Monitors, Goannas, and Earless Monitors (Varanidae)
Monitors, goannas, and earless monitors
Small to very large lizards with long necks and forked tongues
9–122 in (230–3,100 mm) total length
Number of genera, species
2 genera; about 61 species
Vulnerable: 2 species
Africa, Asia, Southeast Asia, and Australia
Evolution and systematics
The Borneon earless monitor Lanthanotus is the sister group to all Varanus. Recent phylogenetic studies suggest that African monitors are the sister group to all other monitors. There is a large Asian clade and two clades of Australian monitor lizards, one consisting of pygmy monitors and the other of larger species. Monitors may well be more closely related to snakes than to most other lizards.
The varanid lizard body plan has been exceedingly successful as it has been around since the late Cretaceous. Komodo dragons are dwarfed by a closely related, extinct gigantic varanid, Megalania prisca, originally placed in the genus Varanus (to which it must be returned). This Australian Pleistocene species is estimated to have reached more than 20 ft (6 m) in total length and to have weighed over 1,320 lb (600 kg). Megalania fossils have been dated at 19,000–26,000 years before present.
Large monitor lizards evolved large body size in response to availability of very large prey (pygmy elephants in Indonesia for V. komodoensis and rhinoceros-sized diprotodont marsupials in Australia for Megalania). At the end of the Pleistocene, much of the megafauna went extinct. Without large prey to support them, large predators also died out. Fortunately, Varanus komodoensis did not die out when pygmy elephants went extinct at the end of the Pleistocene.
There two living genera: Varanus (about 60 species) and Lanthanotus (1 species). Two subfamilies are recognized: Varanines and Lanthanotines.
Varanus are morphologically conservative but vary widely in size, which makes this genus ideal for comparative studies of the evolution of body size. Small body size has evolved at least twice, once in Australia and another time in an Asian clade. Large body sizes appear to have evolved in several lineages of varanids. These lizards range from the diminutive Australian pygmy monitor Varanus brevicauda (only about 6.7–7.9 in or 17–20 cm in total length and 0.28–0.71 oz or 8–20 g in mass) to Indonesian Komodo dragons (Varanus komodoensis), which attain lengths of 9.9 ft (3 m) and weights of 331 lb (150 kg). The largest living lizard, Varanus komodoensis, was not officially described scientifically as a species until 1912, but Chinese and Dutch travelers must have known of its existence centuries before. Flicking their long forked yellow tongues, surely these great reptiles must have given rise to the very concept of dragons breathing fire. Ancient cartographers marked the Lesser Sunda Islands on their maps of southeastern Asia with an ominous warning: "Here be dragons."
The teeth of most varanids are serrated along the rear edge, which facilitates cutting and tearing skin and flesh of prey as these big lizards pull back on their bite. V. komodoensis routinely kills deer and pigs this way, and one Komodo dragon actually eviscerated a water buffalo. Komodo dragons and Megalania are/were ecological equivalents of large saber-toothed cats, using their slashing bite to disembowel large mammals.
Varanids have more aerobic capacity and a greater metabolic scope than other lizards and range over larger areas. Because of
their body size, large individual monitor lizards retain body heat in their nocturnal retreats and can emerge the next morning with body temperatures well above ambient air temperatures. Their mass confers a sort of "inertial homeothermy," which has implications for understanding the evolution of endothermy.
About 60 species of Varanus are currently recognized worldwide, and all occur in Africa, Asia, Southeast Asia, and Australia (the new world is now sadly impoverished, although fossil varanids are known from North America). The largest adaptive radiations have occurred in Australia, where they are commonly known as "goannas," and where about 30 species are found, including one very interesting clade (subgenus Odatria) that has evolved dwarfism. The single species of Lanthanotus is found only in Sarawak in northern Borneo.
Monitors live in a wide variety of habitats, ranging from man-grove swamps to dense forests to savannas to arid deserts. Some species are aquatic, some semiaquatic, others terrestrial, while still others are saxicolous or semiarboreal or truly arboreal.
