| Varanus (Varanus) Temporal range: | |
|---|---|
 | |
| Komodo dragon in Indonesia | |
Scientific classification  | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Order: | Squamata |
| Family: | Varanidae |
| Genus: | Varanus |
| Subgenus: | Varanus Ouwens, 1912 [1] |
| Species | |
Varanus, commonly known as true monitors, is one of the 11 subgenera of the genus Varanus . All of its species are found in the Australasian realm, among which are the world's largest lizards. [3]
True monitors are characterized by their wide skulls and strong jaws, with the nostrils cranially positioned on the sides of their snout. Their teeth are curved, serrated and concealed by thick lips, making them invisible even if their mouths are open. Their tongues are forked like a snake's, which they use to pick up scent. Like all monitors, they have slender, elongated necks. The tails are heavy and muscular, being thick at the base and laterally compressed towards the end. [4] Most true monitors have lean bodies with long tails that can take up over half of their entire body length, but the largest species are very robust and have proportionally short tails. The perentie, however, has exceptionally long limbs with their bodies lifted high above the ground, giving them astonishing maneuverability and allowing them to truly run on all fours. [5] True monitors are usually born with thick stripes of highly contrasting colours on their bodies, which would slowly morph into the loose speckles or stripes of an adult. The tail usually remains striped into adulthood, and many members have a long section at the end of their tails being white or lightly coloured, which is especially common in Australian species.
True monitors gave rise to the largest known terrestrial squamates ever to exist. Most species are medium-sized among monitors, with adults measuring 1 to 2 m (3 ft 3 in to 6 ft 7 in) in length and rarely exceeding weights of 10 kg (22 lb). [6] In contrast, modern perenties can grow to lengths of 2 m (6 ft 7 in) and, although being exceptionally lean among large lizards, still exceeds weights of over 17 kg (37 lb), easily making them the largest extant lizard in Oceania. [7] Komodo dragons, doubtlessly the largest extant lizard, has their largest known specimen measuring 3.13 m (10.3 ft) in length and, with a full stomach of food, weighs an astounding 166 kg (366 lb). [8] Because Komodo monitors can eat up to 60% their own body weight, this weight record is considered invalid, while other specimens suggest a maximum weight range of 90 to 100 kg (200 to 220 lb). This maximum size is surpassed by large snakes like the green anaconda (up to 97.5 kg (215 lb) in the wild [9] ) and the reticulated python (up to 150 kg (330 lb) in captivity [10] ), but considering that these exceptionally large specimens are reported at a much lower frequency, it is still arguable that the Komodo dragon is the largest extant squamate on Earth. Megalania, which went extinct in the Pleistocene around 50,000 years ago, measures over 5.5 m (18 ft) in length and weighs more than 575 kg (1,268 lb) according to estimates made by Wroe et al. in 2009, making it easily the largest terrestrial squamate ever. [11] Among all the squamates, this size is only known to be surpassed by the semi-aquatic boids such as Titanoboa from the Paleocene and the fully aquatic mosasaurs such as Mosasaurus from the Maastrichtian.
True monitors have eyes with retinas that are almost entirely composed of cone cells, giving them excellent colour vision during daytime but nearly no night vision due to the lack of rod cells, which are vital for seeing in low-light environments. Like most lizard, true monitors are oviparous, and some are capable of parthenogenesis. [12] They have been occasionally seen guarding their nests.
While most reptiles are considered to have three-chambered hearts, the hearts of true monitors – as with those of other monitors – have a well developed ventricular septum that completely separates the pulmonary and systemic sides of the circulatory system during systole. [13] This allows their hearts to temporarily function like a four chambered heart, which in turn ensure that oxygenated blood is quickly distributed to the body without also flooding the lungs with high-pressure blood. [14] The highly efficient circulatory system, combined with their strong limbs, allow true monitors to become powerful sprinters. However, the primitive sprawling limbs would limit both the speed and stamina of the larger species. The perentie is the fastest of all monitors with a recorded speed of up to 10 m/s (33 ft/s) or 36 km/h (22 mph), rivalling the aquatic leatherback turtle as the fastest extant reptile. [5] The Komodo dragon is significantly slower, reaching speeds of just 5–6 m/s (16–20 ft/s), although that is enough for it to chase goats and deers, which it mostly preys on. [15] Being the largest true monitor, Megalania is yet slower, with its estimated top speed no more than 3 m/s (9.8 ft/s). [16]
True monitors are most well known by their maximum size, but what really distinguishes them from other monitors is their great evolutionary potential to kill large prey in relation to their own body size, allowing them to occupy the niche of apex predators in most of their biomes. As the ruler of modern Australia, perenties have to prey on other lizards nearly their own size since birth, and the species adapted by developing an unusually large head-to-body ratio. Komodo dragons are the only extant lizards to hunt large mammals, regularly preying on water buffalos that are even heavier than them. Megalania, being the most massive terrestrial reptile back then, would have competed with predators like Thylacoleo to prey on marsupials as big as Procoptodon and even the Diprotodon , the largest of them that ever existed.
