Mitchell's water monitor (''Varanus mitchelli'') is a

semiaquatic In biology, being semi-aquatic refers to various macroorganisms that live regularly in both aquatic and terrestrial environments. When referring to animals, the term describes those that actively spend part of their daily time in water (in ...

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

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 ...

in the

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

Varanidae The Varanidae are a family of lizards in the superfamily Varanoidea and order Anguimorpha. The family, a group of carnivorous and frugivorous lizards, includes the living genus '' Varanus'' and a number of extinct genera more closely related ...

. The species is native to Australia. The species is native to the northern regions of Australia and is on the IUCN Red List as a critically endangered species. They can be distinguished by the orange or yellow stripes along their neck and dark spots along their back. They are mainly carnivorous and eat small prey such as lizard, birds and insects.

Etymology

The

specific name Specific name may refer to: * in Database management systems, a system-assigned name that is unique within a particular database In taxonomy, either of these two meanings, each with its own set of rules: * Specific name (botany), the two-part (bino ...

, ''mitchelli'', as well as the common name, Mitchell's water monitor, are in honor of Australian

herpetologist Herpetology (from Ancient Greek ἑρπετόν ''herpetón'', meaning "reptile" or "creeping animal") is a branch of zoology concerned with the study of amphibians (including frogs, salamanders, and caecilians (Gymnophiona)) and reptiles (in ...

Francis John Mitchell Francis John Mitchell (1929–1970) was a biologist and curator with a special interest in herpetology. Active from South Australia, he was vice president of the state's Royal Society and employed at South Australian Museum. His initial positions a ...

(1929–1970) of the

South Australian Museum The South Australian Museum is a natural history museum and research institution in Adelaide, South Australia, founded in 1856 and owned by the Government of South Australia. It occupies a complex of buildings on North Terrace in the cultur ...

.Beolens, Bo; Watkins, Michael; Grayson, Michael (2011). ''The Eponym Dictionary of Reptiles''. Baltimore: Johns Hopkins University Press. xiii + 296 pp. . (''Varanus mitchelli'', p. 179). The classification ''Varanus'' is a Latinization of the Arabic word for monitor lizard, ''waran''. The species (along with other Australian monitor lizard species) is colloquially referred to as a

goanna A goanna is any one of several species of lizard of the genus ''Monitor lizard, Varanus'' found in Australia and Southeast Asia. Around 70 species of ''Varanus'' are known, 25 of which are found in Australia. This varied group of carnivorous r ...

in Australia. The name likely came from the word

iguana ''Iguana'' (, ) is a genus of herbivorous lizards that are native to tropical areas of Mexico, Central America, South America, and the Caribbean. The genus was first described by Austrian naturalist Josephus Nicolaus Laurenti, J.N. Laurenti in ...

as the lizards looked like the iguanas of South America to the first European settlers of Australia.

Description

Mitchell's water monitor reaches a length of up to . It has a long, slender neck and a pointed head. It is generally dark brown or black and has small yellow and white spots covering the limbs and head, and

ocelli A simple eye or ocellus (sometimes called a pigment pit) is a form of eye or an optical arrangement which has a single lens without the sort of elaborate retina that occurs in most vertebrates. These eyes are called "simple" to distinguish the ...

with dark centers as well as yellow stripes along its side.Australia Department of Environment, Parks and Water Security, ''Threatened species of the Northern Territory,'' November 2021.Goannas (lizard family Varanidae)
Komododragon.biz. Retrieved on 2012-08-22. The skin is rough, with many wrinkles. Its underside is a light cream color. Monitor lizards (including Mitchell's water monitor) have excellent eyesight, but are "night blind" in the dark. The lizards' retinas do not have rods, the receptor cells for night vision.

Geographic range

Mitchell's water monitor resides along all northern river systems in the Kimberly Region of

Western Australia Western Australia (WA) is the westernmost state of Australia. It is bounded by the Indian Ocean to the north and west, the Southern Ocean to the south, the Northern Territory to the north-east, and South Australia to the south-east. Western Aust ...

and the

Northern Territory The Northern Territory (abbreviated as NT; known formally as the Northern Territory of Australia and informally as the Territory) is an states and territories of Australia, Australian internal territory in the central and central-northern regi ...

. There is no data to suggest that they live on any islands surrounding Australia.

