''Varanus'', commonly known as true monitors, is one of the 11

subgenera In biology, a subgenus ( subgenera) is a taxonomic rank directly below genus. In the International Code of Zoological Nomenclature, a subgeneric name can be used independently or included in a species name, in parentheses, placed between the ge ...

of the genus '' Varanus''. All of its species are found in the

Australasian realm The Australasian realm is one of eight biogeographic realms that is coincident with, but not (by some definitions) the same as, the geographical region of Australasia. The realm includes Australia, the island of New Guinea (comprising Papua Ne ...

, among which are the world's largest lizards. Within this subgenus, the

monophyletic In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria: # the grouping contains its own most recent co ...

clade comprised '' V. panoptes'', '' V. gouldii'', '' V. rosenbergi'', and '' V. spenceri'' are collectively known as sand monitors.

Description

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. 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 The perentie (''Varanus giganteus'') 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 ar ...

, 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. 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.

Size

True monitors gave rise to the largest known terrestrial

squamates Squamata (, Latin ''squamatus'', 'scaly, having scales') is the largest order of reptiles; most members of which are commonly known as lizards, with the group also including snakes. With over 11,991 species, it is also the second-largest order ...

ever to exist. Most species are medium-sized among monitors, with adults measuring in length and rarely exceeding weights of . In contrast, modern perenties can grow to lengths of and, although being exceptionally lean among large lizards, still exceeds weights of over , easily making them the largest extant lizard in Oceania. Komodo dragons, doubtlessly the largest extant lizard, has their largest known specimen measuring in length and, with a full stomach of food, weighs an astounding . 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 . This maximum size is surpassed by large snakes like the

green anaconda The green anaconda (''Eunectes murinus''), also known as the giant anaconda, emerald anaconda, common anaconda, common water boa, or southern green anaconda, is a semi-aquatic boa species found in South America and the Caribbean island of Trin ...

(up to in the wild) and the

reticulated python The reticulated python (''Malayopython reticulatus'') is a Pythonidae, python species native to South Asia, South and Southeast Asia. It is the world's List of largest snakes, longest snake, and the list of largest snakes, third heaviest snake. I ...

(up to in captivity), 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 Megalania (''Varanus priscus'') 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, but the fragmentary na ...

, which went extinct in the

Pleistocene The Pleistocene ( ; referred to colloquially as the ''ice age, Ice Age'') is the geological epoch (geology), epoch that lasted from to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fin ...

around 50,000 years ago, measures over in length and weighs more than according to estimates made by Wroe ''et al.'' in 2009, making it easily the largest terrestrial squamate ever. Among all the squamates, this size is only known to be surpassed by the semi-aquatic

boids Boids is an artificial life program, developed by Craig Reynolds in 1986, which simulates the flocking behaviour of birds, and related group motion. His paper on this topic was published in 1987 in the proceedings of the ACM SIGGRAPH confere ...

such as ''

Titanoboa ''Titanoboa'' (; ) is an extinct genus of giant boid (the family that includes all boas and anacondas) snake that lived during the middle and late Paleocene. ''Titanoboa'' was first discovered in the early 2000s by the Smithsonian Tropica ...

'' from the

Paleocene The Paleocene ( ), or Palaeocene, is a geological epoch (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), ...

and the fully aquatic

mosasaurs Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Greek ' meaning 'lizard') are an extinct group of large aquatic reptiles within the family Mosasauridae that lived during the Late Cretaceous. Their first fossil remains were discovered in ...

such as ''

Mosasaurus ''Mosasaurus'' (; "lizard of the Meuse (river), Meuse River") is the type genus (defining example) of the mosasaurs, an extinct group of aquatic Squamata, squamate reptiles. It lived from about 82 to 66 million years ago during the Campanian an ...

'' from the

Maastrichtian The Maastrichtian ( ) is, in the International Commission on Stratigraphy (ICS) geologic timescale, the latest age (geology), age (uppermost stage (stratigraphy), stage) of the Late Cretaceous epoch (geology), Epoch or Upper Cretaceous series (s ...

Ecology

True monitors have eyes with

retinas The retina (; or retinas) is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then proce ...

that are almost entirely composed of

cone cells Cone cells or cones are photoreceptor cells in the retina of the vertebrate eye. Cones are active in daylight conditions and enable photopic vision, as opposed to rod cells, which are active in dim light and enable scotopic vision. Most vertebr ...

, giving them excellent colour vision during daytime but nearly no night vision due to the lack of

rod cells Rod cells are photoreceptor cells in the retina of the eye that can function in lower light better than the other type of visual photoreceptor, cone cells. Rods are usually found concentrated at the outer edges of the retina and are used in pe ...

, which are vital for seeing in low-light environments. Like most lizard, true monitors are

oviparous Oviparous animals are animals that reproduce by depositing fertilized zygotes outside the body (i.e., by laying or spawning) in metabolically independent incubation organs known as eggs, which nurture the embryo into moving offsprings kno ...

, and some are capable of

parthenogenesis Parthenogenesis (; from the Greek + ) is a natural form of asexual reproduction in which the embryo develops directly from an egg without need for fertilization. In animals, parthenogenesis means the development of an embryo from an unfertiliz ...

