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Varanoid Lizards of the World
By Eric R. Pianka, Dennis R. King, Ruth Allen King
Indiana University PressCopyright © 2004 Indiana University Press
All rights reserved.
Eric R. Pianka and Dennis R. King
In Latin, "monere" means "to warn." The word "monitor" comes from the Latin noun "monitio," which means someone who is a warner. According to ancient belief, these lizards were supposed to warn people that crocodiles were in an area. The generic name Yaranus comes from the Arabic word "waran" the Egyptian name for the Nile monitor Yaranus niloticus, which basically means "monitor" in Arabic. Many large varanids commonly adopt a bipedal stance in which the lizard raises itself vertically and supports itself on its hind legs and tail. Because a lizard then has a clear and elevated view of its surroundings, this upright posture may have given rise to the name "monitor" (Fig. 1.1).
Monitor lizards (genus Yaranus) have attracted a great deal of interest; these large and impressive lizards are often the centerpieces of reptile house exhibits. Around the world, dedicated varanophiles keep and breed these magnificent lizards in captivity. Monitors tend to be fairly wary and difficult to observe; therefore, they are not particularly tractable research subjects, but they have nevertheless received an extraordinary amount of attention from devoted students. Often, each scrap of information about these lizards requires perseverance and hard work or simply a substantial amount of luck. Yaranus enthusiasts tend to cooperate freely and exchange data among themselves. This has been emphatically demonstrated again and again during production of this book, as we have received willing assistance from monitor enthusiasts worldwide. The founder of varanid systematics was Robert Mertens, most of whose work was published in German. He wrote in 1942, "Since I — about 30 years ago — got my first living Nile monitor and became acquainted with his life habits in a terrarium, the monitor lizards have fascinated me all the time, these 'proudest, best-proportioned, mightiest and most intelligent lizards as Werner strikingly called them" (Mertens 1942, p. 4). Two varanid symposia have been held in Germany (Böhme and Horn 1991; Horn and Böhme 1999). Walter Auffenberg of the University of Florida spent many years in the field following monitor lizards, studying their ecology. This dedicated student of varanids published a monumental trilogy of extremely important books containing a vast amount of information about the ecology and behavior of three species: V. komodoensis, V. olivaceus, and V. bengalensis (Auffenberg 1981, 1988, 1994). Two other extremely useful compendia of varanid biology are the books by Bennett (1998) and King and Green (1999).
Fifty-three species of Varanus are now currently recognized worldwide. All occur in Africa, Asia, Southeast Asia, and Australia (the New World is sadly impoverished, although fossil varanids and the sister group to monitors, Helodermatids, are known from North America). A rash of new species from remote Indonesian and Philippine Islands in Southeast Asia has recently been described (cerambonensis, caerulivirens, finschi, melinus, mabitang, macraei, and yuwonoi), several others have been recently elevated to species status (kordensis, spinulosus, and ornatus), and others doubtlessly remain to be described. Some "species" (gouldii, indicus, tristis, and sea laris) actually represent species complexes that will require revision and recognition of new species. The largest adaptive radiations have occurred in Australia, where they are commonly known as "goannas," and where 24 endemic species have been named (several other new Australian species remain to be described). Several Asian species, including V indicus and V. prasinus, have also reached northern Australia. One very interesting Australian clade, the subgenus Odatria, has evolved dwarfism.
"Goanna" is believed to be a corruption of the name "iguana," which belongs to a family of lizards found mainly in North and South America, and the name is used to describe varanids of all sizes in Australia. Some Australians often also use the word incorrectly for large members of another family, the Scincidae.
Similarly, in South Africa in the old Dutch language Afrikaans, monitors are called "leguaan which is related to the German "Leguan" for iguana. Because Australia was also colonized by Dutchmen ("New Holland"), the origin of the South African and the Australian names for monitors may have the same origins.
