Varanus giganteus (Perentie)

• Up to 2.5 meters. The Perentie is the largest monitor lizard in Australia. It has a robust body with a long, slender tail, and is known for its speed and powerful build. Found in arid and semi-arid regions across central Australia, particularly in rocky outcrops and desert areas.

Varanus varius (Lace Monitor)

• Up to 2 meters. The Lace Monitor is one of the larger goannas, with distinctive dark bands across its body and a long, slender tail. It is often arboreal and is known for its climbing ability. Common in eastern Australia, particularly in forested areas, including eucalyptus forests and woodlands.

Varanus gouldii (Gould’s Goanna)

• Up to 1.6 meters. Gould’s Goanna is an active forager with a long tail and sharp claws, which it uses to dig for prey. It has a robust build and is highly adaptable. Widely distributed across Australia, except in the wettest parts. It thrives in arid and semi-arid regions, but can also be found in grasslands and woodlands.

Varanus rosenbergi

• Up to 1 meter. This species is similar in appearance to the Lace Monitor but is generally smaller. It is known for its burrowing behavior and often takes over rabbit burrows. Inhabits far southwestern and southern Australia, including coastal heathlands and semi-arid woodlands.

Varanus mertensi

• Up to 1 meter. Mertens’ Water Monitor is semi-aquatic and has a flattened tail adapted for swimming. It is dark in color with pale spots. Commonly found near water bodies, including rivers, swamps, and wetlands in northern Australia.

Varanus panoptes (Yellow-spotted Monitor)

• Up to 1.5 meters. A large, terrestrial monitor with a yellow-spotted pattern on its dark body. It is an aggressive predator and scavenger.
• Habitat: Found in northern and eastern Australia, particularly in wetlands, floodplains, and grasslands.

Varanus tristis

• Up to 0.8 meters. Known as the Black-headed Monitor, this species is arboreal and has a dark head with a lighter body. It is agile and often seen in trees. Inhabits rocky hills, forests, and arid interior regions across northern and central Australia.

Varanus acanthurus (Ridge-tailed Monitor)

• Up to 0.7 meters. A smaller monitor with a distinctive spiny tail. It is known for its ability to wedge itself into tight crevices for protection. Found in rocky outcrops and arid regions of northern Australia.

Varanus eremius (Rusty Desert Monitor)

• Up to 0.5 meters. This small monitor has a reddish-brown coloration, which helps it blend into its desert surroundings. It is highly active and fast-moving. Can be found in the arid deserts of central Australia, particularly in sandy and spinifex-covered areas.

Varanus scalaris (Spotted Tree Monitor)

• Up to 0.6 meters. A small, arboreal species with a distinctive spotted pattern. It is agile and spends much of its time in trees. Found in northern Australia, particularly in tropical forests and woodlands.

Varanus brevicauda

• Up to 0.25 meters. The smallest Australian monitor, with a short tail and a stocky body. It is highly secretive and rarely seen. Lives in sandy deserts with spinifex grass in central and western Australia. It is adapted to burrowing and living in harsh, arid environments.

A Bit of History of the Nile Monitor

The Nile Monitor, known scientifically as Varanus niloticus, was the first of its kind to be named by Linnaeus back in 1758, originally as Lacerta monitor. Fast forward to 1790, and the colonist whites described the first Aussie species, Varanus varius, initially called Lacerta varia. This specimen was likely collected around Botany Bay in New South Wales. Unfortunately, the original holotype seems to have gone missing over time.

In 1802, Daudin made another reference to Varanus varius, and a new holotype from Sydney, New South Wales, found its way to the Museum National d’Histoire Naturelle in Paris. Then in 1820, Merrem coined the generic name Varanus.

Throughout the 19th and 20th centuries, as more of Australia opened up to scientific exploration, additional species were discovered and named. Up until 1869, all known species were found along the coast. However, between 1885 and 1903, several new species were identified from specimens collected during major expeditions into central Australia’s deserts. Most of the species described post-1942 hail from northern Australia or were renamed during revisions of earlier species. Interestingly, a number of species recognised today in Australia still haven’t been formally described.

Characteristics

The snout of these creatures is quite elongated, with their nostrils (or nares) positioned anywhere from near the tip of the snout to close to the eyes, as noted by Bellairs in 1949. Their long, forked tongues snugly fit into a sheath located on the floor of their mouths, according to Smith’s 1986 observations. The tail can be either round or flattened sideways in cross-section, and its length compared to the snout-vent length (SVL) can vary widely, ranging from about 100% to 220% in Aussie species, as per Cogger in 1975.

