PEOPLE bitten by their pet lizard can suffer a painful swelling and prolonged bleeding. Infection by bacteria in the lizard鈥檚 mouth was always assumed to be the cause, but it turns out that many lizards, including some that are common pets, are actually venomous. The finding is rewriting the evolutionary family tree of lizards and snakes.
鈥淭o find the classic rattlesnake toxins in the bearded dragon 鈥� a hugely popular pet 鈥� was a huge surprise,鈥� says Bryan Fry at the University of Melbourne, Australia, who led the work.
Venom was considered the preserve of advanced snakes and just two species of lizard, the gila monster (Heloderma suspectum) and the Mexican beaded lizard (Heloderma horridum). It was thought that these lizards evolved the ability to produce venom independently of the snakes.
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Now Fry鈥檚 team has found that two other groups, the monitor lizards and iguanians, which includes iguanas and chameleons, are also venomous. They share nine of the toxins produced by snakes, but make others that have not been identified before (Nature, DOI: 10.1038/nature04328).
In a related paper published in the journal Comptes Rendus Biologies this week, Nicolas Vidal and Blair Hedges of Pennsylvania State University in University Park, two of Fry鈥檚 co-authors, dub this new family of toxic reptiles Toxicofera. And they suggest a complete overhaul of the whole classification system is needed. 鈥淭hese papers threaten to radically change our concepts of lizard and snake evolution, and particularly of venom evolution,鈥� says Harry Greene, a herpetologist at Cornell University in New York.
DNA analysis by Vidal and Hedges suggests that the closest relatives of snakes are iguanians, of which there are about 1440 species, and anguimorphs, a group that includes the two lizard species already known to be venomous, and monitor lizards. This means iguanas and their close relatives probably evolved much later than was previously thought and suggests that venomous lizards and snakes are descended from a common ancestor that lived around 200 million years ago. This pushes back the evolution of venom by 100 million years, coinciding with the rapid spread of small mammals.
The paper also suggests that the method of classifying reptiles that has been used for the past 80 years, which is based partly on the texture of the tongue, should be changed. It is an unreliable method for inferring evolutionary relationships, Hedges says. Characteristics such as venom production and egg teeth, which hatchlings use to escape from their egg, are more useful, he says.
鈥淭o find the classic rattlesnake toxins in the bearded dragon, a hugely popular pet, was a huge surprise鈥�
But how could venom production in these lizards have been overlooked for so long? Fry suggests that blaming bacteria had become dogma. Komodo dragons, for example, are monitor lizards that eat carrion, and their mouths are blooming with bacteria. 鈥淚t was the classic red herring,鈥� he says. Also, while the toxins produced by these lizards might kill their usual prey, they have less potent and so less noticeable effects on people.
