Varanus (Hapturosaurus)
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''Hapturosaurus,'' sometimes known as the tree monitors, is a
subgenus 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 lizards, consisting of slender-bodied
arboreal Arboreal locomotion is the locomotion of animals in trees. In habitats in which trees are present, animals have evolved to move in them. Some animals may scale trees only occasionally (scansorial), but others are exclusively arboreal. The hab ...
monitor lizard 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 West African Nile monitor, one species is also found in south America as an invasive species. A ...
s mostly found in the tropical rainforests of
Indonesia Indonesia, officially the Republic of Indonesia, is a country in Southeast Asia and Oceania, between the Indian Ocean, Indian and Pacific Ocean, Pacific oceans. Comprising over List of islands of Indonesia, 17,000 islands, including Sumatra, ...
and
Papua New Guinea Papua New Guinea, officially the Independent State of Papua New Guinea, is an island country in Oceania that comprises the eastern half of the island of New Guinea and offshore islands in Melanesia, a region of the southwestern Pacific Ocean n ...
.


Taxonomy

The type species is the green tree monitor, originally designated by
Hermann Schlegel Hermann Schlegel (10 June 1804 – 17 January 1884) was a German ornithologist, herpetologist and ichthyologist. Early life and education Schlegel was born at Altenburg, the son of a brassfounder. His father collected butterflies, which stimulated ...
in 1844 as ''Monitor prasinus''.
Robert Mertens Robert Friedrich Wilhelm Mertens (1 December 1894 – 23 August 1975) was a German herpetologist. Several taxa of reptiles are named after him.Beolens, Bo; Watkins, Michael; Grayson, Michael (2011). ''The Eponym Dictionary of Reptiles''. Baltimore ...
placed tree monitors within the subgenus ''
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 ...
'' in 1942. In 1988, tree monitors were instead placed within '' Euprepriosaurus'' alongside the mangrove monitors. Nevertheless, there was a distinction between mangrove and tree monitors that was clear even then, so ''Euprepriosaurus'' was commonly considered to consist of two species complexes, i.e., the '' V. indicus'' complex and the '' V. prasinus'' complex. In 2016, Yannick Bucklitsch, Wolfgang Böhme, and André Koch found the two species complexes sufficiently morphologically, ecologically, and biologically distinct, and so all species within the ''V. prasinus'' complex were moved under a newly erected subgenus, i.e., ''Hapturosaurus''. ''Hapturosaurus'' diverged from ''Euprepriosaurus'' during the late
Miocene The Miocene ( ) is the first epoch (geology), 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 mea ...
.


Etymology

The name ''Hapturosaurus'' is derived from the Greek words "''haptein"'' (to grasp), "''ouros''" (tail), and "''sauros"'' (lizard), in reference to the
prehensile Prehensility is the quality of an appendage or organ that has adapted for grasping or holding. The word is derived from the Latin term ''prehendere'', meaning "to grasp". The ability to grasp is likely derived from a number of different origin ...
tails of tree monitors.


Ecology


Behaviour

All species are highly arboreal. In captivity, tree monitors demonstrate the capacity to play, in the form of destructive behaviour such as systematically shredding the leaves on plants with teeth and claws. Play-like behaviour is also documented in other less closely related monitor species, such as Komodo dragons.


Feeding habits

Tree monitors are primarily insectivorous, but also consume other small invertebrates such as spiders, or occasionally small mammals, lizards or the nestlings and eggs of birds. In captivity they are occasionally seen eating plants although the gut contents of wild individuals were not reported to contain plant matter. This may however be a result of accidentally ingesting leaves when playing with them, which is documented in captivity. Captive hatchling often refuse food for more than two weeks, although force feeding may be recommended before then and until they begin feeding by themselves. Like other monitor lizards, tree monitors are highly intelligent reptiles, but tree monitors demonstrate particularly complex problem solving abilities, fine motor coordination, and skilled forelimb movements when hunting prey. When they cannot reach prey in tight crevices and holes with their jaws, they instead extract prey by reaching for it with their forelimbs—which are some of the longest and most slender forelimbs of all monitors and end in elongated digits tipped with large claws and adhesive soles—and hooking it out with its claws, allowing them to exploit a wider range of niches. It is recommended that in captivity this behaviour be encouraged by placing food items in places that are only accessible by reaching for them with their forelimbs, as a form of enrichment.