The Varanus genus boasts the broadest size range of any vertebrate genus. Adults of the tiniest species, V. brevicauda, weigh around 17 grams and grow to a total length of 200 millimetres, as per James’s personal communication and Plate 5.13. On the other end of the spectrum, adults of the largest species, V. komodoensis, can tip the scales at over 70 kilograms and reportedly reach a total length of 3.5 metres, as noted by Auffenberg in 1981.

Varanus salvator and V. giganteus follow as the next largest species in terms of body mass. There are claims that V. salvadorii can stretch to a total length of over 4 metres, thanks to an unusually long tail, but such a specimen has yet to be documented.

The skin of these creatures is quite loose and adorned with tiny, closely-set scales. The scales atop the head are notably small and fragmented (Cogger, 1975a). One can observe a distinct and transparent parietal scale sitting on the crown of the head.

Skeletal System

Varanids exhibit cranial kinesis, which means the upper jaw can move independently of the rest of the skull. This unique trait aids in the functionality of their teeth and the swallowing of prey. Their teeth are curved backward, laterally compressed, and some species even have serrated edges (Greer, 1989). The hyoid apparatus in varanids is more mobile than in most lizards (Smith, 1986), which helps in pushing prey down the oesophagus. Unlike other lizards, the roof of their nasal capsule is covered only by skin and connective tissue.

Varanids possess one extra cervical vertebra compared to other lizards, and their vertebrae are elongated. There’s minimal variation in the number of presacral vertebrae, which ranges from 27 to 32. However, the number of post-sacral vertebrae varies significantly, from 56 to 139 in Australian species.

Movement

Getting around on land for these creatures is usually a four-legged affair, marked by a slow, almost swaggering stride. Imagine a hefty varanid strutting through the underbrush, tongue flicking in and out—it’s quite the sight to behold.

Every now and then, these lizards will lift their front half and take off running on just their hind legs. Quite a few species are tree-dwellers when they’re fully grown, especially the smaller ones. Even some of the bigger guys, like V. tristis and V. varius, are pretty good climbers. Youngsters of larger species, including V. komodoensis and V. varius, often take to the trees as well.

Species such as V. varius, V. tristis, V. scalaris, V. gilleni, and V. caudolineatus often find refuge in tree holes and under loose bark. V. prasinus, with its handy prehensile tail, is particularly adept at climbing. On the other hand, V. glauerti, V. glebopalma, and V. kingorum prefer rocky terrains.

Several species, including V. indicus, V. mertensi, V. niloticus, V. salvator, and V. semiremex, are just as comfortable in water as they are on land. In fact, most varanids, even those from the Australian deserts, are capable swimmers. When they hit the water, they tuck their limbs close to their bodies and rely on tail movements and body undulations to move forward. Amphibious species often have tails that are flattened from top to bottom, which seems to help with swimming. For instance, V. mertensi and V. salvator use their tails not only for swimming but also for hunting aquatic prey 

Feeding and Digestive System of Varanids

Their diet is pretty much a free-for-all. They’ll munch on whatever they can find—bugs, molluscs, crustaceans, and sometimes even bigger animals. The hefty ones, like the Komodo dragon (V. komodoensis), are masters of the ambush, targeting large mammals and scavenging for carcasses. On the flip side, the smaller varanids will often swallow their prey whole.

Their eating habits can change a lot, depending on what’s up for grabs, where they’re hanging out, and the time of year. Some, like V. dumerili and V. mertensi, are fond of aquatic prey, while desert dwellers such as V. eremius and V. gouldii have a taste for other lizards. Interestingly, cannibalism isn’t off the table for some species. The differences in diet often mirror the variety in their habitats and what food is available. A standout example is the tree-loving V. olivaceus from the Philippines, which chows down on a lot of fruit at certain times, a trait not commonly seen in its kin.

Varanids are equipped with sizeable Jacobson’s organs situated at the front of their nasal capsules, right up in the roof of their mouths. These organs do more than just complement the nose; they play a significant role in sniffing out prey. Connected via ducts to the nasal sacs, these organs allow the lizard to detect scent particles gathered by the tongue from the air and deposited near the Jacobson’s organs’ openings. You’ll often see a varanid flicking its tongue – a telltale sign of a foraging lizard. Interestingly, the tips of their deeply notched tongues can move independently, which helps them sample more air and gather scent particles more effectively. When extended, a varanid’s tongue can grow by 70 to 90% in length!