Whether true monitors carry toxin is a long debated topic. Experiments have shown that the saliva of Komodo dragons are not considerably different compared to other predators, thus dispelling the theory that monitors kill with bacteria. [17] It is verified that like all monitors or possibly all lizards, true monitors have special glands in their jaws, which is almost certainly a homologous feature for these reptiles. However, not all agree that these can be called venom glands, especially since the effect of them are too mild for prey to most lizard and especially true monitors, who often hunt prey that are simply too large to rely on venom for a kill. [18]
True monitors live in Australia alongside the much smaller and more diverse dwarf monitors. Their dominance was challenged but remained stable in the face of invasive species. Among them, desert-dwelling true monitors (the Perentie, the Sand goanna, the Argus monitor, the Rosenberg's monitor and the Spencer's goanna) all remain a status of least concern on the IUCN Red List with their populations large and stable. [5]
The genus Varanus is believed to have originated in South Asia in the Miocene epoch around 20 million years ago, and the anatomy of its earliest members are thought to resemble today's Indian group, which includes modern Yellow monitors (subgenus Empagusia ) like the Bengal monitor. They then spread across the Old World in two directions: the Afro-Arabian group, including the subgenera Polydaedalus and Psammosaurus moved west towards West Asia and Africa, developing tall skulls and round teeth which aid in crushing shelled prey. Meanwhile, the Indo-Australian group, including subgenera Euprepiosaurus , Hapturosaurus , Odatria , Papusaurus , Phillipinosaurus , Solomonsaurus , Soterosaurus and Varanus moved east towards Southeast Asia, quickly diversifying into a variety of different niches. [19] Subgenus Papusaurus, with their only extant species being the Crocodile monitor, are considered the closest relatives of true monitors. Around 15 million years ago, a land bridge connecting Indochina and Australia appeared, allowing the ancestors of today's true monitors to enter Oceania. They then produced members with colossal sizes such as early Komodo dragons and Megalania, possibly by outcompeting large local marsupial predators such as Thylacinus and Thylacoleo . [3]
The Komodo dragon, also known as the Komodo monitor, is a member of the monitor lizard family Varanidae that is endemic to the Indonesian islands of Komodo, Rinca, Flores, and Gili Motang. It is the largest extant species of lizard, growing to a maximum length of 3 m (9.8 ft), and weighing up to 70 kg (150 lb).
Lizard is the common name used for all squamate reptiles other than snakes, encompassing over 7,000 species, ranging across all continents except Antarctica, as well as most oceanic island chains. The grouping is paraphyletic as some lizards are more closely related to snakes than they are to other lizards. Lizards range in size from chameleons and geckos a few centimeters long to the 3-meter-long Komodo dragon.
Squamata is the largest order of reptiles, comprising lizards and snakes. With over 11,500 species, it is also the second-largest order of extant (living) vertebrates, after the perciform fish. Members of the order are distinguished by their skins, which bear horny scales or shields, and must periodically engage in molting. They also possess movable quadrate bones, making possible movement of the upper jaw relative to the neurocranium. This is particularly visible in snakes, which are able to open their mouths very wide to accommodate comparatively large prey. Squamates are the most variably sized living reptiles, ranging from the 16 mm (0.63 in) dwarf gecko to the 6.5 m (21 ft) reticulated python. The now-extinct mosasaurs reached lengths over 14 m (46 ft).
Monitor lizards are lizards in the genus Varanus, the only extant genus in the family Varanidae. They are native to Africa, Asia, and Oceania, and one species is also found in the Americas as an invasive species. About 80 species are recognized.
