Reptilia · Squamata
Varanus mertensiGlauert, 1951
Mertens's Water Monitor
Photograph by Paul Coddington, CC BY, via iNaturalist
A day-active, meat-eating monitor lizard from the Australo-Pacific region, reaching 560 mm snout to vent.
Reptilia › Squamata › Varanidae › Varanus
- Grows to
- 560mm
- greatest recorded
- Eats
- Carnivorous
- Active
- Diurnal
- Lives up to
- 23years
Longer than 67% of all monitor lizards anyone has measured.
The bar is the rank itself: its length is the share of the family this animal exceeds. The chart below is the family’s real spread, on a logarithmic scale because body length here runs from 116.4 to 1,570 mm. The solid rule is this animal. Every bar is a count of real species, and nothing on this figure is estimated.
What is known
45 sourced facts, each taken from a named scientific dataset. How to read these.
Cited from a dataset unless a row is marked otherwise.
Standing in the world
- Still living
- Extant
- Conservation status
- Endangered
- Status assessed · the year the Red List last looked
- 2017
- Population trend
- Decreasing
- Red List criteria
- A4be
The animal itself
- Limbs
- Four legs
- Fangs
- None
- Longest recorded life
- 23years
- Preferred body temperature · chosen in a laboratory gradientmeasured
- 34.2°C
- Body temperature when active · measured in the wild
- 33.35°C92 °F
- Body temp in the wild · mean, when active
- 29.4–34°C85–93 °F
Carey and Judge 2000, De Magalhaes & Costa 2009, Slavens and Slavens 1999: 20 and 4 months, >20 (Bennett 1995), 23 (Mendyk 2015)
mean body temperature of active individuals in nature
body temperature of active animals in nature
Size
- Length snout to vent · greatest recorded
- 560mm22 in
- Length snout to vent · adult female mean
- 411.8mm16 in
- Largest female · snout to vent
- 366mm14.5 in
- Hatchling length · snout to vent
- 101.5mm4 in
- Largest juvenile · snout to vent
- 101.5mm4 in
- Body mass · greatestcomputed
- 3885g8.6 lb
- Body mass · a typical adult
- 1121.2g2.5 lb
- Body mass at hatchingmeasured
- 27.7g0.98 oz
Shai Meiri, unpublished
snout-vent length; the source column mixes this with straight carapace length
computed from length by a family-level allometric equation, not weighed
AnAge rates this record "acceptable"
How it lives
- Diet
- Carnivorous
- How it hunts
- Active foraging
- Active
- Diurnal
- Where it lives
- Semi-aquatic
- Habitat
- Wetlands
- Biome
- Tropical & Subtropical Grasslands, Savannas & Shrublands
- Active season · months of the year
- 12months
- Region
- Australo-Pacific
- Mainland or island
- Mainland
- Elevation · above sea level
- 40m
- Range size
- 1,201,713km²
Vertebrates and Invertebrates: freshwater crabs, eggs, fish, mammals, birds, aquatic and terrestrial Arthropods, mostly crabs, also fish and eggs, fewer frogs, many spiders but little by volume (Mayes et al. 2005), "crabs, fish, frogs and turtle eggs together with small numbers of insects." (Bennett 1995), mostly (in descending order) crabs, fish andreptile eggs, also frogs and spiders, then insects, reptiles, carrion and fruit (Mayes 2006 p74, 89), "preys on wide range of aquatic and terrestrial animals e.g., insects, crayfish, fish, amphibians, reptiles, birds and smaller mammals plus turtle eggs" (Auliya and Koch 2020)
e.g., Mayes et al. 2005, Mayes 2006
Crepuscular and Diurnal / diurnal and heliothermic (Clusella-Trullas et al. 2008)/ diurnal (Mayes 2006, Auliya and Koch 2020, Black 1983)
Aquatic and sometimes Arboreal / terrestrial and semi aquatic (Thompson et al. 2008), aquatic (Black 1983)
Breeding
- Reproduction
- Oviparous
- Sex decided by
- Genes, ZW/ZZ
- Average clutch
- 9.7eggs
- Eggs or young per clutch
- 3–18eggs
- Mean clutch
- 6.8–12.6eggs
- Clutches in a year
- 2.5per year
- Clutches a year
- 1–5per year
- First breeds at
- 24–48months
5-9 (Mendyk 2011), mean 87.5 (Du et al. 2014, did I mean to write 8.75?), max 11, 14 in captivity (Bennett 1995), 3-18 (Auliya and Koch 2020)
Naming and history
- Suborder
- Sauria
- Infraorder
- Anguimoprha
- Subspecies
- No
- Described by
- Glauert
- Moved to another genus · since it was first described
- No
- Described in
- 1951
- Type locality
- Australia
Keeping one
Mertens's Water Monitor is assessed as Endangered on the IUCN Red List. Species at risk in the wild are often covered by CITES and by national law, which decide what may be bought, sold, moved across a border or kept at all — and captive-bred and wild-caught animals are usually treated differently. Check the rules where you are before acquiring one. Nothing below is a judgement about keeping this species; it is what a published welfare standard says about space, and what the animal itself was measured doing.
