Mertens' goanna, Bulliwallah.
Mertens' goanna is perhaps the most amphibious member
of the monitor lizard family. It is found in northern
Australia, from Western Australia east to western
Queensland. According to Schmida (1985) they are
t:ommon on waterways thoughout northern Australia. Gow (1981) records them from
Groote Eylandt and Schurer & Horn (1978) and Storr (1980) provide location data on the
mainland . Mertens' goanna has a long tail (up to 183% of SVL) which shows extreme lateral
t:ompression . The nostrils are situated on the top of the head (Mertens 1958) and Cogger
(1959) notes that they can be sealed whilst the lizard is submerged. In Western Australia
Storr (1980) gives a maximum size of 47 .5cm SVL (113cm TL). Maximum size according to
Horn (in Eidenmuller 1990) is 160cm TL. Bratzler (1965) records a 126cm TL specimen in
captivity that weighed 4900g. A pair of long term captives maintained by Eidenmuller
measured 85cm (female) and 80cm (male) TL after nine years. Gow (1981) records snout
vent lengths of 30 and 40cm on Groote Eylandt. At Jabiru, Northern Territory, they reach
sexual maturity at 28cm (for females) and 32cm SVL (for males). There is no apparent
difference in size between the sexes. The largest male was 41cm SVL and the largest female
measured 46cm SVL (Shine 1986). In this area they were most commonly seen around small
t:reeks (lying either in very close to the water) rather than larger bodies of water and were
often sympatric with estuarine crocodiles (Crocodylus porosus
). They feed largely on crabs,
fish, frogs and turtle eggs together with small numbers of insects. Gravid females (containing
up to 11eggs) were collected in April and June and a female about to ovulate was collected
in the wet season (December). This suggest that most breeding occurs in the dry season, but
that some reproduction may occur at other times of the year. Egg size is given as 6 × 3.Scm,
volume 77cc. In captivity clutch sizes of up to 14 eggs have been reported (Irwin 1986).
Hatchlings measure 24-27cm TL and weigh about 24-28g (Brotzler 1965).
Mertens' goanna is rarely found more than a metre or so from water. They are frequently
found in rocky gorges and have been recorded along slow and fast moving rivers, reservoirs
and billabongs (Bustard 1970: Schurer & Hom 1978). They are described by several authors
as being crocodilian in habit, basking on the banks of lakes and rivers and sliding into the
water at the first sign of danger. Swanson (1976) notes that when walking underwater they
will keep their eyes wide open. During the wet season a great deal more habitat is available
to them, and many move to temporary bodies of water at this time (Stammer 1970; Shine
1986). Hermes (1981) saw a 100cm long Mertens' goanna collecting fish from a shallow pool
by sweeping its tail in an arc to concentrate the fish close to its mouth and grabbing them from above. Like V.salvator
, Mertens' monitor has to leave the water in order to swallow
large food items.
Another important adaptation for an amphibious lifestyle in V.mertensi
appears to be its
ability to maintain activity at low body temperatures. Schurer & Horn (1978) caught a
specimen during the winter in water of only 17°C. The goanna had a body temperature of
18°C. Eidenmuller (1990) records specimens active in water of 26-32°C during the summer.
Green & King (1993) record activity temperatures of 32.7°C for a wild specimen and
for a captive one. These goannas are often found in large numbers along water courses.
Schurer & Horn (1978) report seeing 12 specimens along a 50m stretch of riverbank.
Eidenmuller (1990) saw five adults along 200m of the bank of Lake Argyll and Stammer
(1970) saw eight specimens along 200m of Lake Moondara. Despite their close proximity to
each other they are not reported to routinely share basking sites.
Not surprisingly, most of Mertens' goannas food is taken from the water. Crabs appear to be
its favourite food , together with fish, frogs, crayfish, shrimps and amphipods. They also eat
reptile eggs birds, mammals and a number of terrestrial invertebrates such as orthopterans,
spiders, dragonflies, beetles and bugs (Shine 1986; Stewart 1982; Losos & Greene 1988).
They are known to scavenge among human rubbish (showing a particular taste for sausages)
and probably eat carrion when the opportunity arises (Shine 1986; Philippen 1994b). Stewart
(1982) records that one ate almost a kilo of fresh buffalo meat in less than a minute. He also
comments on the initial wariness of the goanna to humanity, how it eventually began to
solicit him for bits of food and how, after two months, it eventually disappeared after having
been tormented . Like other large goannas, V.mertensi
adopts a bipedal stance when
threatened and engages in bipedal ritual combat (Murphy & Lamoreaux 1978; Braithwaite (in
Greer 1989); Green & King 1993). Murphy & Lamoreaux (1978) also describe other
aggressive/defensive behaviours exhibited by these elegant creatures. Although little is
known of their breeding habits in the wild, specimens kept outside in Queensland have laid
eggs in March, buried in a vertical burrow 50cm deep (Irwin 1986). Another source
(Swanson 1976) suggests that eggs are laid in leaf litter deposited at the end of a burrow,
after which the entrance is tightly sealed.
There are several reports concerning successful maintenance and breeding of this species in
captivity (Brotzler 1965; Irwin 1986; Eidenmuller 1990, 1991). The size of the enclosure
does not appear to be particularly important, so long as it contains a body of water large
enough for the lizards to immerse themselves. Eidenmuller (1990) had great success with a
pair kept in an aquarium of 150 × 60 × 4Ocm, comprised of about 70%
water (heated to
28°C and 30% land. He reports that the animals are very tolerant of each other and Irwin
(1986) also conunents on the lack of aggression between specimens housed together. The
enclosure should contain plenty of hiding places (hollow logs are ideal). These goannas will
often rest on branches above the ground when given the opportunity. A diet of freshwater
fishes, small rodents and insects is suitable. Schurer & Horn (1978) note that they have
difficulty swallowing larger rodents and refuse small adult birds, although they will accept
chicks. Worrel (1956) considered their favourite foods in captivity to be fish and frogs.
Courtship in this species appears to be a very gentle affair. In captivity at least, it may occur
at any time of the year. Eggs incubated at 29-32°C hatch after 182-277 days (Brotzler 1965;
Irwin 1986). At 27 -28°C they hatch after 265-316 days (Eidenmuller 1990). The hatchlings,
which can be raised together, will enter the water immediately and take small fish and rodents.
A Ithough this species is very rarely seen on dealers' lists it is hoped that the
significant number of captive bred specimens produced in the last few years will result in this
beautiful monitor lizard being more widely available. Under good conditions they can live for
over 20 years (Snider & Bowler 1992).