Varanus Giganteus

Gray 1845 Perentie

The perentie is the king of the Australian goannas. It is the largest lizard on the continent and one of the most beautiful animals in the world. It was once thought to be an uncommon creature but in recent years the number of sightings has greatly increased and the known range of the species has expanded. Despite their large size perenties are extremely shy and wary animals. Their pattern provides excellent camouflage and it is quite possible to walk within a metre or so of a large adult without ever being aware of its presence. Because of their ability to live undetected it is not clear whether perenties have expanded their range in the last few decades (see below) or whether they have succeeded in remaining unnoticed in many areas until recently. The perentie is found in arid areas of all mainland Australian states except Victoria and inhabits many islands off the western coast (Storr 1980). It is absent from the east coast and the tropical forests of the north , with northern and southern limits at about 18° and 30° longitude (Gow 1981a; Storr 1980; Houston 1978). They are said to be particularly common on Barrow Island off Western Australia (Smith 1976; King, et al 1989).

The maximum size of the perentie is probably not as great as many authors claim King & Green (1993) provide a useful discussion of size in this species. Stirling (1912) suggested a maximum size of almost 230cm. The longest found on Barrow Island by King et al (1989) had a total length of 196cm (88cm SVL) and the heaviest weighed 11.7kg. Butler (1970) record s a specimen of 17kg with a total length of 193cm from the same island. Stokes (1846) records that two specimens collected on Barrow Island in 1840 had total lengths of 213cm. Strimple ( 1988) suggested that one of these animals was the type specimen used by Gray, which has a total length of only 202cm (Mertens 1958). Many perenties do not grow to such an enormous size, and the specimens on Barrow Island may grow larger than the mainJand population (Case & Schwaner 1993). The limited data available suggests that females reach a smaller maximum size than males, rarely exceeding 140cm TL. Bredl (1987) records males of 180 and 190cm TL, and a female of 130cm TL. A male kept for 14 years in captivity had a total length of 135cm and weighed 2.9kg, whilst a female kept for seven years measured 116cm TL and weighed 1.55kg (Banks, pers.comm.). A specimen I saw in the Great Victoria desert measured 52cm SVL (l23cm TL) and weighed 1.8kg when its stomach was empty. It appears that perenties approaching 200cm in length are the exception rather than the rule, and that in general few specimens grow larger than 150cm. If specimens of 240cm or more have ever existed, none seem to have survived to the present.

Perenties are associated with desert regions and particularly with rock outcrops. However they are also found on grasslands and shrublands devoid of exposed rock. Where caves and crevices are available the perenties will use them for shelter, otherwise they take refuge in burrows. Pianka (1994) suggests that the spread of rabbits through the interior of Australia may have enabled the perenties to occupy many areas that were previously unable to support them. The perenties certainly prey on mammals to a greater extent than any other Australian monitor lizard. Stirling (1912) and McPhee (1959) both record that they are able to kill kangaroos and dismember those that are too large to be swallowed whole with the powerful forelimbs and claws. Lizards probably account for most of their food especially large and dwarf goannas including weaker members of the same species (Pianka 1994). They are al so recorded as having eaten skinks. agamids, seagulls, orthopterans, centipedes and chilopods and are known to dig up the eggs and young of turtles from sandy beaches (Losos & Greene 1988; Green et al 1988, James, Losos & King 1992; Butler 1970). Much of the perenties' prey may be caught in open pursuit; they are reported to catch gulls by hiding under vehicles until the birds come close enough to allow them to be chased down before they can take to the air (Losos & Greene 1988; King et al 1989). Prey is usually killed by violent shaking. Horn & Visser (1988) suggest that perenties and sand goannas rarely occur in the same area, with sand goannas occupying open areas in contrast to the rock outcrops favoured by perenties. However a perentie we found on a flat spinifex plain in the Great Victoria Desert had swallowed a desert sand goanna almost the same length as itself and weighing about a quarter as much as its predator (pianka 1994; Bennett in press).

