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

semiaquatic In biology, semiaquatic can refer to various types of animals that spend part of their time in water, or plants that naturally grow partially submerged in water. Examples are given below. Semiaquatic animals Semi aquatic animals include: * Ve ...

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

of monitor lizard in the

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

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

. The species is native to Australia. The species is native to the Northern regions of Australia, and is on IUCN's 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, ''mitchelli'', as well as the common name, Mitchell's water monitor, are in honor of Australian

herpetologist Herpetology (from Greek ἑρπετόν ''herpetón'', meaning "reptile" or "creeping animal") is the branch of zoology concerned with the study of amphibians (including frogs, toads, salamanders, newts, and caecilians ( gymnophiona)) and rep ...

Francis John Mitchell (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 cultu ...

.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 Mitchell's Water Monitor, as well as other species of Monitors in Australia, are colloquially referred to as

goanna A goanna is any one of several species of lizards of the genus '' 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 reptiles ranges ...

s 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 in 1768 by Austrian naturalist Josephus Nicolaus Laurenti in his ...

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

Description

''Varanus mitchelli'' 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 (sometimes called a pigment pit) refers to a form of eye or an optical arrangement composed of a single lens and without an elaborate retina such as occurs in most vertebrates. In this sense "simple eye" is distinct from a multi-l ...

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. Varanus lizards, including ''V. mitchelli'' 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 (commonly abbreviated as WA) is a state of Australia occupying the western percent of the land area of Australia excluding external territories. It is bounded by the Indian Ocean to the north and west, the Southern Ocean to th ...

and the

Northern Territory The Northern Territory (commonly abbreviated as NT; formally the Northern Territory of Australia) is an Australian territory in the central and central northern regions of Australia. The Northern Territory shares its borders with Western Aust ...

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

Habitat

''V. mitchelli'' inhabit swamps,

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'') ...

s, inland

river A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of w ...

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, including only woody plants with secondary growth, plants that are ...

s near the water. It prefers to climb trees to shelter in tree hollows or under bark. If disturbed, ''V. mitchelli'' will head to the water. ''V. mitchelli'' frequently basks on rocks near the water.

Reproduction

''Varanus mitchelli'' is oviparous. ''V. Mitchelli'' breed 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 Varanus lizards. They live around 10 years.Shea, G., Woinarski, J. & Cogger, H. 2018. ''Varanus mitchelli''. ''The IUCN Red List of Threatened Species'' 2018: e.T83778268A101752345. https://dx.doi.org/10.2305/IUCN.UK.2018-1.RLTS.T83778268A101752345.en. Accessed on 26 September 2022.

In captivity

The 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. This lizard grows to a over 2ft 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 the best onset for breeding although 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. 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 or near the ground, as opposed to ...

and aquatic origin. Its prey includes: smaller lizards, small mammals, nestling

bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...

s, reptile

eggs Humans and human ancestors have scavenged and eaten animal eggs for millions of years. Humans in Southeast Asia had domesticated chickens and harvested their eggs for food by 1,500 BCE. The most widely consumed eggs are those of fowl, especial ...

and terrestrial

invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...

s ( orthopterans,

arachnid Arachnida () is a class of joint-legged invertebrate animals (arthropods), in the subphylum Chelicerata. Arachnida includes, among others, spiders, scorpions, ticks, mites, pseudoscorpions, harvestmen, camel spiders, whip spiders and vinegar ...

beetle Beetles are insects that form the order Coleoptera (), in the superorder Endopterygota. Their front pair of wings are hardened into wing-cases, elytra, distinguishing them from most other insects. The Coleoptera, with about 400,000 describ ...

s etc.). Aquatic prey include:

fish Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of ...

, crabs and

frogs A frog is any member of a diverse and largely carnivorous group of short-bodied, tailless amphibians composing the order Anura (ανοὐρά, literally ''without tail'' in Ancient Greek). The oldest fossil "proto-frog" '' Triadobatrachus'' is ...

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

Conservation

Threats

The current most significant threat to the Mitchell's Water Monitor is the spread of Cane Toads across the Northern Territory in Australia.

Toad Toad 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. A distinction between frogs and toads is not made in scient ...

s and

frog A frog is any member of a diverse and largely carnivorous group of short-bodied, tailless amphibians composing the order Anura (ανοὐρά, literally ''without tail'' in Ancient Greek). The oldest fossil "proto-frog" ''Triadobatrachus'' is ...

s are a part of ''V. Mitchelli's'' diet, though the Cane Toads are poisonous to many species of Water Monitors, including ''V. Mitchelli''. Cane Toads have become an invasive species in Australia since their introduction to the area in 1935, and Australia is currently facing an overpopulation of Cane Toads, which presents a problem for the species who mistake the poisonous Cane Toads for

endemic Endemism is the state of a species being found 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 found else ...

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

climate change In common usage, climate change describes global warming—the ongoing increase in global average temperature—and its effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes to ...

, and deaths occurring from contact with humans, such as death on roads. ''V. mitchelli'' have also been reported to be exported live to be sold in global markets.

Status

Mitchell's Water Monitor is currently classified as critically endangered according to IUCN's Red List.Shea, G., Woinarski, J. & Cogger, H. 2018. ''Varanus mitchelli''. ''The IUCN Red List of Threatened Species'' 2018: e.T83778268A101752345. https://dx.doi.org/10.2305/IUCN.UK.2018-1.RLTS.T83778268A101752345.en. Accessed on 26 September 2022. Its population is decreasing. The decrease of ''V. mitchelli'' following the introduction of the Cane Toads 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 liv ...

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

Efforts

Researches are trying to find ways to make monitor lizards avoid the 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 Garcia effect is that the aversion develops more strongly for stimuli that cause nausea than other stimuli. Th ...

Therapy, or CTA. This is where the animal associates a certain food with illness. The animal will avoid this food in the future because of the association. Researchers at the

University of Sydney The University of Sydney (USYD), also known as Sydney University, or informally Sydney Uni, is a public research university located in Sydney, Australia. Founded in 1850, it is the oldest university in Australia and is one of the country's ...

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 cane toads march across 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 cane 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

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