On the basis of morphological evidence, Mertens (1942, 1958, 1963) recognized 10 subgenera, 5 of which survive today. Ziegler and Böhme (1997) examined hemipeneal structure and revised and extended Mertens's classification, erecting a new subgenus Soterosaurus for V salvator and its subspecies. Two subgenera, Odatria and Varanus, both of which are speciose, have undergone adaptive radiations in Australia. The nine currently recognized subgenera (Böhme 2003) and the species belonging in each are listed in Table 1.1.
Monitor lizards live in a wide variety of habitats, ranging from mangrove swamps to dense forests to savannas to arid deserts. Some species are aquatic, some semiaquatic, others terrestrial, whereas still others are saxicolous (rock dwelling), semiarboreal, or truly arboreal.
The varanid lizard body plan appears to have been exceedingly successful: it has been around since the late Cretaceous, 80 million years ago, as evidenced by the Mongolian fossil Estesia. Varanus are morphologically conservative but vary widely in size, which makes this genus ideal for comparative studies of the evolution of body size. No matter what size a monitor happens to be, it always looks like a monitor! These lizards range from the diminutive Australian pygmy monitor Varanus brevicauda (~17-20 cm in total length and 8-20 g in mass) to Indonesian Komodo dragons (Varanus komodoensis), which attain lengths of 3 m and weights of 150 kg. Komodo monitors, however, are themselves dwarfed by a closely related, extinct, gigantic varanid Varanus priscus (formerly Megalania prisca). This Australian Pleistocene species is estimated to have reached more than 6 m in total length and to have weighed over 600 kg. Varanus priscus fossils have been dated at 19,000 to 26,000 years BP.
Monitors may well be more closely related to snakes than to most other lizards. Many monitor lizards are active predatory species that raid vertebrate nests and eat large vertebrate prey (some smaller species also feed extensively on invertebrates, including centipedes, large insects, earthworms, crustaceans, and snails). Many monitor lizards are the top predators in the communities in which they live.
Varanids are by far the most intelligent of all lizards. At the National Zoo in Washington, D.C., individual Komodo monitors have their own personalities and recognize each keeper. These big lizards exhibit curiosity: one lizard walked up to its keeper and gently climbed up on him before taking a notebook from the keeper's shirt pocket in its mouth! The Komodo then dropped down, put the notebook on the ground, and tongue flicked it, as if to ask, "Why is this of such interest?" Recent experiments on captive V. albigularis by John Phillips at the San Diego Zoo suggest that some varanids can actually count. Lizards were conditioned by feeding them groups of four snails in separate compartments with movable partitions, which were opened one at a time to allow monitors to eat each batch of four snails. Upon finishing the fourth snail, lizards were allowed into another chamber containing four more snails. After such conditioning, one snail was removed from some snail groups. Lizards searched extensively for the missing fourth snail, even when they had access to the next group. Similar experiments with varying numbers of snails showed that these varanids can count up to six, but with groups of snails larger than six, the monitors seemed to stop counting and merely classified them as "lots," eating them all before moving on to the next chamber (King and Green, 1999, p. 43). Such an ability to count probably evolved as a consequence of raiding nests of reptiles, birds, and mammals, because average clutch or litter size would be around six.
Teeth of most varanids are serrated along the rear edge, which facilitates cutting and tearing the skin and flesh of prey as these big lizards pull back on their bite. Varanus komodoensis routinely kill deer and pigs in this way, and one Komodo monitor actually eviscerated a water buffalo (Auffenberg 1981). Varanus komodoensis and the extinct V. priscus are ecological equivalents of large saber-toothed cats, using their slashing bite to disembowel large mammals. V. priscus, which was contemporary with aboriginal humans, probably ate Homo, although strangely, Australian aborigines seem not to have any dreamtime stories about gigantic, fierce, man-eating lizards.