. They have been occasionally seen guarding their nests. Perentie at Sydney Wildlife World

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 The lungs are the primary organs of the respiratory system in many animals, including humans. In mammals and most other tetrapods, two lungs are located near the backbone on either side of the heart. Their function in the respiratory syste ...

and systemic sides of the circulatory system during

systole Systole ( ) is the part of the cardiac cycle during which some chambers of the heart contract after refilling with blood. Its contrasting phase is diastole, the relaxed phase of the cardiac cycle when the chambers of the heart are refilling ...

. 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.Pianka, E.R., Vitt, L.J. (2003). ''Lizards: Windows to the Evolution of Diversity.'' California. University of California Press. 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

is the fastest of all monitors with a recorded speed of up to or , rivalling the aquatic leatherback turtle as the fastest extant reptile. The

Komodo dragon The Komodo dragon (''Varanus komodoensis''), also known as the Komodo monitor, is a large reptile of the monitor lizard family Varanidae that is endemic to the Indonesian islands of Komodo (island), Komodo, Rinca, Flores, Gili Dasami, and Gili ...

is significantly slower, reaching speeds of just , although that is enough for it to chase goats and deers, which it mostly preys on. Being the largest true monitor,

is yet slower, with its estimated top speed no more than . Varanoid skulls

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 An apex predator, also known as a top predator or superpredator, is a predator at the top of a food chain, without natural predators of its own. Apex predators are usually defined in terms of trophic dynamics, meaning that they occupy the hi ...

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.

, being the most massive terrestrial reptile back then, would have competed with predators like ''

Thylacoleo ''Thylacoleo'' ("pouch lion") is an extinct genus of carnivorous marsupials that lived in Australia from the late Pliocene to the Late Pleistocene (until around 40,000 years ago), often known as marsupial lions. They were the largest and last mem ...

'' to prey on

marsupials Marsupials are a diverse group of mammals belonging to the infraclass Marsupialia. They are natively found in Australasia, Wallacea, and the Americas. One of marsupials' unique features is their reproductive strategy: the young are born in a ...

as big as '' Procoptodon'' and even the ''

Diprotodon ''Diprotodon'' (Ancient Greek: "two protruding front teeth") is an extinct genus of marsupial from the Pleistocene of Australia containing one species, ''D. optatum''. The earliest finds date to 1.77 million to 780,000 years ago but most speci ...

'', the largest of them that ever existed. Whether true monitors carry

toxin A toxin is a naturally occurring poison produced by metabolic activities of living cells or organisms. They occur especially as proteins, often conjugated. The term was first used by organic chemist Ludwig Brieger (1849–1919), derived ...

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. 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. 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 An invasive species is an introduced species that harms its new environment. Invasive species adversely affect habitats and bioregions, causing ecological, environmental, and/or economic damage. The term can also be used for native spec ...

. Among them, desert-dwelling true monitors ( perenties, sand goannas, Argus monitors, Rosenberg's monitors and Spencer's goannas) all remain a status of

least concern A least-concern species is a species that has been evaluated and categorized by the International Union for Conservation of Nature (IUCN) as not being a focus of wildlife conservation because the specific species is still plentiful in the wil ...

on the

IUCN Red List The International Union for Conservation of Nature (IUCN) Red List of Threatened Species, also known as the IUCN Red List or Red Data Book, founded in 1964, is an inventory of the global conservation status and extinction risk of biological ...

with their populations large and stable.

Taxonomy

The genus '' Varanus'' is believed to have originated in South Asia in the

Miocene epoch The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recent" ...

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 The Bengal monitor (''Varanus bengalensis''), also called the Indian monitor, is a species of monitor lizard distributed widely in the Indian subcontinent, as well as parts of Southeast Asia and West Asia. Description The Bengal monitor can ...

. They then spread across the

Old World The "Old World" () is a term for Afro-Eurasia coined by Europeans after 1493, when they became aware of the existence of the Americas. It is used to contrast the continents of Africa, Europe, and Asia in the Eastern Hemisphere, previously ...

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 ''Odatria'', commonly known as dwarf monitors, consists of small monitor lizards found in Australia and Indonesia. Species in this subgenus include the smallest monitor species in the world, the tiny 16 gram Dampier Peninsula monitor, but also i ...

'', '' Papusaurus'', '' Phillipinosaurus'', '' Solomonsaurus'', '' Soterosaurus'' and ''Varanus'' moved east towards Southeast Asia, quickly diversifying into a variety of different niches. Subgenus ''Papusaurus'', with their only extant species being the

crocodile monitor The crocodile monitor (''Varanus salvadorii''), also known as the Papuan monitor or Salvadori's monitor, is a species of monitor lizard Endemism, endemic to New Guinea. It is the largest monitor lizard in New Guinea and is one of the List of lar ...

, are considered the closest relatives of true monitors. Around 15 million years ago, a land bridge connecting

Indochina Mainland Southeast Asia (historically known as Indochina and the Indochinese Peninsula) is the continental portion of Southeast Asia. It lies east of the Indian subcontinent and south of Mainland China and is bordered by the Indian Ocean to th ...

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 ''Thylacinus'' is a genus of extinct carnivorous marsupials in the family Thylacinidae. The only recent member was the thylacine The thylacine (; binomial name ''Thylacinus cynocephalus''), also commonly known as the Tasmanian tiger or ...

'' and ''

''. During the Miocene, Komodo dragons hybridized with the common ancestor of ''V. panoptes'', ''V. gouldii'', ''V. rosenbergi'', and ''V. spenceri'', supporting fossil evidence that Komodo dragons once inhabited Australia.

References

{{taxonbar, from= Q21439252 Varanus