Habitat

Mitchell's water monitor inhabits

swamp A swamp is a forested wetland.Keddy, P.A. 2010. Wetland Ecology: Principles and Conservation (2nd edition). Cambridge University Press, Cambridge, UK. 497 p. Swamps are considered to be transition zones because both land and water play a role in ...

lagoon A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into ''coastal lagoons'' (or ''barrier lagoons'') an ...

s, inland

river A river is a natural stream of fresh water that flows on land or inside Subterranean river, caves towards another body of water at a lower elevation, such as an ocean, lake, or another river. A river may run dry before reaching the end of ...

s, and other bodies of water and are often found on

tree In botany, a tree is a perennial plant with an elongated stem, or trunk, usually supporting branches and leaves. In some usages, the definition of a tree may be narrower, e.g., including only woody plants with secondary growth, only ...

s near the water. It prefers to climb trees to shelter in tree hollows or under

bark Bark may refer to: Common meanings * Bark (botany), an outer layer of a woody plant such as a tree or stick * Bark (sound), a vocalization of some animals (which is commonly the dog) Arts and entertainment * ''Bark'' (Jefferson Airplane album), ...

. If disturbed, it will head to the water. ''V. mitchelli'' frequently basks on rocks near the water.

Reproduction

Mitchell's water monitor is

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 ...

. It breeds during the dry season, with the females laying eggs between April and June. Clutch sizes have been recorded from 3 to 12. Breeding behaviors are found to be similar to other species of monitor lizards. They live around 10 years.

In captivity

Mitchell's water monitor is reported to be nervous and shy. It is rarely kept in captivity. Average clutch sizes for ''V. mitchelli'' are between 3-11 eggs, though they can lay up to 20 in captivity. They are found to be easily bred in captivity. Mitchell's water monitor grows to an over 2 feet in length and requires a large bioactive enclosure,; recommendations of 8x4x4 ft are not uncommon, especially if attempting to breed. Providing a seasonal environment and food to mimic natural behaviors is said to be the best onset for breeding; however, these requirements can be difficult to achieve and reduces their appearance in captivity.

Diet

Mitchell's water monitor preys on smaller animals of both

terrestrial Terrestrial refers to things related to land or the planet Earth, as opposed to extraterrestrial. Terrestrial may also refer to: * Terrestrial animal, an animal that lives on land opposed to living in water, or sometimes an animal that lives on o ...

and

aquatic Aquatic means relating to water; living in or near water or taking place in water; does not include groundwater, as "aquatic" implies an environment where plants and animals live. Aquatic(s) may also refer to: * Aquatic animal, either vertebrate ...

origin. Its prey includes smaller

lizard Lizard is the common name used for all Squamata, squamate reptiles other than snakes (and to a lesser extent amphisbaenians), encompassing over 7,000 species, ranging across all continents except Antarctica, as well as most Island#Oceanic isla ...

s, small

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

s, nestling

bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...

s, reptile

eggs An egg is an organic vessel in which an embryo begins to develop. Egg, EGG or eggs may also refer to: Biology * Egg cell, the female reproductive cell (gamete) in oogamous organisms Food * Eggs as food Places * Egg, Austria * Egg, Switzerland ...

, and terrestrial

invertebrate Invertebrates are animals that neither develop nor retain a vertebral column (commonly known as a ''spine'' or ''backbone''), which evolved from the notochord. It is a paraphyletic grouping including all animals excluding the chordata, chordate s ...

s (

orthoptera Orthoptera () is an order of insects that comprises the grasshoppers, locusts, and crickets, including closely related insects, such as the bush crickets or katydids and wētā. The order is subdivided into two suborders: Caelifera – gras ...

ns,

arachnid Arachnids are arthropods in the Class (biology), class Arachnida () of the subphylum Chelicerata. Arachnida includes, among others, spiders, scorpions, ticks, mites, pseudoscorpions, opiliones, harvestmen, Solifugae, camel spiders, Amblypygi, wh ...

beetle Beetles are insects that form the Taxonomic rank, order Coleoptera (), in the superorder Holometabola. Their front pair of wings are hardened into wing-cases, elytra, distinguishing them from most other insects. The Coleoptera, with about 40 ...

s, etc.). Aquatic prey include

fish A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...

crab Crabs are decapod crustaceans of the infraorder Brachyura (meaning "short tailed" in Greek language, Greek), which typically have a very short projecting tail-like abdomen#Arthropoda, abdomen, usually hidden entirely under the Thorax (arthropo ...

s and

frogs A frog is any member of a diverse and largely semiaquatic group of short-bodied, tailless amphibian vertebrates composing the order Anura (coming from the Ancient Greek , literally 'without tail'). Frog species with rough skin texture due to ...

. Its diet changes seasonally according to flooding during the wet season.