Scientists have recently discovered useful pharmacological agents in varanoid lizards. Venoms of Heloderma are complex mixtures of over a dozen small peptides, neurotransmitters, proteins, and other molecules, which have powerful effects on mammalian physiology (Raufman 1996). Natural selection has invented molecular analogs that mimic important mammalian hormones such as the neurotransmitter serotonin, secretin, and a variety of peptides and proteins. One lowers blood pressure, another regulates insulin release, and still another attacks certain cancers. Yet another, a peptide called gilatide, improves memory in rats and is a candidate for development of a drug to treat Alzheimer's disease. Such molecules could prove to be useful drugs to control hypertension, diabetes, and cancer. One such drug. Exendin-4, derived from Gila monster venom, is currently being evaluated for treatment of type 2 diabetes (Edwards et al. 2001; Seppa 2001). Some, but not all, of these molecules are also found in snake venoms, which tend to be much more toxic.
Although Komodo dragons are not actually considered to be venomous, they virtually are. Their saliva harbors over 50 different strains of bacteria, some of which are highly septic. If a Komodo does not kill its prey outright, its bite introduces germs potent enough to kill its quarry with a massive infection in a few days. When these big lizards fight each other, their bites do not become infected by the bacteria of other dragons, suggesting that Komodo dragons may possess a natural immunity to bacterial infection. These observations prompted an investigation of blood plasma of Komodo dragons. Recent preliminary work by Dr. Gill Diamond at a Medical School in New Jersey has identified a powerful antibacterial agent in the blood plasma of Komodo dragons. It could be developed as a new useful antibiotic in our ongoing worldwide battle against the evolution of antibiotic-resistant microbes (Diamond, personal communication).
To compile this reference book, we have assembled 35 experts on various aspects of the biology of varanoid lizards. These people are of many different nationalities and their writing styles vary (we tried to standardize their treatments as much as possible). We attempt to provide a comprehensive account of virtually everything important known about monitors. We begin with a review of fossil varanoids, followed by a discussion of biogeography and phylogeny. The heart of the book consists of 56 detailed species accounts, most written by people with hands-on field experience with a particular species, although a few had to be based on literature reviews. Species accounts are separated into three sections, one for African species, another for Asian and Southeast Asian species (a few of which reach tropical northern Australia), and finally a section for endemic Australian species, some of which have reached New Guinea. Following this, we proceed to various comparative analyses and a chapter on the art of keeping these lizards healthy and breeding them in captivity, and the book concludes with a new taxonomy for Varanus.
In the process of reporting what is known, we also identify what remains to be learned about these lizards. A great deal is known about some species, but barely anything is known about others. Such a diverse monophyletic group can be exploited both to identify and to understand the actual course of evolution.CHAPTER 2
The Long and Honorable History of Monitors and Their Kin
Ralph E. Molnar
Monitors are the most impressive of lizards. Unlike smaller scincids, iguanids, and agamids that flee if they believe they are noticed, monitors have a deliberate, unafraid demeanor. They are large and they can be fierce — and no one who has had a picnic visitation from a monitor will easily forget their presence. Thus, we are naturally curious about their history and ancestry. Today they are found through most of Africa, southern Asia — including Arabia, Iran, India, Indochina, and Indonesia — New Guinea, and Australia. In the past, monitors and their relatives inhabited central Asia, Europe, and North America, and they ranged into the South Pacific as far as New Caledonia. Their fossil record begins about 90 mya (Turonian), but their history must be longer than that.
The modern varanoid lizards, Varanus, Lanthanotus, and Heloderma, give little clue to the number of forms that existed in the past. This diversity is manifested in the group known as platynotans, which includes varanoids together with several extinct groups. Unlike crocodilians, whose history reveals forms remarkably different from those surviving today, most fossil varanoids would probably be readily recognized as relatives of Varanus (or Heloderma). However, there were also marine forms rather different from the living monitors, although if a modern herpetologist could see these creatures alive, they would be recognized as kin of monitors. The traditional view, supported by some recent phylogenetic work (Lee 1997), is that snakes are derived from platynotan stock.
Excerpted from Varanoid Lizards of the World by Eric R. Pianka, Dennis R. King, Ruth Allen King. Copyright © 2004 Indiana University Press. Excerpted by permission of Indiana University Press.
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