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Monitor Lizards


Varanus griseus Print E-mail

The diet of the Caspian monitor is similar to that of the nominate race. In many areas they feed on hatchling tortoises which emerge in the spring and excavate and consume tortoise eggs which are laid in May. They also eat small mammals, including the giant gerbil and young hares. In Turkmenistan adults prey extensively on large snakes, cobras and vipers up to 140cm are swallowed whole. Shammakov (1981) lists 10 species of lizards, six snakes, four birds and six mammals that are regularly eaten by the monitor lizards.  In Kyzulkhum their diet consists mainly of rodents (especially the giant gerbil) and far fewer numbers of lizards and invertebrates than are consumed in the Karakhum. In the former area the lizards regularly enter burrows in search of mammalian prey, whilst in the latter region they tend to forage mainly on the surface (Tsellarius et al 1991). In the Surhandarja Baisin they feed largely on young tortoise, small mammals and invertebrates (especially tenebrionid beetles) (Yadgarov 1968). Caspian monitors will also prey on smaller members of their own species (Makarov 1985). The lizards ability to prey on snakes that are extremely dangerous to man  is particularly interesting. Experiments to determine the Caspian monitors' ability to tolerate viper and cobra venom by injecting them with venom sufficient to kill up to 4,000 adults humans seem to indicate that the lizards have considerable resistance to both haemotoxic and neurotoxic venoms (Rjumin 1968).

In Turkmenistan Caspian monitors commence activity in late March and early April. Like other desert monitors they are active during the middle of the day in spring, but as temperatures rise they adopt a bimodal activity pattern, foraging early in the morning and late in the afternoon but spending the hottest part of the day below ground. By September and October temperatures are cool enough for them to resume a single period of activity, but they commence hibernation earlier than most other lizards and have virtually disappeared by early October (Shammakov 1981). Activity temperatures of 31.7-40.6oC have been recorded (Sokolov et al 1975; Tsellarius et al 1991). The Caspian monitor is capable of running at speeds of up to 20km per hour over short (100-150m) distances. Home ranges of over 1km2 have been determined for adults (Tsellarius et al 1991). The same study suggested that Caspian monitors may mark their territory in spring and early summer, in contrast to the work of Stanner in Israel which found no evidence of scent marking. Foraging trips may extend to over 10km per day (Tsellarius & Cherlin 1991).

ImageCaspian monitors are strong diggers. In clay desert where the substrate is too hard to excavate they shelter in mammal burrows and in river valleys they often utilise the burrows of ground dwelling birds. In sandy desert burrows are often more than 500cm long and typically 50-120cm deep. Burrows used during the spring and summer tend to be in more open areas than those used for hibernation, which are dug under bushes Yadgarov 1968; Makayev 1982; Bennett 1992b). Mating occurs during April and May. Up to 34 eggs are laid in a burrow 70-114cm deep usually situated on a slope during June and July. Sometimes the burrows of rodents are used as nesting sites. Females spend up to a week digging test holes and preparing nests, guard the eggs for several weeks after egglaying and have been reported to return to the vicinity of their eggs around hatching time. The eggs hatch in September or October but the youngsters remain together in the nest and do not commence activity until the following spring. Eggs laid in captivity weigh 32-35g (Yadgarov 1968; Shammakov 1981; Tsellarius & Menshikov 1995; Dujsebayeva 1995; Makayev, pers.comm; Kudryatsev pers. comm.). Sexual maturity is attained within three years (Shammakov 1981). Tsellarius and Cherlin (1991) note that many Caspian monitors have scars on the back, which they attribute to attempted predation by birds, but are interpreted in other species as wounds received during ritual combat. Bipedal combat occurs in the typical fashion of large monitor lizards (Tsellarius 1994).

Tsellarius and Menshikov (1994) suggest that female Caspian monitors in the western Kyzylkum desert often do not produce eggs until they are at least five years old, although they are courted by males from the age of two or three. They divide the year of the Caspian monitor into a period of hibernation, an establishing period (during which males show increased interest in all other monitors), a very short breeding period of about ten days (in which courtship and mating occur), and a private period (when the lizards ignore each other and gorge themselves with food prior to hibernation). Their data, gathered by direct observations and by following footprints shed light on an almost completely unknown facet of monitor life (Chapter 3). In this area the adults live in loose communities, including individuals of at least 15 years old, where they rarely meet outside the breeding season, but are acquainted with each other through encountering each others' tracks. Whilst in some areas the monitors' paths cross each other only every 500m or so, in other areas they cross every 50m. Thus the activity areas of the lizards are largely overlapping and all resident animals quickly become aware of the presence of newcomers. Whilst actual encounters with known individuals elicit ritualised behaviour (Chapter 4), meeting an animal whose smell is unfamiliar is more likely to result in aggression. Therefore newcomers frequently mark the paths of resident animals, presumably to acquaint them with their smell before they meet, reducing the chances of an unwelcoming reception.



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