrambled across Australia. Surprisingly, despite the press’ love of prehistoric superlatives, I can’t recall hearing a word of this hypothesis when the news reports about the PNAS study came out, but the researchers pointed out that a gargantuan, extinct monitor lizard called Varanus priscus may have also had a venomous bite.

Paleontology fans may know this huge reptile better as “Megalania”, although the lizard was recently reshuffled in with other monitor lizards in the genus Varanus. This carnivore is most often restored as a bigger version of the Komodo dragon. Old estimates placed this extinct lizard at about 23 feet long, but modern estimates settle around 15 feet (with the lizard’s tail often adding or subtracting from the total length based upon how different researchers restore the tail). Whether 23 feet or 15 feet long, though, Varanus priscus was a giant, and, so far as we know, was the largest lizard of all time. If the team behind the venomous Komodo dragon hypothesis is correct, Varanus priscus may have also been the largest venomous vertebrate ever.

Unfortunately, there aren’t any titanic Varanus priscus around to cut into and look for venom glands. The last of these huge reptiles disappeared around 40,000 years ago, handing down the “largest lizard” title to the Komodo dragon. Even worse, the remains of Varanus priscus are so scrappy that we don’t yet have a truly complete understanding of this lizard’s skeleton. Still, if Varanus priscus had venom glands like the Komodo dragon, then there should be osteological clues – a pocket in the lower jaw for where the venom gland sat, and openings between the teeth for the venom to ooze out. Provided that someone has collected, or will collect, a well-preserved lower jaw and has access to high-resolution scanning technology, researchers may be able to look for these clues and investigate whether the huge prehistoric lizard “was a combined-arsenal predator” like its living cousin.

But we should bear in mind that such comparisons require great care. The same year that the Komodo dragon venom paper came out, a different team of researchers led by Enpu Gong published a PNAS paper claiming that the feathered dinosaur Sinornithosaurus had a venomous bite. (Keep in mind that, despite being called “reptiles”, dinosaurs were only distantly related to lizards and snakes. They were part of a different and distinct lineage, the Archosauria, represented by crocodylians and avian dinosaurs today.) Gong and colleagues cited long teeth, a pocket for a venom gland, and grooved teeth as evidence, but a later analysis indicated that Gong and co-authors had misconstrued the evidence. The various features cited as signs that Sinornithosaurus was venomous were either common among theropod dinosaurs (e.g. grooved teeth) or were artifacts of preservation (e.g. teeth that had slipped from their sockets to give the appearance of fangs). As yet, there’s no good evidence for venomous dinosaurs, and the brief episode reminds us how tricky it can be to reconstruct the biology of prehistoric creatures.

As it stands now, the idea that Varanus priscus was venomous rests on evolutionary inference. Based on the fact that Varanus priscus is closely related to other venomous monitor lizards, and that venom was an ancient lizard trait that seems to go back to the last common ancestor of lizards and snakes, it’s entirely possible that the extinct giant was venomous, too. Paleontologists use the same logic to predict which dinosaur groups had feathers, for example, or whether the seagoing lizards called mosasaurs (distant cousins of the monitor lizards) had forked tongues. Based on the accumulated evidence, the idea that Varanus priscus was a sharp-toothed, venomous predator is reasonable, but this idea has yet to be tested against the anatomical evidence of the animal itself.

The puzzles that remain about these gargantuan reptiles feed their legendary status. The Komodo dragon and its extinct cousin have only become known to us in the last century and a half. Both are reptiles of such stature and imposing nature that they approach the mythological creatures we invented to fill the dark forests of our minds, and we are only just beginning to understand the biology of these magnificent lizards.

References:

Bull JJ, Jessop TS, & Whiteley M (2010). Deathly drool: evolutionary and ecological basis of septic bacteria in Komodo dragon mouths. PloS one, 5 (6) PMID: 20574514

D’ Amore, D., Blumenschine, R. (2009). Komodo monitor (Varanus komodoensis) feeding behavior and dental function reflected through tooth marks on bone surfaces, and the application to ziphodont paleobiology Paleobiology, 35 (4), 525-552 DOI: 10.1666/0094-8373-35.4.525

D’Amore DC, Moreno K, McHenry CR, & Wroe S (2011). The effects of biting and pulling on the forces generated during feeding in the Komodo dragon (Varanus komodoensis). PloS one, 6 (10) PMID: 22028837

Fry BG, Wroe S, Teeuwisse W, van Osch MJ, Moreno K, Ingle J, McHenry C, Ferrara T, Clausen P, Scheib H, Winter KL, Greisman L, Roelants K, van der Weerd L, Clemente CJ, Giannakis E, Hodgson WC, Luz S, Martelli P, Krishnasamy K, Kochva E, Kwok HF, Scanlon D, Karas J, Citron DM, Goldstein EJ, McNaughtan JE, & Norman JA (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 of the United States of America, 106 (22), 8969-74 PMID: 19451641

King, D., Pianka, E., Green, B. 2002. Biology, Ecology, and Evolution. pp 23-41in Komodo Dragons, Murphy, J., Ciofi, C., de La Panouse, C., and Walsh, T., eds. Washington: Smithsonian Institution Press.