Monitor lizards adopt characteristic defensive postures, flattening themselves from side to side and extending their gular pouches to make themselves appear as large as possible. Often they hiss loudly and flick their tongues. Big species lash their tails like whips with considerable accuracy. Some species stand up erect on their hind legs during such displays.
Varanids appear to be much more intelligent than most lizards. At the National Zoo in Washington, D.C., individual Komodo dragons have their own "personalities" and recognize each keeper. 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, opened one at a time allowing monitors to eat each of the 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 could see the next group. Such experiments 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. Such an ability to count probably evolved as a consequence of raiding the nests of reptiles, birds, and mammals, since average clutch or litter size of prey would usually be less than six. A pair of V. niloticus were observed to cooperate when raiding a crocodile nest in Africa. After one monitor lured a female crocodile guarding her nest away into the water, another monitor dug into the nest and began eating eggs and hatchlings. Soon it was joined by its accomplice. Similar observations have been made of monitors raiding bird nests.
Some varanids, such as Varanus niloticus and V. mertensi, are highly aquatic, leaving water only to bask nearby or to dig nest burrows and lay their eggs. Others, such as V. indicus, V. mitchelli, and the so-called water monitor, V. salvator, are more terrestrial but are strong swimmers, very much at home in the
water. Komodo dragons also swim well, which may facilitate colonization of islands. Even desert monitors swim with ease.
Using high-tech radiotelemetry, Auffenberg followed movements and body temperatures of Varanus olivaceus in the Philippines. Surprisingly, he found that this large, rare, previously unstudied, arboreal monitor feeds primarily on fruit. His similar extensive study of Varanus bengalensis, primarily in India and Pakistan, showed that this wide-ranging terrestrial monitor feeds primarily on a wide variety of arthropods (earthworms, crustaceans, snails, many other vertebrates, as well as their eggs and young, are also eaten when available).
One of the most arboreal of all monitor lizards is the beautiful green V. prasinus from New Guinea and Cape York, Australia. These small climbing monitors have strongly prehensile tails and spend most of their lives above ground in trees. Several new species have recently been described that belong in the prasinus group.
In the Australian desert, as many as six or seven species of Varanus occur together in sympatry. All are exceedingly wary, essentially unapproachable, and unobservable lizards. Fortunately, however, they leave fairly conspicuous tracks, and one may deduce quite a lot about their biology from careful study of this spoor. Each species leaves its own distinct track. The largest species, the perentie, V. giganteus, reaches 6.6 ft (2 m) or more in total length, whereas some smaller "pygmy goannas," such as the ubiquitous and very important lizard predator V. eremius, achieve total lengths of only about 15.8 in (40 cm). Two other species, V. gouldii and V. tristis, are intermediate in size. Individuals of all four of these species range over extensive areas and consume very large prey items, particularly other vertebrates (especially lizards). Daily forays typically cover distances of a kilometer or more. V. tristis and two other little-known small species, V. caudolineatus and V. gilleni, which have strongly curved very sharp claws, are semiarboreal. Four other species (V. brevicauda, V. eremius, V. gouldii, and V. giganteus) are terrestrial.
Feeding ecology and diet
Almost all 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 top predators in the communities in which they live. Varanids are very active lizards that forage widely, using their forked tongues extensively to locate and discriminate among their prey by scent (vision and sound are also used).
When foraging, V. gouldii hunt by smell, swinging their long necks and heads from side-to-side, constantly flicking out their long forked, very snake-like, tongues, searching for scent trails and sweeping over as big an arc as possible so that they cover as much ground as possible. Upon detecting a scent signal, these monitors follow the trail to the source, usually a burrow, and dig up the intended prey. Digging is methodical, using the forearms and sharp claws of the forefeet with the pointed snout, mouth, and sharp teeth right in between ever ready to snatch up prey as they dash to escape. Sand monitors consume many geckos captured in their diurnal retreats, dead-end burrows. Many diurnal species of lizards are also eaten (Ctenophorus, Ctenotus, Lerista, Lialis, Menetia, Moloch, Pogona, as well as other Varanus, including V. brevicauda, V. caudolineatus, V. gilleni, and V. gouldii). They also eat reptile eggs, baby mammals, and baby birds. These lizards very probably eat any other lizard that they can catch. Among specimens examined, the largest relative prey mass was a Pogona minor estimated to weigh about 0.88 oz (25 g) eaten by a V. gouldii that weighed 6.3 oz (180 g), or about 13.9% of its weight.
