Chemicals in Dragon’s Glands Stir Venom Debate

The Komodo dragon is already a terrifying beast. Measuring up to 10 feet long, it is the world’s largest lizard. It delivers a devastating bite with its long, serrated teeth, attacking prey as big as water buffaloes.

But in a provocative paper to be published this week, an international team of scientists argues that the Komodo dragon is even more impressive. They claim that the lizards use a potent venom to bring down their victims.

Other biologists have greeted the notion of giant venomous lizards with mixed reactions. Some think the scientists have made a compelling case, while others say the evidence is thin.

Biologists have long been intrigued by the success Komodo dragons have at killing big prey. They use an unusual strategy to hunt, lying in ambush and then suddenly delivering a single deep bite, often to the leg or the belly. Sometimes the victim immediately falls, and the lizards can finish it off.

But sometimes a bitten animal escapes. Biologists have noted that the lizard’s victims may collapse later, becoming still and quiet, and even die. For decades, many scientists have speculated that the dragons infected their victims with deadly bacteria that lived in the bits of carrion stuck in their teeth.

Yet others have always been skeptical of the bacteria hypothesis. “Your average lion has a much dirtier bite,” said Bryan Fry, a biologist at the University of Melbourne. “It’s complete voodoo.”

Dr. Fry suspected that Komodo dragons were using venom instead. In 2006, Dr. Fry and his colleagues published evidence suggesting that some lizards share the same venom genes as snakes. They concluded that venom evolved about 200 million years ago in the common ancestors of the lizards and snakes. Studying an Australian lizard called the lace monitor, Dr. Fry found proteins in its mouth that were produced by those shared venom genes. When he tested the proteins, he found that some could cause a rapid drop in blood pressure, and that others stop blood from clotting.

Komodo dragons, like the lace monitor, are closely related to snakes, which suggested to Dr. Fry that the largest of all lizards might be venomous too. To test his hypothesis, Dr. Fry climbed into a Komodo dragon’s cage at the Singapore Zoo. “I dangled a rat above it and got it really excited,” he said. As mucus drooled out of the lizard’s mouth, he used a test tube on a pole to collect it.

Dr. Fry did not find any venomlike proteins in the mucus, but later he realized mucus was the wrong place to look. Studying other lizards, he discovered that the proteins were coming from a separate set of glands in their mouths.

A medical disaster offered Dr. Fry a chance to take another look at the Komodo dragons. During a mysterious outbreak at the Singapore Zoo, most of the Komodo dragons died. Dr. Fry and his colleagues were given permission to dissect jaw tissue from a terminally ill lizard and preserve the heads of two dead lizards for later study.

The researchers found the second set of glands in the Komodo dragon heads, and inside they found venomlike proteins. Tests showed that one protein keeps blood from clotting. Another one relaxes blood vessel walls. “It drops the blood pressure like a stone,” Dr. Fry said.

He argues that Komodo dragons depend on these venoms for their success. A Komodo skull is light, so it cannot generate a powerful bite or hold down a struggling victim, the way alligators do. “You’d expect them to be strong, but they’re not,” he said.

Instead, Dr. Fry argues, Komodo dragons slice open their victims, adding venom to the open wound. “If you keep it bleeding and lower its blood pressure, it’s going to lose consciousness, and then you can tear its guts out at your leisure,” he said.

Some experts praised the new study, which is published this week in the Proceedings of the National Academy of Science. “This paper clearly demonstrates that the Komodo dragon is a venomous predator,” wrote Nicolas Vidal, an evolutionary biologist at the National Museum of Natural History in Paris, in an e-mail message.

But Kurt Schwenk, an evolutionary biologist at the University of Connecticut, is not so impressed.

Dr. Schwenk finds the new mouth glands intriguing, but he considers most of the evidence for venom in the study to be “meaningless, irrelevant, incorrect or falsely misleading.” Even if the lizards have venomlike proteins in their mouths, Dr. Schwenk argues, they may be using them for a different function.

Dr. Schwenk also doubts that venom is necessary to explain the effect of a Komodo dragon bite. “I guarantee that if you had a 10-foot lizard jump out of the bushes and rip your guts out, you’d be somewhat still and quiet for a bit,” he said, “at least until you keeled over from shock and blood loss owing to the fact that your intestines were spread out on the ground in front of you.”