Episode 143 – Monitor Lizards

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These lizards are famous for their diverse lifestyles, their widely-ranging body sizes, and their somewhat mysterious evolutionary past. This episode, we discuss Monitor Lizards.

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Many Monitors

Modern monitor lizards are widespread across the tropics and subtropics of the Old World. Despite all having a similar body plan, and all being classified in the genus Varanus, they are quite diverse: there are arboreal monitors, rock-dwelling monitors, swimming monitors, desert-dwelling monitors, and more. They come in an incredible range of sizes, from pygmy monitors only 20cm long to three-meter Komodo dragons, the largest lizards on the planet.

Monitor lizards tend to have a pretty standard body shape, but they come in a variety of sizes and habits.
Top left: Asian water monitor (Varanus salvator). Image by Carlos Delgado, CC BY-SA 4.0
Top right: Kimberly rock monitor (Varanus glauerti). Image by Haplochromis, CC BY-SA 3.0
Bottom left: Emerald tree monitor (Varanus prasinus). Image by L. Shyamal, CC BY-SA 3.0
Bottom right: Komodo dragon (Varanus komodoensis). Image by Mark Dumont, CC BY 2.0

Monitors are mostly carnivorous, apparently venomous (at least to some degree), and noted for their curiosity and intelligence. Their closest relatives include beaded lizards, glass lizards, and others in a group called Anguimorpha. This group altogether is also likely closely related to snakes.

A Mysterious Past

A variety of ancient lizards have been identified as close relatives of monitors, though many have been re-identified or reconsidered over the years. These include a diversity of monitor-like lizards of the Cretaceous, including mosasaurs (although some studies suggest mosasaurs are more closely related to snakes).

The best-studied ancient monitor cousin is Saniwa from the Eocene of North America. These lizards were very similar to modern monitors, and while many other maybe-monitors have been identified, Saniwa is perhaps the most widely-agreed-upon example of a monitor ancestral cousin.
Left: Complete skeleton of Saniwa. Image by Jonathan Chen, CC BY-SA 4.0
Right: Skull of Saniwa. Image by Smokeybjb, CC BY-SA 3.0

Monitors in the genus Varanus go back at least to the Miocene, nearly 20 million years ago, and they include the largest terrestrial lizards in history. Giant monitors include our modern Komodo dragons, a number of ancient species of similar sizes, and the record-holder: Megalania (Varanus priscus) from Pleistocene Australia, the largest land-dwelling lizard of all time with estimates of at least 4-6 meters and 70-80kg.

Megalania (Varanus priscus) skeletal reconstruction at the Melbourne Museum. It’s likely that these giant lizards shared Australia with ancient humans for thousands of years.
Image: Cas Liber, Public Domain

Learn More

Komodo dragon osteoderms

Chemicals in Dragon’s Glands Stir Venom Debate
Dobson et al. 2019. Monitor venom (technical, open access)

Florida’s Dragon Problem

Play behavior by captive tree monitors, 2019 (technical, open access)

Archaeovaranus identified in 2022
The four eyes of Saniwa, 2018

Last lizard standing: The enigmatic persistence of the Komodo dragon (technical, open access)

Dragon’s Paradise Lost: Palaeobiogeography, Evolution and Extinction of the Largest-Ever Terrestrial Lizards (technical, open access)

If you enjoyed this topic and want more like it, check out these related episodes:

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