Varanus

Varanus mertensi GLAUERT 1951
Mertens’ Water Monitor

 
Varanus mertensi
Adelaide River, NT
mertensi
Varanus mertensi
Wangi Falls, NT
mertensi
Varanus mertensi
Wangi Falls, NT
mertensi
Varanus mertensi
Wangi Falls, NT

Varanus mertensi is a member of the subgenera Varanus. With a total length up to 150 cm, this species is one of the smaller members within this subgenera. The greatest members of the monitors are combined in this subgenera.
V. mertensi is very strongly adapted to the life in water, which can be recognize at the strongly depressed, with an clearly seen keel tail, which is approximately 1.5 times as long as SVL. Also the position of the nostrils, positoned on the top of the snout, are a sign, that this goanna prefers to live in the water. The distance between nuchalia and eye is approximately twice as big as the distance between nuchalia and tip of the snout. As soon as the animals dive, the nostrils are automatically blocked by flaps.
The major color of V. mertensi is a dark olive green. Some pale yellow spots, surrounded by black scales are irregularly distributed over the back. The throat is bright yellow. An small bluish stripe stretches along the upper jaw, under the ear opening along the neck up to the shoulder belt. The head scales are moderately large, regularly ordered and smooth. 150-190 scale rows are around midbody. The tail scales are slightly keeled, they are not ordered in regular rings since the scales on the underside are larger than the one on top.


Distribution, habitat and behavior

mertensi

mertensiThe distribution of V. mertensi is restricted to the tropical north of Australia. It goes from Broome in the west up to Cape York Peninsula in the east. The animals are very strongly bound to water. Through this the distribution area isn't continuous. Despite this partly strong isolation of single populations the pheno type hardly varies. Own observations made in the different habitats showed, that monitors from the Mt. Isa area in western Queensland had more rounded snouts than animals from the western distribution area. No subspecieses have been described yet (MERTENS 1958, mertensiKEAST 1959, SWANSON 1976, WORRELL 1966, BUSTARD 1968, STORR 1980, STORR et al. 1983b, WILSON & KNOWLES 1988, HOSER 1989, COGGER 1992, EHMANN 1992).
V. mertensi lasts in the free country. almost exclusively in the water. Only occasionally the animals climb onto rocks or tree-trunks lying at the shore for sunbasking. The animals frequently lie within aquatic plants to sunbask. The diet is also adapted strongly to the life in the water. The main food consists of fish, crabs, frogs, insects and the larvae (SHINE 1986), which were studied from stomach contents examinations. HERMES (1981) could observe some monitors at a billabong in the Kakadu National Park, as they trapped fishes with their tails. They chased them into shallow water to grab and swallow them).


Keeping and breeding

mertensiThe keeping of V. mertensi is a bit more difficult, because these monitors need a fairly big watertank. We choose an aquarium as basis for the enclosure, corresponding to the size and the special request of this species. The rock formation, which is used for basking consist of styrofoam with epoxy resin. The water should have a temperature of 27-29°C. Observations and measurings in the wild have shown that the animals like to go into areas in which these temperatures are reached or surpassed insignificantly.
Own experiences at the long-terrm captive care have shown that you can keep one pairs of V. mertensi together year round. Aggressive behavior could be never observed. In the wild the monitors have a hibernation like phase during the dry season between July and September, which should be simulated in the enclosure. Since the seasonal rhythm of the south hemisphere is different to the rhythm of the northern hemisphere however, the activity and quiet phases will change continuously the rhythm of the northern hemoisphere. However, this does not cause any problems with the care. Just let the animals adapt to the rhythm. Both monitors should accept rhythm for the same since otherwise perhaps a sexual synchronization does not take place. If the propagation cycles within the partners are not identical, the egg developing does not show any fertilization capability. Through this a successful breeding would not be possible.
mertensiAs food we will offer several different items, including locusts, cockroaches, mice, fish and occasional fresh water crabs according to the size of the animals.
The first described captive breeding outside Australia happened in the Stuttgart Wilhelma (BROTZLER 1965). Within the last years the captive breeding of V. mertensi was very successful. Breeding up to the F3 generation was already described (EIDENMÜLLER & WICKER 1995, POLLECK 1999). These lovely monitors need larger enclosures to be able behave normally (see above). If both adults are sexual active, a mating behavior of the animals will start. These activities can proceed very turbulent, if the female is not willing to mate. Mostly the mating starts in the water and will continue on the rocks. The copulation can happen under water as well as on the land. As a rule, only one mating takes place during that period. In other monitor species the time between mating and egg-laying is approximately six weeks (own experiences). In this species it take exactly 22 days. To avoid stress for the female in front of the egg-laying and to avoid that the male will swallow the eggs just just after deposition, we will move the male into another enclosure. As soon as the female has dropped the eggs, we will remove them from the nesting box and incubate them in separate boxes, prepared with damp vermiculite or perlite, in an incubator. We adjust the breeding temperature to 27-29°C.
After an incubation period of 193 -327 (mean value 275) days the babies will hatch. They measure about 30 cm in total length, about 18 cm are part of the tail. The average mass is 29 g. They are an exact replicate of the adults, however the coloration is more brighter in contrast. You can raise the babies together in one enclosure. There were problems with the offspring till now. Sometimes the incubation period and the size of the babies occasionally were very different anyway, but you can keep the young animals of one clutch together. The enclosure should be built up similar to that one of the parents. The offspring can swim immediately. Therefore a big water basin is very important. The youngsters start feeding after approx. 5 days. We offer them crickets, house crickets, cockroaches, larvae of the big mealworm (Zophobas morio), fish and little fresh water crabs. The fish are hunted under water and got away after some time. We always powder the insects with a vitamin mineral mixture in front of the feeding. With this diet the offspring will reach the sexually maturity after 3-4 years.

   
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