Male monitor lizards engage in ritualized combat, fighting over females. Larger species wrestle in an upright posture, using their tails for support, grabbing each other with their forelegs and attempting to throw their opponent to the ground. Smaller species grapple each other while lying horizontally, with legs wrapped around each other, rolling over and over on the ground. The victor then courts the female, first flicking his tongue all over her, and if she concurs, climbing on top of her and mating by curling the base of his tail beneath hers and inserting one of his two hemipenes into her cloaca. Male varanids have a cartilaginous or bony support structure in each hemipenis called a hemibaculum.
All monitor lizards lay eggs. Clutch sizes vary widely among species from two to three in the smallest monitors such as V. brevicauda to 35–60 in large African species like V. albigularis and V. niloticus. Some monitors (V. rosenbergi, V. niloticus, V. prasinus, and V. varius) excavate termitaria for nesting burrows. Termites close off the entrance, sealing the eggs inside in an almost ideal, protected, humid environment of nearly constant temperature. Sometimes, females return to the same termitarium nine months later to open it and free their hatchlings. In some species of monitor lizards, hatchlings are much more vividly marked and colorful than adults (V. dumerilii, V. griseus, V. rosenbergi, V. tristis).
The IUCN lists two species as Vulnerable, the Indonesian Komodo dragon and the Philippine Gray's monitor. In Africa and Asia, habitat loss and the skin trade pose serious threats to these magnificent lizards. All varanids are on the CITES endangered list, even though some, such as Australian desert species, are not really in any immediate danger of going extinct.
Significance to humans
Monitor comes from the Latin noun "monition," 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 Varanus comes from the Arabic word "waran." Waran is the Egyptian name for the Nile monitor, Varanus niloticus, which basically means "monitor" in Arabic. Large monitor lizards, especially V. salvator, are hunted for their skins, especially in Southeast Asia. The largest of all lizards, Komodo dragons, are popular centerpieces of reptile exhibits at zoos.
List of SpeciesEarless monitor lizard
Earless monitor lizard
Lanthanotus borneensis Steindachner, 1878, Sarawak, Borneo.
other common names
One of the least known of all lizards, earless monitor lizards are medium-sized, with adults averaging 16.5–21.6 in (42–55 cm) total length with a relatively long cylindrical body, long neck, and long tail. They have short legs but long, curved, sharp claws. They can wrap their muscular bodies and prehensile tails around a branch in a manner that suggests that they might climb. Most of their scales are small, but six longitudinal rows of enlarged scales run from the head down the back, and two central rows run out on to the tail. Earless monitor lizard tails do not regenerate. They shed their skin in one piece as do some other anguimorphans and snakes. The brain case is similar to snakes in that it is more solidly encased than it is in varanids. The upper temporal arch has been lost and there is a hinge joint in the middle of the lower jaw, as in snakes. Also, like snakes, these lizards have sharp recurved teeth on their premaxillaries, maxillaries, palatines, pterygoids, and dentaries. Lanthanotus is the only species among anguimorphans with translucent windows in its lower eyelids, which could be a precursor to the "spectacle" covering the eyes of snakes. Like snakes, Lanthanotus (Lanthan = hidden, otus = ear)
have no external ear openings and they have deeply forked tongues. Indeed, the earless monitor lizard could well be closely related to snakes. Viewed from below, males have blunt, rectangular jaws, whereas jaws of females are more pointed. Because of the hemipenes in males, the base of the tail is broader than in females.
These very secretive lizards are found only in lowland riverine regions of Sarawak on northern Borneo (there are unsubstantiated reports from nearby Kalimantan, Indonesia).
These lizards are found along banks of rivers and ponds.