Stirling (1912) records that perenties will mistake stationary people or horses for trees and climb up them in their attempts to escape. This behaviour has been observed many times (e.g. Pianka 1994b, Bennett in press). Perenties do not appear to be active throughout the year. According to Stirling (1912) they are inactive from May to August and pairs of animals may share the same burrow. This is confirmed by Hom & Visser (1988) who found a pair of perenties in a cave at the beginning of October. Pianka (1982) found a large specimen in a burrow I m deep which he believed had been underground for at least a week.

Perenties are extremely wary animals in the desert, although on Barrow Island they are said to be accustomed to people and relatively easy to observe. When threatened they often remain motionless, either in a stiff, extended pose (Bustard 1970) or lying flat on the ground (Swanson 1976; Hom & Visser 1988). They do not appear to adopt a bipedal stance for defence, but when running may take to the back legs for short periods (Stirling 1912). Barrett (1950) published a picture of two perenties engaged in bipedal ritual combat. Similar photographs by Waite (1929) attributed to V.giganteus are actually of V.spenceri (Horn 1981).

When chased perenties will seek refuge in trees, fallen logs, burrows or even in water (Pianka 1982; Hom & Visser 1988). According to Cogger (in Strimple 1988) they adopt an arboreal existence in some parts of Western Australia. Between September and June perenties are only active in early morning and late afternoon in Western Australia (Heger & Heger 1993). A similar bimodal activity pattern has been observed during the summer on Barrow Island (King, Green & Butler 1989). Perenties maintain active body temperatures of 33-39°C and can drop to 27°C at night (pianka 1982; King et al 1989). Breeding is said to occur in the spring (September - October) (Hom & Visser 1988) or summer (November to January) (Heger & Heger 1993). On Barrow Island mating occurs in Spring and hatchlings appear the following November (King et al 1989, Butler 1970). Here, activity ranges over ten days of up to 0.2km² have been recorded and the same specimen has been found at locations over 5km apart. In the desert the lizards are likely to Cover much larger areas. I followed perentie tracks made in one afternoon and the early part of the next morning which extended over 2.5km. Pianka (1982) reports that tracks often exceed 1 km. They are able to accumulate large fat reserves. Stirling (1912) reports that an adult that died after a three month fast still weighed 7.7kg and contained fat bodies weighing almost 1kg. Details of metabolism are given by Thompson (1995).

Perenties have been maintained at a number of zoos in the Northern Hemisphere, but captive reproduction has only been achieved in outdoor enclosures in Australia (Bredl 1987 & pers.comrn: Bredl & Horn 1987). Up to four adult specimens can be housed in an enclosure with 150m² of noor space. The substrate should allow the lizards to dig and large rocks provided to help them feel secure. Males may be very intolerant of each other but once compatibility between pairs has been established they may live together for many years without any violent incidents. However, perenties practise cannibalism regularly in the wild and care must be taken that only animals of compatible sizes are housed together. Mating and egglaying occur in late spring and early summer. Up to 11eggs, each weighing around 80g, are laid in a single clutch, which hatch after about 8 months at 30-32°C with 85% humidity. Hatchlings weigh 30-50g and measure 14.5-15.5cm SVL (37-38cm TL). If incubated with higher humidity they would doubtless weigh more. After a month they increase in weight by ahout 50%. They should be fed a mixed diet of reptiles, birds, mammals, freshwater fish and large insects, together with vitamin and mineral supplements. A short period of inactivity during the winter is probably beneficial. Green et al (1986) estimated that during the summer, when food is abundant (in this case baby turtles containing about 36kJ of energy per gramme of body weight), adults eat about 36g of prey per kg of body weight, using 118kJ of energy per kg per day to sustain themselves. During another summer when food was less abundant they used only about 57kJ per kg per day and consumed 7.3g of food per kg per day. Perenties have survived in captivity for almost 20 years (Snider & Bowler 1992).

Attribution / Courtesy: Daniel Bennett. 1995. A Little Book of Monitor Lizards. Viper Press U.K.