Megalania is an extinct species of giant monitor lizard, part of the megafaunal assemblage that inhabited Australia during the Pleistocene. It is the largest terrestrial lizard known to have existed, reaching an estimated length of 3.5 to 7 metres, and weighing between 97–1,940 kg (214–4,277 lb), but the fragmentary nature of known remains make estimates highly uncertain.
The term Australian megafauna refers to the megafauna in Australia during the Pleistocene Epoch. Most of these species became extinct during the latter half of the Pleistocene, and the roles of human and climatic factors in their extinction are contested.
A goanna is any one of several species of lizard of the genus Varanus found in Australia and Southeast Asia.
The Nile monitor is a large member of the monitor family (Varanidae) found throughout most of Sub-Saharan Africa, particularly in drier regions, and along the Nile River and its tributaries in East Africa. Additionally, there are modern, invasive populations in North America. The population found in West African forests and savannahs is sometimes recognized as a separate species, the West African Nile monitor. While it is dwarfed by its larger relatives, such as the Komodo dragon, the Asian water monitor or the crocodile monitor, it is still one of the largest lizards in the world, reaching Australia’s perentie in size. Other common names include the African small-grain lizard, as well as iguana and various forms derived from it, such as guana, water leguaan or river leguaan.
The perentie is a species of monitor lizard. It is one of the largest living lizards on earth, after the Komodo dragon, Asian water monitor, and the Crocodile monitor. Found west of the Great Dividing Range in the arid areas of Australia, it is rarely seen, because of its shyness and the remoteness of much of its range from human habitation. The species is considered to be a least-concern species according to the International Union for Conservation of Nature.
Toxicofera is a proposed clade of scaled reptiles (squamates) that includes the Serpentes (snakes), Anguimorpha and Iguania. Toxicofera contains about 4,600 species, of extant Squamata. It encompasses all venomous reptile species, as well as numerous related non-venomous species. There is little morphological evidence to support this grouping; however, it has been recovered by all molecular analyses as of 2012.
The desert monitor is a species of monitor lizard of the order Squamata found living throughout North Africa and Central and South Asia. The desert monitor is carnivorous, feeding on a wide range of vertebrates and invertebrates.
The Asian water monitor is a large varanid lizard native to South and Southeast Asia. It is one of the most common monitor lizards in Asia, ranging from coastal northeast India, Bangladesh, Sri Lanka, mainland Southeast Asia, and southern China to Indonesian islands where it lives close to water. It is listed as Least Concern on the IUCN Red List. It was described by Laurenti in 1768 and is among the largest squamates in the world.
The lace monitor, also known as the tree goanna, is a member of the monitor lizard family native to eastern Australia. A large lizard, it can reach 2 metres (6.6 ft) in total length and 14 kilograms (31 lb) in weight. The lace monitor is considered to be a least-concern species according to the International Union for Conservation of Nature.
The sand goanna, also known commonly as Gould's monitor, the racehorse goanna, and the sand monitor, is a species of large Australian monitor lizard in the family Varanidae.
The crocodile monitor, also known as the Papuan monitor or Salvadori's monitor, is a species of monitor lizard endemic to New Guinea. It is the largest monitor lizard in New Guinea and is one of the longest lizards, verified at up to 255 cm (100 in). Its tail is exceptionally long, with some specimens having been claimed to exceed the length of the Komodo dragon, however less massive.
Varanoidea is a superfamily of lizards, including the well-known family Varanidae. Also included in the Varanoidea are the Lanthanotidae, and the extinct Palaeovaranidae.
The Anguimorpha is a suborder of squamates. The group was named by Fürbringer in 1900 to include all autarchoglossans closer to Varanus and Anguis than Scincus. These lizards, along with iguanians and snakes, constitute the proposed "venom clade" Toxicofera of all venomous reptiles.
Hapturosaurus, sometimes known as the tree monitors, is a subgenus of lizards, consisting of slender-bodied arboreal monitor lizards mostly found in the tropical rainforests of Indonesia and Papua New Guinea.
Polydaedalus, commonly referred as African monitors, is one of the 11 subgenera of the genus Varanus. Its species are native to Africa and West Asia, among which are some of the World's largest lizards.