Despite its impressive capabilities, the tongue doesn’t assist in handling prey. Instead, varanids rely on inertial feeding, using neck movements and the hyoid apparatus to push prey down into their simple stomachs. Smaller prey are swallowed whole, while larger prey or carrion are torn apart. The lizard holds the larger prey with its forefeet and uses its sharp teeth and jerking head movements to rip pieces off the carcass. During feeding, the tongue is safely tucked away in its sheath on the mouth’s floor. Once the food is in the throat, the hyoid apparatus shoves it back into the oesophagus. The neck then twists from side to side to nudge the food further down. For particularly large food items, varanids might resort to ramming – they press the food against a solid object and push against it to force it down their throats.

Internal Organs

Varanids boast respiratory systems that are quite unique among reptiles. Their lungs are not only larger but also more intricate, featuring intra-pulmonary ducts that are reinforced with cartilage. These ducts connect to numerous small chambers, creating a vast surface area for gas exchange. Such a sophisticated lung design enables them to breathe efficiently while using very little energy.

These lungs are attached directly to the body wall beneath the ribcage, which allows for effective ventilation through costal breathing alone, without the need for active deflation. This method of convection-driven gas exchange, paired with efficient blood buffering, helps varanids maintain low levels of lactic acid in their blood and quickly recover their oxygen levels after physical exertion.

Compared to other reptiles, varanids have a superior aerobic capacity, allowing them to stay active for extended periods without relying on anaerobic metabolism. The chemical control of lung ventilation is both swift and effective, underpinning their high respiratory performance. These adaptations empower varanids to maintain an active, wide-ranging foraging lifestyle without easily tiring out.

Interestingly, they do not have a diaphragm. Their heart is located further back in the body cavity and is protected by a thick pericardium.

One notable characteristic of the varanid heart is its enhanced oxygen transport, thanks to the unique arrangement of its chambers and valves. This layout reduces the mixing of oxygen-rich and oxygen-poor blood, effectively creating a four-chambered heart during systole. As a result, varanids can maintain higher systemic blood pressure compared to pulmonary blood pressure.

Uniquely, varanids are adept at regulating blood pH during temperature changes, ensuring efficient oxygen transport and aerobic activity. They also have higher levels of myoglobin in their muscles, which facilitates the rapid transfer of oxygen from the blood to the muscles, supporting their active lifestyle.

Adaptions

While some critters are pretty much tied to water, most varanids spend a good chunk of their lives in dry conditions. They’re pretty good at keeping their water loss to a minimum, much better than other reptiles, which lets them save water.

Research on species like V. rosenbergi and V. gouldii shows they’re quite adept at tweaking their kidney functions to suit their surroundings. They can adjust things like their glomerular filtration rate, tubular reabsorption, and ureteral urine flow. The urine from their ureters heads to the coprodaeum of the cloaca, where more water gets reabsorbed thanks to sodium transport. This whole shebang is mainly controlled by the anti-diuretic hormone arginine vasotocin (AVT), which changes based on the lizard’s environment. In the end, the urine pellets that varanids pass out have very little water in them, making their water-saving abilities even better than desert mammals.

Extra cations, especially sodium, get kicked out through nasal salt glands found in several varanid species. These glands help reabsorb water from the kidney tubules, cloaca, and rectum by secreting hyperosmotic sodium. Field studies show that there are clear seasonal trends in water use, with tropical species generally having higher water fluxes compared to those in semi-arid or arid zones. These nifty adaptations let varanids manage their water and electrolyte balance efficiently, which is crucial for surviving in their diverse habitats.

Reproduction

Varanid lizards, those interesting creatures, show off a whole array of intricate behaviours, especially when it comes to courtship, mating, and staking out their territory. When the blokes are courting, they’ll do a bit of licking, stroking, and even scratching to keep the sheilas interested. The bigger species are known for their bipedal combats during mating rituals, where the males will wrestle and try to outmuscle each other. Sometimes, these tussles get pretty brutal and can lead to nasty injuries.

During the whole mating shindig, these lizards communicate in various ways – they use touch, smells, and visual cues. Males have this unique way of using both hemipenes for copulation. They might even pin down the females’ legs to stop them from getting away. After the deed is done, the females lay their eggs in nests, often choosing termite mounds, and seal them up nice and tight. In some species, the little ones get a helping hand from their parents to break out of the nest, but others are on their own right from the get-go.

