In 1969, an American biologist named Walter Auffenberg moved to the Indonesia island of Komodo to study its most famous resident—the Komodo dragon. This huge lizard—the largest in the world—grows to lengths of 3 metres, and can take down large prey like deer and water buffalo. Auffenberg watched the dragons for a year and eventually published a book on their behaviour in 1981. It won him an award. It also enshrined a myth that took almost three decades to refute, and is still prevalent today.

Auffenberg noticed that when large animals like water buffalo were injured by the dragons, they would soon develop fatal infections. Based on this observation, and no actual evidence, he suggested that the dragons use bacteria as a form of venom. When they bite prey, they flood the wounds with the microbes in their mouths, which debilitate and kill the victim.

This explanation is found in textbooks, wildlife documentaries, zoo placards, and more. It’s also wrong. “It’s an enchanting fairy tale, which has been taken as gospel,” says Bryan Fry from the University of Queensland.

In 2009, Fry discovered the true culprit behind the dragon’s lethal bite, by putting one of them in a medical scanner. The dragon has venom glands, which are loaded with toxins that lower blood pressure, cause massive bleeding, prevent clotting and induce shock. Rather than using bacteria as venom, the dragons use, well, venom as venom.

Based on a thorough analysis of the dragon’s skull, Fry thinks that they kill with a grip, rip and drip tactic. They bite down with serrated teeth and pull back with powerful neck muscles. The result: huge gaping wounds. The venom then quickens the loss of blood and sends the prey into shock.

That doesn’t discount the possibility that the dragons might also rely on oral microbes. To study these microbes, Fry contacted Ellie Goldstein from the UCLA School of Medicine—an expert on microbes an animal bites. Goldstein has advised people around the world on treating unusual bite wounds, including at least one from a Komodo dragon. “The bacteria-as-venom model seemed to be based on faulty and dated studies,” he says. “There was no really good data on the topic.”

Goldstein tried calling several zoos with captive dragons. “Many would not respond and sometimes actively tried to deter our research for reasons unclear to me,” he says. “The detractors [said] this study had already been done and no new info would result,” adds Kerin Tyrrell, who is part of Goldstein’s team. Fortunately, three zoos in Los Angeles, Honolulu and Houston were more cooperative, and the team managed to swab the mouths of 10 adults and 6 hatchlings.

They found… nothing special. All the microbes they found were common in the skin and guts of their recent meals. There were no virulent species at all, and certainly nothing capable of causing a quick, fatal infection. And the species that were there weren’t particularly abundant. “The levels of bacteria in the mouth are lower than you’d get for a captive mammalian carnivore, such as a lion or Tasmanian devil,” says Fry. “Komodos are actually remarkably clean animals. This is another nail in the coffin to the idea of them using bacteria as a weapon.”

View Images The Komodo dragon: surprisingly clean. Photograph by Bryan Fry

Of course, you might argue that wild dragons might harbour deadlier bacteria. But the captive animals aren’t living in a sterile environment nor eating sterile food. If wild dragons are truly using bacteria as weapons, the captive ones should at the very least have some way of encouraging bacteria to grow in their mouths. “If they were facilitating the growth of bacteria in their mouths in the wild, they should be doing it in captivity,” says Fry. “They don’t. Their mouths were not dramatically different from the mouth of any other captive carnivore.”

Aside from Auffenberg’s book, the only other support for the bacteria-as-venom hypothesis comes from a team at the Universtiy of Texas at Arlington. In 2002, they found a wide range of bacteria in the saliva of 26 wild dragons and 13 captive ones, including 54 disease-causing pathogens. When they injected the saliva into mice, many of them died and their blood was rich in one particular microbe—Pasteurella multocida.

But Fry thinks the study is laughable. Sure, they studied wild dragons, but the microbes in Fry’s captive animals were actually closer to those from the wild ones in the Texan study. The so-called pathogens they discovered are just normal non-virulent members of an animal’s microbial entourage. And despite making a big deal of Pasteurella, they only found it in 2 of their 39 dragons. Goldstein never saw it in his captive ones.

And worst of all, no single species of microbe has ever been consistently identified in all dragons. How could these lizards rely on a strategy that’s so variable? “It’s evolutionary implausible,” says Fry.

The only remaining lifeline for the bacteria-as-venom hypothesis, says Tyrrell, is that the team only identified the bacteria that they could grow in laboratory cultures. Some species can’t be identified in this way, so one of these might contribute to the dragon’s killing bite.

Fry thinks that bacteria do help to kill the largest of the dragon’s victims, but not in the way that Auffenberg suggested. When the dragons tackle natural prey—medium-sized mammals like deer or pigs—the victims die very quickly from blood loss. The venom helps, but it’s the wounds that are important. But water buffalos are a different story.

These creatures were introduced to Komodo by humans. They’re too big to kill outright and always escape the initial attack. In their natural environment, they’d disappear into wide marshlands, but there’s nothing like that in Komodo. Instead, the buffalos seek refuge in rank water holes, stagnant and contaminated with their own faeces. In this microbial wonderland, their wounds soon become infected. “It’s the same as if you dumped a whole bunch of cow dung in your pool during the peak heat of summer, shaved your legs with a very old razor, and then went and stood in the water for a day,” says Fry. “You’d end up with some very tasty infections!”

Reference: Golstein, Tyrrell, Citron, Cox, Recchio, Okimoto, Bryja & Fry. 2013. Anaerobic and aerobic bacteriology of the saliva and gingiva from 16 captive komodo dragons (varanus komodoensis): New implications for the "Bacteria as Venom" model. Journal of Zoo and Wildlife Medicine