Mertens' water monitor

Mertens' water monitor (Varanus mertensi), often misspelled Mertin's water monitor, is a species of monitor lizard. The species is endemic to northern Australia, and is a wide-ranging, actively foraging, opportunistic predator of aquatic and riparian habitats.^[2] It is named after German herpetologist Robert Mertens.^[3]

Mertens' water monitor
Smithsonian National Zoological Park
Conservation status
Endangered  (IUCN 3.1)[1]
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Order:	Squamata
Family:	Varanidae
Genus:	Varanus
Subgenus:	Varanus
Species:	V. mertensi
Binomial name
Varanus mertensi Glauert, 1951

Description

Northern Territory, Australia

Mertens' water monitor grows to a total length (including tail) of about 2.0 m (6.6 ft). It is dark brown to black above, with many cream to yellow spots. The underparts are paler – white to yellowish – with grey mottling on the throat and blue-grey bars on the chest. The tail is strongly compressed laterally, with a high median dorsal keel, and is about 1.5 times the length of head and body.^[4]

Wyndham, Western Australia

Amongst all monitor species, including even the water monitors of Soterosaurus, Mertens' water monitor is morphologically the most well adapted to an aquatic lifestyle, being able to seal its upwards facing nostrils when underwater.^[5] It is capable of swallowing prey underwater, which is an ability not reported in any other monitor species other than the Borneo earless monitor (Lanthanotus borneensis).^[6] It is also the only monitor species reported capable of using its sense of smell to locate and capture prey underwater.^[7]

Distribution and habitat

Mertens' water monitor is found in coastal and inland waters across much of northern Australia, from the Kimberley region of Western Australia, across the Top End of the Northern Territory and the Gulf Country, to the western side of the Cape York Peninsula in Far North Queensland.^[4]

Behaviour

Underwater, Northern Territory, Australia

Mertens' water monitor is semiaquatic, a strong swimmer, and seldom far from water. It is often seen basking on midstream rocks and logs, and on branches overhanging swamps, lagoons, and waterways throughout its range. When disturbed, it drops into the water, where it can stay submerged for long periods.^[4] It has been observed sleeping underwater.^[5]

Feeding

Mertens' water monitor feeds both on land and in the water, mainly on fish, frogs, crabs, crayfish, shrimps, amphipods, and carrion, also taking terrestrial vertebrates, insects, spiders, and human rubbish when available.^[4][8] It's diet consists mainly of freshwater Holthuisana crabs, which are reported to make up 29-83.7% of the biomass found in gut and scat samples. 11.5% of prey biomass was found to be fish. While eggs and frogs are infrequently eaten, large amounts of them are eaten when found. Arthropods including spiders, beetles and water bugs, while frequently eaten, make up a small proportion of ingested prey biomass.^[7] It has a good sense of smell and may dig up prey when foraging, including the eggs of freshwater turtles.^[9]

Breeding

Mertens' water monitor lays eggs in a burrow, usually with egg-laying taking place early in the dry season and hatching in the following wet season. The eggs hatch within 200–300 days after laying, depending on temperature, with the hatchlings able to enter the water and swim immediately.^[9][10]

Conservation and status

Mertens’ water monitor is threatened by the spread of cane toads through its range, through poisoning after eating them. Because of this V. mertensi is listed as Vulnerable under Northern Territory legislation.^[9]

Gallery

• 
  Portrait of Mertens' water monitor in the Northern Territory
• 
  Mertens' water monitor at the Grotto waterhole near Wyndham, Western Australia
• 
  Australia Zoo
• 
  Australia Zoo
• 
  Featherdale Wildlife Park
• 
  Bronx Zoo
• 
  Healesville Sanctuary

References

1. Shea, G.; Woinarski, J.C.Z.; Macdonald, S.M.; Cogger, H. (2018). "Varanus mertensi". IUCN Red List of Threatened Species. 2018: e.T83778246A101752340. doi:10.2305/IUCN.UK.2018-1.RLTS.T83778246A101752340.en. Retrieved 19 November 2021.
2. Mayes, Phillip James (2006). The ecology and behaviour of Varanus mertensi (Reptilia: Varanidae) (Dissertation submitted for the Degree of Doctor of Philosophy). Perth, Western Australia: Faculty of Natural Sciences, Edith Cowan University.
3. 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 mertensi, p. 176).
4. Cogger HG (1979). Reptiles and Amphibians of Australia. Sydney: Reed. p. 257. ISBN 0-589-50108-9.
5. Auliya, Marc; Koch, André (2020). Visual Identification Guide for the Monitor Lizard Species of the World (Genus Varanus) (552 ed.). DE: Bundesamt für Naturschutz. doi:10.19217/skr552. ISBN 978-3-89624-290-7.
6. Mendyk, Robert; Shuter, Avishai; Kathriner, Andrew (2015). "Historical Notes on a Living Specimen of Lanthanotus borneensis (Squamata: Sauria: Lanthanotidae) Maintained at the Bronx Zoo from 1968 to 1976". BIAWAK. 9 (2): 44–49.
7. Mayes, P. J.; Thompson, G. G.; Withers, P. C. (2005). "Diet and foraging behaviour of the semi-aquatic Varanus mertensi (Reptilia:Varanidae)". Wildlife Research. 32 (1): 67. doi:10.1071/WR04040. ISSN 1035-3712.
8. "Varanus Mertensi".
9. "Threatened Species of the Northern Territory: Mertens Water Monitor" (PDF). Simon Ward, John Woinarski, Tony Griffiths and Lindley McKay (compilers). Department of Natural Resources, Environment and the Arts, Northern Territory. November 2006. Archived from the original (PDF) on 9 October 2009. Retrieved 10 January 2010.
10. OzAnimals.com: Mertens’ Water Monitor.

External links

• Monitor Lizards: Varanus mertensi (includes notes on keeping and breeding in captivity)