Mitchell's water monitor

Mitchell's water monitor (Varanus mitchelli) is a semiaquatic species of monitor lizard in the family Varanidae. The species is native to Australia.[4] 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.

Mitchell's water monitor
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Squamata
Family: Varanidae
Genus: Varanus
Subgenus: Odatria
Species:
V. mitchelli
Binomial name
Varanus mitchelli

Etymology

The specific name, mitchelli, as well as the common name, Mitchell's water monitor, are in honor of Australian herpetologist Francis John Mitchell (1929–1970) of the South Australian Museum.[5][6] The classification Varanus is a Latinization of the Arabic word for monitor lizard, waran.[7] The Mitchell's Water Monitor, as well as other species of Monitors in Australia, are colloquially referred to as goannas in Australia. The name likely came from the word iguana, 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 70 cm (28 in). 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 with dark centers as well as yellow stripes along its side.[8][2] The skin is rough, with many wrinkles. Its underside is a light cream color.[8]

Varanus lizards, including V. mitchelli have excellent eyesight, but are "night blind" in the dark.[9] The lizards' retinas do not have rods, the receptor cells for night vision.

The Mitchell's Water Monitor has yellow and orange stripes down its sides and neck.

Geographic range

Mitchell's water monitor resides along all northern river systems in the Kimberly Region of Western Australia and the Northern Territory.[8][2][3] There is no data to suggest that they live on any islands surrounding Australia.[8]

Habitat

V. mitchelli inhabit swamps, lagoons, inland rivers, and other bodies of water and are often found on trees near the water. It prefers to climb trees to shelter in tree hollows or under bark.[4][5] If disturbed, V. mitchelli will head to the water.[4] V. mitchelli frequently basks on rocks near the water.[6]

The geographic range of the Mitchell's Water Monitor.

Reproduction

Varanus mitchelli is oviparous.[10] V. Mitchelli breed during the dry season, with the females laying eggs between April and June.[11] Clutch sizes have been recorded from 3 to 12. Breeding behaviors are found to be similar to other species of Varanus lizards.[11] They live around 10 years.[12]

In captivity

The Mitchell's water monitor is reported to be nervous and shy. It is rarely kept in captivity.[13] Average clutch sizes for V. mitchelli are between 3-11 eggs, though they can lay up to 20 in captivity.[14] They are found to be easily bred in captivity.

[14]

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 and aquatic origin. Its prey includes: smaller lizards, small mammals, nestling birds, reptile eggs and terrestrial invertebrates (orthopterans, arachnids, beetles etc.). Aquatic prey include: fish, crabs and frogs.[15] Its diet changes seasonally according to flooding during the wet season.[16]

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.[17]Toads and frogs are a part of V. Mitchelli's diet, though the Cane Toads are poisonous to many species of Water Monitors, including V. Mitchelli.[17] 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 toads native to Australia that make up the lizards' usual diet.[18] The geographic range of the cane toads completely overlaps the geographic range of V. mitchelli.[8] 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, and deaths occurring from contact with humans, such as death on roads.[7] V. mitchelli have also been reported to be exported live to be sold in global markets.[19]

Status

Mitchell's Water Monitor is currently classified as critically endangered according to IUCN's Red List.[20] Its population is decreasing.[20] 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.[21] At Kakadu National park, 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.[22] 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.[8]

Efforts

Researches are trying to find ways to make monitor lizards avoid the cane toads. One current strategy is called Conditioned Taste Aversion 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 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.[23] 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.[24] 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.[24] 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.[8]

The Northern Territory Government has set up the Island Arks program to help the conservation of species affected by the cane toads.[25]