Earless monitor lizards dig burrows in banks along watercourses and retreat into the water when threatened. By some accounts, the earless monitor is aquatic (individuals have been captured in fish seines and traps), by others, it is a burrower. The species does seem to prefer cool moist habitats. A number of these unusual lizards were collected after severe flooding in Sarawak in 1963. They could have been inactive in underground retreats and emerged when it flooded. They may also climb. Some reports of captive lizards suggest that the earless monitor could be nocturnal. In captivity, earless monitors appear to prefer relatively low ambient temperatures of about 75.2–82.4°F (24–28°C). In captivity, these are sluggish lizards that spend most of their time lying in water, seldom moving. Captives shed their skins very infrequently, less than once per year. Such observations could be mere artifacts of the unusual environmental conditions in captivity and could be largely irrelevant to behaviors of free-ranging wild lizards.
feeding ecology and diet
Captives have eaten squid, small bits of fish, earthworms, liver, and even beaten eggs. Earthworm setae have been found in stomachs of museum specimens, but their natural diet remains largely unknown.
Little is known about reproduction in the earless monitor lizard. One lizard was found with six large eggs in September 1976, but these eggs may not have been Lanthanotus eggs. Eggs have never been laid in captivity. Clutch sizes from dissected females is three to four. Eggs are large, about 1.2 in (30 mm) long.
The earless monitor lizard is not threatened, but should be considered gravely endangered as it occurs only in lowland riverine areas of Sarawak that have been dramatically altered by human activity.
significance to humans
As the sister species to varanids, this living fossil may offer hints as to what the ancestors of monitor lizards were like.
Varanus brevicauda Boulenger, 1898, Sherlock River, Nickol Bay, Western Australia.
other common names
English: Short-tailed pygmy monitor.
The short-tailed monitor is the smallest varanid. Adult size is reached at a snout-vent length (SVL) of 3.5–4.3 in (90–110 mm) and a weight of 0.35–0.6 oz (10–17 g). Hatchlings are about 1.8 in (45 mm) SVL and weigh only 0.07–0.1 oz (2–3 g).
Central and Western Australia.
Red sandy desert dominated by spinifex (Triodia) grasses.
Short-tailed monitors are terrestrial, spending most of their time within tussocks of spinifex grass. These small monitors probably climb around within these grass tussocks. Their tails are very muscular and prehensile and they "hang on for dear life" when inside a spinifex grass tussock using their legs as well as their tail. The typical monitor lizard threat posture and behavior has been conserved in the evolution of these diminutive monitors, which hiss and lunge with their throat inflated as if they are a serious threat. This tiny varanid is seldom encountered active above ground; the vast majority of specimens are collected in pit traps. Two were dug up in shallow burrows
during August (one must have been active immediately prior to being exhumed, as crisp, fresh tail lash marks were at the burrow's entrance and the lizard had a body temperature of 95.7°F [35.4°C], 10 degrees above ambient air temperature). Mark-recapture studies show that short-tailed monitors do not move very far. Dozens have been pit trapped on a flat sand-plain covered with large, long unburned, clumps of spinifex, possibly the preferred habitat of this monitor. One female weighing 0.32 oz (9.1 g) contained an adult 0.05-oz (1.5-ml) Ctenotus calurus skink; this prey item constituted 16.5% of the short-tailed monitor's body weight.
feeding ecology and diet
Since these sedentary lizards seldom leave the protective cover of spinifex tussocks, they must forage within tussocks. They eat large insects and, occasionally, small lizards.
In the Great Victoria Desert, the smallest male V. brevicauda with enlarged testes was 3.2 in (82 mm) SVL and the smallest gravid female was 3.7 in (94 mm) SVL. In central Australia, sexual maturity is reached in males at about 2.75 in (70 mm) SVL and in females at about 3.3 in (83 mm) SVL. Males may become reproductive at an age of about 10 months, but females probably do not mature until their second spring at an age of about 22 months. A male fell into the same pit only hours after a female was removed from that pit trap, suggesting that males may follow scent trails to find females. Clutch size is usually two to three eggs, although larger clutches have been reported. Relative clutch masses of two females with oviductal eggs was 16.7% of a female's body weight. Mating occurs in the spring (September–October) and eggs are laid in November. Hatchlings emerge in late January to February and are about 1.6–1.8 in (42–45 mm) SVL and weigh only about 0.07 oz (2 g). Incubation takes about 70–84 days at 64.4–77°F (18–25°C).
significance to humans
As the smallest of all monitors, this diminutive species offers insight into the evolution of small body size. German herpetoculturists have successfully bred this species in captivity.
