Desert monitor

Varanus griseus monitor lizards normally display a variety of body coloration from light brown and yellow to grey. They average about one meter in length, but can reach total body lengths of almost two meters. These lizards can also have horizontal bands on either their backs or tails, along with yellow spots across their backs. Their young are normally a brightly colored orange and have distinctive bands across their backs which may be lost as they mature. Their nostrils are slits located farther back on their snouts (closer to the eyes than the nose), and their overall body size is dependent on the available food supply, the time of year, environmental climate, and reproductive state. Males are generally larger and more robust than females, but females have a more gentle look about them. Those differences allow males to be distinguished from females from a distance without careful inspection. Adult monitor lizards also go through periods of molting in which they shed their outer layer of skin to expand their overall body size. This process can take several months and happens around three times per year. Their skin is adapted to the desert environment where they live, and they are excellent swimmers and divers and have been known to enter the water occasionally to hunt for food.[4]

Three subspecies have been described:[5]

• V. g. griseus (grey monitor)
• V. g. caspius (Caspian monitor)
• V. g. koniecznyi (Indian desert monitor)

The V. g. griseus subspecies (grey monitor) has five to eight narrow grey bands on its back, as well as 19-28 bands on its tail. Its tail is more rounded that those of the other subspecies, and the final size of the adult depends on which habitat they are living. Their coloration can be from simple grey (in desert-like ecosystems) to brilliantly colored (in areas with large amounts of plant growth). Their most common prey is lizards and snakes, but can also include ground-nesting birds and other small mammals.[3]

The V. g. caspius subspecies (Caspian monitor) has five to eight bands on its back, 13-19 bands on its tail, a plain tail tip, and about 143 rows of scales in the middle section of its body. The Caspian monitor is found from the eastern shore of the Caspian Sea and east to the Central Asian plateaus, as well as islets of the Aral Sea. They can be found at elevations up to 800 meters in the Kopet Dag Mountains, northern Iran, western and southern Afghanistan and as far as south as western Pakistan. It is best-known from the secretions of Turkmenistan, Kazakhstan, Uzbekistan, Kyrgyzstan and Tajikistan.

They are found in barren areas of mainly sand and/or clay soils, and occasionally in wooded areas. Their diets include numerous invertebrates, small lizards and birds, bird eggs, young turtles and tortoises (and their eggs), rodents, and even cobras and vipers. They are strong diggers and can easily build burrows that are several feet long.[3]

The V. g. koniecznyi subspecies (Indian desert monitor) has three to five bands on its back, 13-19 bands on its tail, a plain tail tip, 108-139 rows of scales on its midsection, and a broader and flatter head when compared to the other subspecies. This subspecies has the smallest body of the three. It is mainly found in Pakistan and west-central India, including the states of Gujarat, Rajasthan, Madhya Pradesh and Maharashtra, and probably others.

Due to climatic variations, the Indian subspecies has, reportedly, not been observed engaging in hibernation over the winter, but rather decreasing physical activity, becoming lethargic and inactive. They tend to not feed between December and March. When they resume their normal diets, prey will consist mainly of larger invertebrates and insects, but will also include smaller lizards, rodents, birds, and their chicks and eggs, and various other small vertebrates.[3]

V. griseus goes into hibernation from about September to April. In April is a mass exodus from their hibernation, and they become most active between May and July. During the middle of the day, the lizards mainly stay in their burrows and only come to the desert surface to search for food. The monitor lizards require approximately 3 to 4 full hibernation periods (years) to reach their full size (about 55–65 cm excluding their tails) and at least 3 hibernation periods before they become sexually mature. The overall lifespan of V. griseus in the wild does not normally exceed around 8 years in both males and females.[6]

V. griseus is a cold-blooded ectotherm whose behaviors therefore depend on the outside temperatures. Many lizards become sluggish in cold weather and even may become inactive if the temperature decreases substantially. Their olfactory and nerve signals significantly slow down, which severely limit the lizard from either catching potential prey or escaping from predators. The body temperature of V. griseus is directly proportional to its running speed between the temperatures of 21 and 37 °C. Between those temperatures, the running speed of the monitor lizard increases from a little over 1 m/s at 21 °C to around 3 m/s at 37 °C. Over 37 °C, its running speed does not increase, and below 21 °C, the lizards are extremely sluggish. If they are being pursued by a predator while their body temperature is less than 21 °C, they will not flee, but will instead hold their ground and become extremely aggressive.

The body temperature of V. griseus depends mainly on the outside environment (time of day, season, etc.). Their internal temperatures begin warming up before they even leave their burrows through conductive heat gain, and their temperatures rapidly rise once they begin basking in the morning sun and reach their highest point in the noonday heat. The specific body temperature of V. griseus can vary depending on the average temperatures of the country they live in, but their maximum body temperature does not usually exceed 38.5 °C even when basking in the sunlight. Male monitor lizards are generally more active and have a higher average body temperature than the females. The body temperature of the lizard during hibernation is 15.0 to 30.5 °C, but in many areas, the average body temperature during hibernation is around 16-18 °C.[7]

The species is one of the few monitor lizards that tolerate relatively cold temperatures, being present as far north as south-west Kazakhstan.[8]

Desert monitor reproduction normally takes place between May and July. Copulation occurs in May and June, and the lizards normally lay their eggs from the latter part of June through the beginning of July.[6] The eggs are incubated at temperatures from 29 to 31 °C, and hatch after an average of 120 days. At birth, the baby lizards have a total length of around 25 cm.[9]

Like most members of the genus Varanus, V. griseus is a carnivore. The preferred prey of the species is mice, eggs, or fish, but it will also prey on smaller mammals, reptiles, birds, amphibians, insects, or other invertebrates, if the opportunity presents itself.[10]

The possibility of venom in the genus Varanus is widely debated. Previously, venom was thought to be unique to Serpentes (snakes) and Heloderma (venomous lizards). The aftereffects of a Varanus bite were thought to be due to oral bacteria alone, but recent studies have shown venom glands are more likely in the mouths of several, if not all of the species. V. griseus has not yet been specifically tested, but its bites have shown aftereffects consistent with the venomous bites from other varanid lizards. The venom can be used as a defensive mechanism to fend off predators, to help digest food, to sustain oral hygiene, and possibly to help in capturing and killing prey.[10]

These lizards rarely do well and, at most, live only a few years in captivity. On occasion, when their living requirements can be specifically met, they have been documented to live for more than 17 years, although they never become docile, and never become accustomed to being handled.[3] In captivity, their environments should mirror those of many ground-dwelling animals, as well as their natural desert habitat. They require lower temperatures to hibernate during the winter, along with warmer temperatures during the summer months, and their diets in captivity should be similar to their diets in the wild.[9]