'') True monitors have eyes with
retinas that are almost entirely composed of
cone cells, giving them excellent colour vision during daytime but nearly no night vision due to the lack of
rod cells, which are vital for seeing in low-light environments. Like most lizard, true monitors are
oviparous, and some are capable of
parthenogenesis. They have been occasionally seen guarding their nests. ''), showing its proportionately long limbs. While most reptiles are considered to have three-chambered hearts, the hearts of true monitors – as with those of other monitors – have a well developed
ventricular septum that completely separates the
pulmonary and
systemic sides of the circulatory system during
systole. This allows their hearts to temporarily function like a four chambered heart, which in turn ensure that oxygenated blood is quickly distributed to the body without also flooding the lungs with high-pressure blood. The highly efficient circulatory system, combined with their strong limbs, allow true monitors to become powerful sprinters. However, the primitive sprawling limbs would limit both the speed and stamina of the larger species. The
perentie is the fastest of all monitors with a recorded speed of up to or , rivalling the aquatic
leatherback turtle as the fastest extant reptile. Being the largest true monitor,
Megalania is yet slower, with its estimated top speed no more than . '' (4th row on the right) compared with other
varanoids '' True monitors are most well known by their maximum size, but what really distinguishes them from other monitors is their great evolutionary potential to kill large prey in relation to their own body size, allowing them to occupy the niche of
apex predators in most of their biomes. As the ruler of modern Australia,
perenties have to prey on other lizards nearly their own size since birth, and the species adapted by developing an unusually large head-to-body ratio.
Komodo dragons are the only extant lizards to hunt large mammals, regularly preying on
water buffalos that are even heavier than them.
Megalania, being the most massive terrestrial reptile back then, would have competed with predators like
Thylacoleo to prey on
marsupials as big as
Procoptodon and even the
Diprotodon, the largest of them that ever existed. Whether true monitors carry
toxin is a long debated topic. Experiments have shown that the saliva of
Komodo dragons are not considerably different compared to other predators, thus dispelling the theory that monitors kill with bacteria. It is verified that like all monitors or possibly all lizards, true monitors have special glands in their jaws, which is almost certainly a homologous feature for these reptiles. However, not all agree that these can be called venom glands, especially since the effect of them are too mild for prey to most lizard and especially true monitors, who often hunt prey that are simply too large to rely on venom for a kill. True monitors live in Australia alongside the much smaller and more diverse
dwarf monitors. Their dominance was challenged but remained stable in the face of
invasive species. Among them, desert-dwelling true monitors (
perenties,
sand goannas,
Argus monitors,
Rosenberg's monitors and
Spencer's goannas) all remain a status of
least concern on the
IUCN Red List with their populations large and stable. ==Taxonomy==