Range

Most species are
allopatric Allopatric speciation () – also referred to as geographic speciation, vicariant speciation, or its earlier name the dumbbell model – is a mode of speciation that occurs when biological populations become geographically isolated from ...
and confined to single islands in
Indonesia Indonesia, officially the Republic of Indonesia, is a country in Southeast Asia and Oceania, between the Indian Ocean, Indian and Pacific Ocean, Pacific oceans. Comprising over List of islands of Indonesia, 17,000 islands, including Sumatra, ...
and
Papua New Guinea Papua New Guinea, officially the Independent State of Papua New Guinea, is an island country in Oceania that comprises the eastern half of the island of New Guinea and offshore islands in Melanesia, a region of the southwestern Pacific Ocean n ...
, with the exception of the
canopy goanna ''Varanus keithhornei'', commonly known as the canopy goanna, Keith Horne's monitor, blue-nosed tree monitor, or Nesbit River monitor, is a species of monitor lizards native to northeast Australia. It is a member of the '' Varanus prasinus'' sp ...
which is endemic to northeast
Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...
and the green tree monitor which has a much wider distribution than other tree monitors. The green tree monitor has even been purportedly sighted on the
Cape York Peninsula The Cape York Peninsula is a peninsula located in Far North Queensland, Australia. It is the largest wilderness in northern Australia.Mittermeier, R.E. et al. (2002). Wilderness: Earth's last wild places. Mexico City: Agrupación Sierra Madre, ...
on multiple instances, which if true would make it
sympatric In biology, two closely related species or populations are considered sympatric when they exist in the same geographic area and thus frequently encounter each other. An initially interbreeding population that splits into two or more distinct spe ...
with the canopy goanna. Their colonization of the
Indo-Pacific The Indo-Pacific is a vast biogeographic region of Earth. In a narrow sense, sometimes known as the Indo-West Pacific or Indo-Pacific Asia, it comprises the tropical waters of the Indian Ocean, the western and central Pacific Ocean, and the ...
islands occurred during 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 ...
glacial periods, when lower global sea levels caused the
Sahul Shelf Geology, Geologically, the Sahul Shelf () is a part of the continental shelf of the Australia (continent), Australian continent, lying off the northwest coast of mainland Australia. Etymology The name "Sahull" or "Sahoel" appeared on 17th c ...
to form a continuous land-bridge out of the shallow waters around the
Torres Strait Islands The Torres Strait Islands are an archipelago of at least 274 small islands in the Torres Strait, a waterway separating far northern continental Australia's Cape York Peninsula and the island of New Guinea. They span an area of , but their tot ...
. This allowed the exchange and dispersal of
fauna Fauna (: faunae or faunas) is all of the animal life present in a particular region or time. The corresponding terms for plants and fungi are ''flora'' and '' funga'', respectively. Flora, fauna, funga and other forms of life are collectively ...
between New Guinea and Australia each time the land-bridge formed.


Venom

Like all other monitor lizards, tree monitors possess venom glands in their lower jaws, giving them a noticeably venomous bite. The venom is an
anticoagulant An anticoagulant, commonly known as a blood thinner, is a chemical substance that prevents or reduces the coagulation of blood, prolonging the clotting time. Some occur naturally in blood-eating animals, such as leeches and mosquitoes, which ...
, and has two known mechanisms for disrupting blood clotting: by
fibrinogenolysis Primary fibrinogenolysis is a medical condition that appears with abnormal production of fibrinogen/fibrin degradation products (FDP), degradation of coagulation factors V, VIII, IX, XI and/or degradation of the fibrin present in any pre-existing ...
(the destructive cleavage of
fibrinogen Fibrinogen (coagulation factor I) is a glycoprotein protein complex, complex, produced in the liver, that circulates in the blood of all vertebrates. During tissue and vascular injury, it is converted Enzyme, enzymatically by thrombin to fibrin ...
) and by blocking
platelet Platelets or thrombocytes () are a part of blood whose function (along with the coagulation#Coagulation factors, coagulation factors) is to react to bleeding from blood vessel injury by clumping to form a thrombus, blood clot. Platelets have no ...
aggregation. The venom also causes
hypotension Hypotension, also known as low blood pressure, is a cardiovascular condition characterized by abnormally reduced blood pressure. Blood pressure is the force of blood pushing against the walls of the arteries as the heart pumps out blood and is ...
. Monitor lizard venom is extremely complex and diverse due to the great range of
ecological niche In ecology, a niche is the match of a species to a specific environmental condition. Three variants of ecological niche are described by It describes how an organism or population responds to the distribution of Resource (biology), resources an ...
s that they occupy. Tree monitors have the most potently fibrinogenolytic venoms of all monitor lizards, matched only by the also arboreal banded monitor from the subgenus ''
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 ...
''. This may be because arboreal monitor species experience strong
selection pressure Evolutionary pressure, selective pressure or selection pressure is exerted by factors that reduce or increase reproductive success in a portion of a population, driving natural selection. It is a quantitative description of the amount of change oc ...
to quickly subjugate prey items before they break free and escape by falling out of the trees or flying away. As fibrinogenolytic activity is not painful, the venom likely has a primarily predatory role in subjugating prey, instead of acting as a predator deterrent. It is suggested that higher prey escape potential may cause increased venom potency, as the lethally toxic
eastern green mamba The eastern green mamba (''Dendroaspis angusticeps'') is a highly venomous snake species of the mamba genus ''Dendroaspis'' native to the coastal regions of southern East Africa. Described by Scottish surgeon and zoologist Andrew Smith in 1849, ...
and
boomslang The boomslang ( or ; ''Dispholidus typus'') is a highly venomous snake in the family Colubridae. The species is native to Sub-Saharan Africa. Etymology Its common name means "tree snake" in Dutch and Afrikaans – ''boom'' meaning "tree", and ...
are also arboreal. This may also apply to the
piscivorous A piscivore () is a carnivorous animal that primarily eats fish. Fish were the diet of early tetrapod evolution (via water-bound amphibians during the Devonian period); insectivory came next; then in time, the more terrestrially adapted rept ...
and highly venomous
cone snail Cone snails, or cones, are highly venomous sea snails that constitute the family Conidae. ''Conidae'' is a taxonomic family (previously subfamily) of predatory marine gastropod molluscs in the superfamily Conoidea. The 2014 classification of ...
s, given their highly mobile fish prey. As the venom is an anticoagulant, monitor lizard bites often bleed far more than what the mechanical damage of the bite alone would have induced.


Species


References

{{Taxonbar, from=Q28950988 Hapturosaurus Taxa named by Wolfgang Böhme (herpetologist)