Conservation

Threats

The current most significant threat to Mitchell's water monitor is the spread of

cane toads The cane toad (''Rhinella marina''), also known as the giant neotropical toad or marine toad, is a large, terrestrial true toad native to South and mainland Central America, but which has been introduced to various islands throughout Oceania ...

across the Northern Territory in Australia. Though

toad Toad (also known as a hoptoad) is a common name for certain frogs, especially of the family Bufonidae, that are characterized by dry, leathery skin, short legs, and large bumps covering the parotoid glands. In popular culture (folk taxonomy ...

s and

frog A frog is any member of a diverse and largely semiaquatic group of short-bodied, tailless amphibian vertebrates composing the order (biology), order Anura (coming from the Ancient Greek , literally 'without tail'). Frog species with rough ski ...

s are a part of their diet, cane toads are poisonous to Mitchell's water monitor and many other water monitor species. Cane toads have become an invasive species in Australia since their introduction to the area in 1935; because of that, Australia is said to be currently facing an overpopulation of cane toads, which presents a problem for monitor lizards that mistake the poisonous amphibians for

endemic Endemism is the state of a species being found only in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also foun ...

toads native to Australia that make up the lizards' usual diet. The geographic range of cane toads completely overlaps the geographic range of Mitchell's water monitor. Not all cane toads are entirely lethal as the smaller ones have non-lethal doses of poison compared to the larger toads; despite this, Mitchell's water monitor and other monitors will go after the larger toads that contain lethal doses. Other threats to Mitchell's water monitor include habitat loss due to land clearing, habitat degradation and change due to

climate change Present-day climate change includes both global warming—the ongoing increase in Global surface temperature, global average temperature—and its wider effects on Earth's climate system. Climate variability and change, Climate change in ...

and deaths occurring from contact with humans (such as death on roads). Mitchell's water monitor have also been reported to be exported live to be sold in global markets.

Status

According to the IUCN Red List, Mitchell's water monitor is currently classified as critically endangered.Shea, G., Woinarski, J. & Cogger, H. 2018. ''Varanus mitchelli''. ''The IUCN Red List of Threatened Species'' 2018: e.T83778268A101752345. . Accessed on 26 September 2022. Its population is decreasing. The decrease of Mitchell's water monitor following the introduction of cane toads to Australia has been estimated to be as high as 97% after only three years. At

Kakadu National Park Kakadu National Park is a protected area in the Northern Territory of Australia, southeast of Darwin. It is a World Heritage Site. Kakadu is also gazetted as a locality, covering the same area as the national park, with 313 people recorded l ...

, almost the entire population of Mitchell's water monitor was taken out by the arrival of cane toads; however, a 2020 survey of the area found some individuals of the species. Despite this, the species has persisted at many habitat spots; however, as sightings are rare, there is no current estimate on the number of individuals in each population of Mitchell's water monitor.

Efforts

Researches are trying to find ways to make monitor lizards avoid cane toads. One current strategy is called

Conditioned Taste Aversion Conditioned taste aversion occurs when an animal acquires an aversion to the taste of a food that was paired with aversive stimuli. The effect explains that the aversion develops more strongly for stimuli that cause nausea than other stimuli. Thi ...

Therapy, or CTA. It is a strategy where an animal associates a certain food with illness; said animal will avoid that food in the future because of the association. Researchers at the

University of Sydney The University of Sydney (USYD) is a public university, public research university in Sydney, Australia. Founded in 1850, it is the oldest university in both Australia and Oceania. One of Australia's six sandstone universities, it was one of the ...

offered small cane toads with a non-lethal amount of poison to monitors in an area where the toads haven't reached yet, and found that the monitors who ate the small toads avoided them in the future. These small toads (or "teacher toads") only make the lizards sick, but it is enough to dissuade them from eating the toads again. Other reports of successful CTA include the use of non-living bait. In order for this prevention strategy to work, scientists would have to identify areas of concentrated biodiversity in order for the results to balance out the cost of the bait. As the invasive cane toad population expands throughout Australia, scientists believe that releasing smaller toads ahead of the larger ones will increase CTA in lizards and prevent them from eating the toads that contain a lethal amount of poison. Other efforts are aimed at protecting the current populations of Mitchell's water monitor at sites already occupied by cane toads as it will be extremely difficult to stop the spread of these invasive toads. The Northern Territory Government has set up the Island Arks program to help the conservation of species affected by the cane toads.Hyatt, Alex. "Battlelines drawn against the cane toad march." 2004.

References

External links

Zipcodezoo.comPhotos at Pbase.com
{{Taxonbar, from=Q2710727 Varanus Monitor lizards of Australia Endemic fauna of Australia Reptiles described in 1958 Taxa named by Robert Mertens