Now, these varanids aren’t just about mating; their activity patterns are quite seasonal and depend heavily on where they live. Those in tropical areas tend to be active all year round, while the desert dwellers might take a break during extreme heat. Even within the same species, activity can differ based on the local climate. In temperate zones, some species slow down in winter and switch up their behaviours between the seasons.

Certain varanid species from Australia and Africa feature sex chromosomes with female heterogamety. All varanids are oviparous, meaning they lay eggs. The clutch size, or number of eggs, is directly related to the body length of the lizard. Some gravid females can become so swollen with eggs that they can’t eat for a few weeks before laying them.

The breeding season varies significantly among species and can also differ within the same species depending on the locality. This variation is especially noticeable in tropical regions.

The age at which varanids reach sexual maturity isn’t well-documented for most species, but larger species likely take longer to mature than smaller ones. For instance, V. komodoensis is estimated to take about five years to reach maturity, while V. albigularis takes four to six years. Varanus olivaceus reaches maturity at around three years old. Captive V. varius have been known to breed at seven to eight years old. It’s estimated that wild V. acanthurus may reach sexual maturity in just one to two years.

Eggs

In captivity, these creatures can produce two clutches a year, but in the wild, they typically breed only once annually. Certain species, like V. rosenbergi, V. varius, V. giganteus, V. gouldii, V. albigularis, V. niloticus, and V. bengalensis, choose to lay eggs in termite mounds. These termite mounds maintain a temperature above 25°C with high humidity throughout winter, creating an ideal environment for the eggs’ development. Other species prefer to dig deep into the soil to bury their eggs, while V. olivaceus is known to lay its eggs in hollow tree trunks and limbs. Some species might even lay their eggs in communal burrows.

The embryology and development of varanid lizards show a fair bit of variation in incubation times and growth rates, swayed by species and environmental factors. Out in the wild, eggs that aren’t laid in termite mounds have incubation periods ranging from about 90 to 120 days for species like V. acanthurus, stretching out to a lengthy 250 to 265 days for V. komodoensis. When kept in captivity, these periods can fluctuate between roughly 75 and 250 days, with longer incubation times generally linked to larger maternal body sizes.

Once they hatch, young varanids grow at a pretty swift pace, especially in the first three months. Smaller species usually hit adult size within a year or two, while their larger counterparts might take up to five years to fully mature. This link between incubation time and body size is a crucial part of varanid development, showing how maternal size and environmental conditions impact the growth and maturation of these lizards.

Habitat

Varanid lizards show off a wide array of ecological tricks, thriving in spots as varied as watery havens, bone-dry deserts, and cool, temperate forests. Most of these critters prefer to go solo and are active during the day, although a few get their groove on at dusk. They’re pretty ambitious foragers, roaming vast stretches to hunt down grub, which ranges from tiny invertebrates to, in some cases, sizeable vertebrates.

You won’t find these lizards packed together; their populations are generally sparse. But they do have well-marked territories, which can overlap with their neighbours. Males, being the wanderers they are, have larger territories and are more on-the-go compared to females. When the chilly season rolls in, they tend to stick closer to their burrows.

Conservation and Protection

EIn Australia, all kinds of varanids are protected by state, federal, and international laws. Even though Aboriginal folks do hunt them here and there, it doesn’t seem to jeopardise their existence. There isn’t any permitted commercial hunting of these lizards.

In contrast, a staggering number of skins from a variety of species (like V. salvator, V. bengalensis, V. indicus, V. flavescens, V. niloticus, and V. exanthematicus) have been shipped out from some South-East Asian countries and Africa over the past decade. We’re talking about figures ranging from 425,000 to 1.3 million per year from Asia and 275,000 to 800,000 per year from Africa, all destined for the leathergoods market (as per Luxmoore, Groombridge & Broad 1988). The sustainable harvest levels for these species remain a big question mark. The major exporters include Indonesia, Singapore, Sudan, Nigeria, and Mali, with Belgium, France, Italy, the USA, and Japan being the main buyers (Luxmoore et al. 1988). Now, even though there are international laws protecting all varanid species, not all nations are on board with these treaties, and the level of legal protection and enforcement varies widely across these regions (Luxmoore et al. 1988).

When it comes to the threats to varanid survival due to habitat changes and industrial pollution in many developing nations, there’s pretty much zilch data available.