Varanus caudolineatus Boulenger, 1885, Champion Bay, Western Australia.
other common names
English: Line-tailed pygmy monitor.
Both sexes appear to mature at about 3.6 in (91 mm) SVL, the size of the smallest male with enlarged testes and the smallest gravid female. One specimen with an SVL of 4.4 in (111 mm), estimated to weigh 0.53 oz (15 g), contained an intact 0.1-oz (3-ml) Gehyra (20% of its mass).
Central and interior Western Australia.
The stripe-tailed monitor is semiarboreal, preferring habitats with mulga trees, which offer small hollows that provide the lizards with tight-fitting safe diurnal and nocturnal retreats.
Movements of stripe-tailed monitors marked with a radioactive tracer were not nearly as extensive as movements observed in other species of varanids, suggesting that these pygmy monitors may be fairly sedentary.
feeding ecology and diet
These monitors descend to the ground to forage, evidenced by the fact that three of the 13 active specimens observed were on the ground when first sighted. Moreover, one stomach contained a ground-dwelling Rhynchoedura ornata gecko. Others contained tails and intact Gehyra, arboreal geckos. Gut contents of another sample consisted largely of scorpions and ground-dwelling spiders. These monitors forage on the ground searching for prey by going down into their burrows. V. caudolineatus (and V. gilleni) actually "harvest" the exceedingly fragile tails of geckos that are too large to subdue intact.
The male combat ritual for the stripe-tailed monitor is similar to that of V. gilleni and most other pygmy varanids in the Odatria group. It involves wrestling, longitudinal rolls while embraced with ventral surfaces adjacent, lateral twisting and flexing, and occasional biting on the flank, limbs, and tail. While embraced with both fore and hind limbs, two adversaries form an arch from their snout to their tail.
Mating behavior is similar to that in other varanids. A male will nudge and tongue-flick a female, particularly around the head and neck. Mating occurs when a male lies along the top of a female, using a hind limb and tail to expose her cloaca, enabling him to insert a hemipene. Male testes were largest during July and August, and a female with enlarged oviducts was collected in late December. Smith (1988) reports that a 1.3 oz (37 g) female V. caudolineatus laid four eggs (total mass of 0.3 oz or 9 g) on October 23, 1986. V. caudolineatus appear to oviposit over an extended period. Clutch size is usually three to four eggs.
significance to humans
Varanus komodoensis Ouwens, 1912, Komodo, Indonesia.
other common names
English: Komodo monitor, ora; French: Dragon des Komodos, Varan de Komodo; German: Komodo-Waran; Spanish: Varano de Komodo.
Indonesian Komodo dragons, which attain lengths of 9.9 ft (3 m) and weights of 331 lb (150 kg), are the largest living lizard.
Found only on several Indonesian islands in the Lesser Sunda Island chain, Flores, Rinca, and Komodo.
Hatchling Komodo dragons are arboreal. Juveniles and subadults are both arboreal and terrestrial. Adult individuals are strictly terrestrial; their large body size hinders climbing trees. Komodo dragons have been occasionally observed swimming for short distances, close to mangrove forest.
feeding ecology and diet
Komodo dragons are versatile predators, and they survived to the present by switching their diet to smaller prey such as large reptiles, mammals, and birds. Only relatively recently, humans introduced the deer and wild pigs that constitute the staple diet of large Komodo dragons today. Komodo monitors have secondarily evolved to become ambush predators, lying in wait along trails for large prey such as small deer or wild pigs. When foraging for smaller prey such as mammals and snakes, Komodo dragons forage widely like other monitors. Juvenile Komodos are active, widely foraging predators. The diet includes insects, small to large vertebrates (lizards, snakes, rodents, monkeys, wild boars, deer, and water buffalo), bird and sea turtle eggs, as well as carrion. Deer make up about 50% of the adult diet. Adults also prey upon the young of their own species. Juvenile Komodos are highly arboreal, which may protect them from being eaten by their larger, less agile brethren. These monitors can detect airborne volatile oils released by decomposition of carcasses, which leads lizards on long feeding excursions.
