Varanus mertensi
Silhouette: Steven Traver · PhyloPic
In 3 days ~60% in a week ~90%
Mertens' water monitor (Varanus mertensi) is a species of monitor lizard in the family Varanidae. The species is native to northern Australia, and is a wide-ranging, actively foraging, opportunistic predator of aquatic and riparian habitats. It is named after German herpetologist Robert Mertens.
Mertens' water monitor grows to a total length (tail included) 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.
Amongst all monitor species, including even the water monitors of the subgenus Soterosaurus, Mertens' water monitor is morphologically the most well adapted to an aquatic lifestyle, being able to seal its upwards facing nostrils when underwater. 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). It is also the only monitor species reported capable of using its sense... Wikipedia →
| # | Park | Country | Daily sighting probability |
|---|---|---|---|
| 1 | Litchfield National Park | 🇦🇺 Australia | |
| 2 | Nitmiluk National Park | 🇦🇺 Australia | |
| 3 | Kakadu National Park | 🇦🇺 Australia |
Possible
About 30% of wildlife watchers report a Mertens's Water Monitor sighting in Kakadu National Park on a given day, based on 27 verified sightings from GBIF citizen-science data from 2019 to 2026. It is the 8th most-seen of the 54 reptiles we can rate there. Over three days the chance rises to about 60%, and over a week to about 90%, if each day is a fresh chance.
February
In that month about 50% of wildlife watchers report a Mertens's Water Monitor in Kakadu National Park on a given day, against under 5% in August and September, the lowest months. That is one of Kakadu National Park's wettest months, with about 290 mm of rain against almost none in August.
Between noon and 3 pm
Across all the parks we cover, that three-hour window holds the most Mertens's Water Monitor sightings with a recorded time, and about 9 in 10 of them come between 6 am and 6 pm: it is a daytime animal.
3rd of 3
Of the 3 parks where we can rate Mertens's Water Monitor sightings, Kakadu National Park ranks 3rd at about 30%. The best odds are in Litchfield National Park (Australia, about 80%) and Nitmiluk National Park (Australia, about 40%).
Same genus: Varanus
The closest relatives of the Mertens's Water Monitor that you can see in Kakadu National Park (genus Varanus, Monitor Lizards) are Argus Monitor (about 30%), Black-headed Monitor (about 20%) and Gould's Goanna (about 20%), with each one's daily chance of a sighting.
GBIF citizen-science records
It is estimated from how often the Mertens's Water Monitor is recorded against all the reptiles logged in Kakadu National Park (27 verified sightings from 2019 to 2026), calibrated against known sighting rates. Figures are rounded to the nearest 10% because citizen-science data supports honest approximations, not decimals, and months with records from fewer than five people show "—". How it works.
