|Four families of lizards and the suborder of snakes share a common ancestor which must have lived some 200 million years ago. Three of the lizard families were assumed not to be venomous, but researchers have now shown that within these five groups there are nine types of venom. An article on this subject appeared in Nature on 17 november 2005, to which a total of 14 researchers from six different countries had contributed. The Leiden biology student Freek Vonk was one of the contributors.
The Leiden biology student Freek Vonk, co-author of the article in Nature, with a black throat monitor lizard (Varanus albigularis)
From among the extant reptiles there are two lineages which have for centuries been known by biologists to have venom-producing glands. These are the suborder of Serpents, the advances snakes of which there are 2500 to 3000 species, and the two members of the lizard family Helodermatidae, the Gila Monster and the Mexican Beaded Lizard. Snakes have venom glands in their upper jaws, and lizards in the lower jaws.
It is generally believed that these two groups have developed their venom systems separately from one another. With snakes this is thought to have happened some 60 to 80 million years ago. Because the two species of lizards mentioned have their venom glands in the lower jaw, and since no close connection was ever made between these two groups, it was assumed that lizards had developed their venom system some 100 million years ago, independently of the snakes.
The authors of the article in Nature present two other lizard families with venom glands, the monitor lizards (Varanidae) and the iguana's (Iguania). 'The Commodore Lizard (Varanus komodoensis), for example, is said to have fifty different types of toxic bacteria swarming around in its mouth,' says Freek Vonk. When a lizard bites into his prey, he could pass these bacteria on and the animal could die within a very short time. 'But,' says Vonk, 'that's very unlikely because such bacteria take at least a few days to become effective, whereas the venom starts to affect on the animal within only a few minutes to a few hours.'
The researchers show that all families of snakes and lizards which have venom glands come from the same origin. This means that the venom system in lizards and snakes must also come from the same origin. In order to demonstrate this, four obstacles have to be overcome. Firstly, the problem of the relationship between snakes and lizards. Secondly, the researchers have to determine which groups have venom glands. Thirdly, whether the venom glands are in the upper or lower jaw, and finally what the types of venom are and what their relationship is in evolutionary terms.
By researching the genetic material and the oral glands of snakes and lizards, the authors have found answers to these questions, and they can show that only the four families of lizards which are most closely related to snakes have venom glands. Three groups of lizards have venom glands in the lower jaw, the Helodermatidae, the Anguidae, the species to which the Dutch slow worm belongs, and the Varanidae. The fourth lizard family has venom glands in both the upper and lower jaw. These are the Iguanians, or the lizard types.
There are apparently nine types of venom in these groups of snakes and lizards. The analysis of the venom from the Lace Monitor Lizard Varanus varius shows that this venom has a dramatic effect on the victim's blood pressure, blood clotting and internal bleeding, and quickly causes lthe prey to lose consciousness.
The lizard-like bearded dragon Pogona vitticeps has a venom system in both its upper and lower jaws. 'On the basis of this information, we assume that these four families of lizards and the suborder of snakes are descended from a common ancestor which must have lived some 200 million years ago,' explains Vonk. 'These creatures must have had a venom-producing system in both jaws.' According to this theory, the iguana are then the most primitive, because they have retained the ancestral system of venom glands in the upper and lower jaw. The other three lizard families went on to develop venom glands in the lower jaw, and snakes in the upper jaw.'