After following Komodos in the field using radiotelemetry for over a year, Auffenberg summed up their ambush thus: "when these animals decide to attack, nothing can stop them." He followed one lizard for 81 days, during which time it made only two verified successful kills. A 110-lb (50-kg) female consumed a 683-lb (310-kg) boar in just 17 minutes. Auffenberg himself was attacked and treed by a "maverick" Komodo dragon.
In captivity, age of first reproduction is reached at about eight or nine years (no data are available for wild specimens). Courtship and mating occur from May to August. Adult males of similar size engage in ritual combats to gain access to females. Using their tails for support, they wrestle in an upright position, grabbing each other with their forelegs, and try to knock down their opponent. In courtship, a male flicks its tongue on the female's snout and then over her body until he reaches her cloaca. He then presses his snout at the base of her tail, scratches her body with his claws, and eventually crawls on her back.
Females deposit eggs in September in burrows located on hill slopes or in nests of megapode birds. These nests consist of heaps of twigs mixed with earth, up to 5 ft (1.5 m) high and 16.4 ft (5 m) in diameter. A hole is dug into the nest and used to lay eggs. Females sometimes lie on the nest for several months, probably guarding their eggs from predators. Eggs hatch in March–April. In captivity, incubation period averages about 220 days. Parental care has not been observed in Komodo dragons. Average clutch size is 18 eggs. The largest clutch recorded is 36 eggs.
Male Komodo dragons tend to grow bulkier and bigger than females. No obvious morphological differences between sexes exist, except for a specific area of precloacal scales. On Komodo island, sex ratio was estimated at three males per female. Hatchlings gain weight rapidly, and after about five years they may weigh 55 lb (25 kg) and have a total length of over 6.6 ft (2 m). Growth continues slowly throughout life. Males reach a larger size than females. Captive records and field observations suggest a longevity of over 30 years.
The IUCN lists the Komodo dragon as Vulnerable due to its small geographic range, small population size, and loss of habitat due to human encroachment. Populations of these spectacular creatures have now been reduced to only a few thousand lizards on a handful of islands. Komodo dragons have recently been successfully bred in captivity.
significance to humans
Blood plasma of Komodo dragons contains powerful antibacterial substances that could be developed as new useful antibiotics in the ongoing worldwide battle against the evolution of antibiotic-resistant microbes. These giant lizards are often the centerpiece of reptile exhibits at zoos.
Varanus salvadorii Peters and Doria, 1878, Southern New Guinea.
other common names
This is an unusual large climbing monitor lizard. Characterized by an extremely long tail and a distinctive arched muzzle and bulbous nose, crocodile monitors could perhaps be allied with the Australian "lace monitor," V. varius, also a large climbing monitor. The largest reliable measurements of individual adult crocodile monitors list snout to vent lengths of 30.7–33.5 in (78–85 cm), tails from 60–65 in (153–166 cm), and total lengths of 91–100 in (231–255 cm), although many anecdotal reports of much larger lizards exist.
Southern New Guinea.
This species is found in lowland rainforest.
Crocodile monitors use their long prehensile tails as counterbalances when climbing in the canopy and as effective whips when threatened. Their teeth are very long and fanglike. Ritualized combat, the clinch-bipedal stance phase, courtship, and copulation have been observed in captive crocodile monitors, which have been successfully bred in Germany several times.
feeding ecology and diet
In captivity, these monitors will eat mice, rats, and chickens. In the wild, they probably prey largely on birds.
Hatchlings are large, measuring 20 in (0.5 m) in total length.
significance to humans
New Guinea natives consider this species to be an "evil spirit that climbs trees, walks upright, breathes fire, and kills men."
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Eric R. Pianka, PhD