7 ft 4 in × 7 ft 4 in × 4 ft 7 in(224 × 224 × 140 cm)
Minimum length × width × height, for one animal. Bigger is always better.
The standard sizes ground-dwelling species at 4 × 4 × 2.5 times snout-vent length.
Mindestanforderungen an die Haltung von Reptilien (Sachverständigengutachten), German Federal Ministry for Nutrition, Agriculture and Forestry, Animal Welfare Division, 10 January 1997. The standard states these floor areas are recommendations rather than binding limits.
From this animal’s own snout-vent length of 22 in (560 mm), at 4 × 4 × 2.5 times it, following the German federal welfare standard — how this is worked out.
In a laboratory thermal gradient this species settled at 94 °F (34.2 °C).
Preferred body temperature, recorded in a laboratory gradient where the animal could choose freely. A measurement of the animal, not a setting for a thermostat.
Feeding
A prey item should be no wider than the space between the animal's eyes.
Widely used across reptile husbandry references. The stated reason is that the gap approximates the width of the throat, so wider prey is associated with impaction. This is established practice, not a measurement.
304kJ per day
Worked out for an animal of 3,885 g, the heaviest recorded for Mertens's Water Monitor — so it is the top of the range, not a figure for a particular animal. Weigh yours and the number will be lower.
An upper estimate, not a feeding target, and not veterinary advice. The equation covers only what an animal burns staying alive, so a growing, gravid or recovering animal needs more — and it was measured on wild animals, which move far more than one in an enclosure. Body condition and a vet who knows the species are what a feeding decision should rest on. Where this comes from.
What nobody knows
These are not gaps in our research. No scientific dataset records them for this species — and saying so is information you cannot get from a reference that simply leaves a field out.
- — Length snout to vent · adult male mean
- — Length snout to vent · adult mean
- — Length nose to tail · greatest recorded
- — Female length · mean, nose to tail
- — Length nose to tail · adult male mean
- — Length nose to tail · adult mean
- — Largest male · snout to vent
- — Largest female · shell length
- — Largest male · shell length
- — Hatchling length · nose to tail
- — Largest juvenile · shell length
- — Egg length
- — Egg width
- — Body mass · adult female, mean
- — Eats arthropods
- — Eats vertebrates
- — Eats plant matter
- — How the young develop
- — Gestation or incubation
- — Hatchling length · smallest recorded
- — Hatchling length · largest recorded
- — Age at first breeding · female
- — Mature from · youngest recorded
- — Mature by · oldest recorded
- — Size at maturity · smallest recorded
- — Size at maturity · largest recorded
- — Venomous
- — Colour on the back
- — Pattern on the back
- — Pupil
- — Density · lowest recorded
- — Density · highest recorded
- — Home range · largest recorded
- — Active in
Others in Varanidae
Take this data
Everything on this page is available as data under CC BY 4.0 — use it freely, credit it, no permission needed.
This species as JSON · the whole dataset
Cite this page
The Reptile & Amphibian Encyclopedia (2026). Varanus mertensi. Retrieved 2026-09-08, from https://www.allanglescreatures.com/species/varanus-mertensi
@misc{varanus_mertensi_2026,
title = {Varanus mertensi},
author = {{The Reptile & Amphibian Encyclopedia}},
year = {2026},
url = {https://www.allanglescreatures.com/species/varanus-mertensi}
}Sources
- Meiri, S. (2024). SquamBase — a database of squamate (Reptilia: Squamata) traits. Global Ecology and Biogeography. Public domain (CC0).
- Oskyrko, O. et al. (2024). ReptTraits: a comprehensive dataset of ecological traits in reptiles. Scientific Data. Used under CC BY 4.0.
- Catalogue of Life (dataset 3LR), used under CC BY.
- AnAge, Human Ageing Genomic Resources (de Magalhães et al.), used under CC BY 3.0.