References
  1. Shea, G.; Woinarski, J.C.Z.; Cogger, H. (2018). "Varanus mitchelli". IUCN Red List of Threatened Species. 2018: e.T83778268A101752345. doi:10.2305/IUCN.UK.2018-1.RLTS.T83778268A101752345.en. Retrieved 18 November 2021.
  2. Goannas (lizard family Varanidae). Komododragon.biz. Retrieved on 2012-08-22.
  3. "GBIF.org". Retrieved 3 July 2010.
  4. Böhme, Wolfgang [in German] (2003), Checklist of the living lizards of the world (family Varanidae), Nationaal Natuurhistorisch Museum, p. 32, ISBN 978-90-73239-86-9
  5. Pianka, Eric R.; King, Dennis; King, Ruth Allen (2004). Varanoid lizards of the world. Indiana University Press. ISBN 978-0-253-34366-6. Retrieved 2010-07-04.
  6. Beolens, Bo; Watkins, Michael; Grayson, Michael (2011). The Eponym Dictionary of Reptiles. Baltimore: Johns Hopkins University Press. xiii + 296 pp. ISBN 978-1-4214-0135-5. (Varanus mitchelli, p. 179).
  7. Macdonald, Stewart (2007). "Mitchell's Water Monitor". AROD.guide. Retrieved October 17, 2022.
  8. Australia Department of Environment, Parks and Water Security, Threatened species of the Northern Territory, November 2021.
  9. Jackson, Timothy (2020-05-28). "Are monitor lizards venomous? (the Tale of Toxicofera, part 4)". School of Biomedical Sciences. Retrieved 2022-10-16.
  10. Species Varanus mitchelli at The Reptile Database . www.reptile-database.org.
  11. Gaikhorst, Glen; McLaughlin, James; Larkin, Brian; McPharlin, Meagan (18 May 2009). "Successful captive breeding of Mitchell's water monitor, Varanus mitchelli (Mertens 1958), at Perth Zoo". Zoo Biology. 29 (5): 615–625. doi:10.1002/zoo.20244. PMID 19459149.
  12. Shea, G., Woinarski, J. & Cogger, H. 2018. Varanus mitchelli. The IUCN Red List of Threatened Species 2018: e.T83778268A101752345. doi:10.2305/IUCN.UK.2018-1.RLTS.T83778268A101752345.en. Accessed on 26 September 2022.
  13. Gaikhorst, Glen; McLaughlin, James; Larkin, Brian; McPharlin, Meagan (18 May 2009). "Successful captive breeding of Mitchell's water monitor, Varanus mitchelli (Mertens 1958), at Perth Zoo". Zoo Biology. 29 (5): 615–625. doi:10.1002/zoo.20244. PMID 19459149.
  14. Gaikhorst, Glen; McLaughlin, James; Larkin, Brian; McPharlin, Meagan (18 May 2009). "Successful captive breeding of Mitchell's water monitor, Varanus mitchelli (Mertens 1958), at Perth Zoo". Zoo Biology. 29 (5): 615–625. doi:10.1002/zoo.20244. PMID 19459149.
  15. "Mampam.com". Retrieved 3 July 2010.
  16. Gaikhorst, Glen; McLaughlin, James; Larkin, Brian; McPharlin, Meagan (18 May 2009). "Successful captive breeding of Mitchell's water monitor, Varanus mitchelli (Mertens 1958), at Perth Zoo". Zoo Biology. 29 (5): 615–625. doi:10.1002/zoo.20244. PMID 19459149.
  17. Doody, J. Sean; Soanes, Rebekah; Castellano, Christina M.; Rhind, David; Green, Brian; McHenry, Colin R.; Clulow, Simon (September 1, 2015). "Invasive toads shift predator–prey densities in animal communities by removing top predators". Ecology. 96 (9): 2544–2554. doi:10.1890/14-1332.1. ISSN 0012-9658. PMID 26594710.
  18. Lower, Brian H.; Shaul, Travis R.; Shaul, Kylienne A.; Weaver, and Ella M. "Cane Toad: Purposefully Introduced, but Instantly a Regret". Environmental ScienceBites.
  19. Biawak. "Monitoring the Trade: Using the CITES Database to Examine the Global Trade in Live Monitor Lizards (Varanus spp.)" 3(2), pp. 37-45 © 2009 by International Varanid Interest Group
  20. Shea, G., Woinarski, J. & Cogger, H. 2018. Varanus mitchelli. The IUCN Red List of Threatened Species 2018: e.T83778268A101752345. doi:10.2305/IUCN.UK.2018-1.RLTS.T83778268A101752345.en. Accessed on 26 September 2022.
  21. Doody, J.S., Green, B., Rhind, D., Castellano, C.M., Sims, R. and Robinson, T. 2009. Population-level declines in Australian predators caused by an invasive species. Animal Conservation 12: 46-53.
  22. Einoder LD, Gillespie GR and Buckley KA (2020) Terrestrial fauna monitoring in Kakadu National Park. Northern Territory Department of Environment, Parks and Water Security, Darwin.
  23. "Researchers stop threatened lizards eating toxic cane toads". The University of Sydney. Retrieved 2022-10-16.
  24. Aiyer, Abhilasha; Bunuba Rangers; Bell, Tina; Shine, Richard; Somaweera, Ruchira; Bruny, Miles; Ward‐Fear, Georgia (June 2, 2022). "Taking the bait: Developing a bait delivery system to target free‐ranging crocodiles and varanid lizards with a novel conservation strategy". Ecology and Evolution. 12 (6): e8933. doi:10.1002/ece3.8933. ISSN 2045-7758. PMC 9163195. PMID 35784020.
  25. Hyatt, Alex. "Battlelines drawn against the cane toad march." 2004.

Further reading
  • Ávalos, J. de & Martínez Carrión, P. (1997). "Warane". Reptilia (Münster) 2 (5): 16–21. (in German).
  • Bennet, D.F. (2003). "Australische Warane". Reptilia (Münster) 8 (5): 18–25. (in German).
  • Bennet, D.F. (2003). "Australian Monitors". Reptilia (Great Britain) (30): 12–19.
  • Bonetti (2002). 100 Sauri. Milan: Mondadori. 192 pp. (in Italian).
  • Bustard, H.R. (1970). Australian Lizards. Sydney: Collins. 162 pp.
  • Cogger, H.G. (2000). Reptiles and Amphibians of Australia, Sixth Edition. Sanibel Island, Florida: Ralph Curtis Publishing. 808 pp.
  • De Lisle, H.F. (1996). Natural History of Monitor Lizards. Malabar, Florida: Krieger Publishing.
  • Eidenmüller, B. (2007). "Small monitors in the terrarium". Reptilia (GB) (50): 12–19.
  • Mertens, R. (1958). "Bemerkungen über die Warane Australiens". Senckenberg. Biol. 39: 229–264. (Varanus mitchelli, new species, p. 256). (in German).
  • Storr, G.M. (1980). "The monitor lizards (genus Varanus Merrem, 1820) of Western Australia". Records of the Western Australian Museum 8 (2): 237–293.
  • Swanson, S. (1976). Lizards of Australia. Sydney: Angus & Robertson. 80 pp.
  • Wilson, Steve; Swan, Gerry (2013). A Complete Guide to Reptiles of Australia, Fourth Edition. Sydney: New Holland Publishers. 522 pp. ISBN 978-1921517280.
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