Monitor lizards are formidable predatory reptiles of family Varanidae, populating tropical areas of Africa, Asia, Australia and Indo-Pacific. There are more than 70 species of monitor lizards recognized and they show the widest size range of any vertebrate genus. The smallest, V. sparnus weighing about 16.3 grams and reaching length of 23 cm. The largest, V. komodoensis weighing up to 70 kg and reaching 3 meters in length. Despite such vast size difference monitor lizards are very similar physiologically, so studying them as a whole will help to understand each species separately and vice versa.
Monitor lizards have adapted very unique for a reptile foraging behavior and cover large territory at a time in a search for food. This very active lifestyle would not be possible without some very significant physiological adaptations, that make monitor lizards stand aside from most other reptiles.
Circulatory system. Without getting deeply into scientific terms, varanids circulatory system is very advanced, compared to other reptiles. Position of the heart in the body cavity, muscle arrangements, chamber and valve arrangements effectively create simulation of 4-chamber heart with all the advantages it provides. Varanids also have higher levels of myoglobin in their tissues, which facilitates rapid oxygen transport from blood to muscles.
Respiratory system. Monitor lizard lungs are larger and more advanced than other reptiles and provide very efficient oxygen transport to the blood. The lungs are attached directly to the body wall under the rib cage, have a strong compliance with body movements, and are efficiently ventilated by costal breathing alone. The gas exchange strategy is convection-dominated and the lungs do not have to be actively deflated like those of most lizards.
Varanids have a greater aerobic capacity than other reptiles and do not function anaerobically for the long periods shown by other reptiles. Varanids can repay their oxygen debt quickly after exertion.
It has also been recently discovered, that monitors possess unidirectional airflow, similar to birds and crocodilians. Monitor lizards have the highest standard metabolic rates of all extant reptiles.
Image courtesy of Emma Schachner.
Sensory organs and activity. Monitor lizards have good eyesight and can easily spot an object at more than 100 meters away. The way their eyes are positioned gives them great field of view, which is only obscured directly in front of them by the tip of their nose.
For hunting and tracking down the prey monitor lizards rely greatly on Jacobson’s vomeronasal organ. Tongue flicking brings small particles directly to the organ, located at the top of their mouth and accessible through two narrow grooves. Importance of Jacobson’s organ for monitor lizards is indicated by deep tongue forking, because it provides better transport of particles to organ sensors.
Most monitor lizards are equally good at various types of activities. They can climb, swim, dive, run, jump, burrow. Some are arboreal specialists, some are aquatic, some are burrowing specialists. But their specialization does not prevent them from being excellent at other types of activities. Arboreal tree monitors have been observed underwater foraging, aquatic monitors can burrow and climb, and so on.
Intelligence. All those physiological enhancements, sensory organs, various prolonged activity
levels and predatory tactics require highly developed central nervous system and brain. Monitor lizards show one of the highest intelligence
levels of all reptiles being equal to mammals and birds. Studies show, that they can solve problems, count up to six, recognize and
accept their keepers, be trained, engage in social behavior while hunting, construct very complex burrows, have distinct personalities.
They are highly adaptable and utilize human-inhabited areas to their advantage.
Monitor lizards occupy the same ecological niche as small and mid-sized mammal predators and due to all the physiological adaptations described above are able to successfully compete with them.
Should be noted that Teiidae, Crocodylidae and Iguanidae families possess similar to Varanidae intelligence levels.
A lot can be said about monitor lizards, and this article provides just enough information for potential monitor lizard keeper to understand the importance of all husbandry steps laid out in Keep section of this website. Monitors are the most rewarding reptiles to keep, but also the hardest and require lots of space, time and financial resources to make sure they thrive in captivity.
Back to Varanus melinus species
1. Fauna of Australia. 30. Family Varanidae. Dennis King & Brian Green Link(PDF)
2. Unidirectional airflow in the lungs of birds, crocs and now monitor lizards!? Link
3. Anim Cogn (2008) 11:267–273 DOI 10.1007/s10071-007-0109-0 Rapid solving of a problem apparatus by juvenile black-throated monitor lizards (Varanus albigularis albigularis) Jennifer D. Manrod, Ruston Hartdegen, Gordon M. Burghardt
4. Monitors, Mammals, and Wallace’s Line. Samuel S. Sweet & Eric R. Pianka Link(PDF)