Monitor Lizards aka Goannas
  • Last updated:
  • 25 Sep 2018

At our place the Kookaburras give a special call to warn of a monitor lizard on the prowl. Even the dog recognises the call and gets a bit edgy.  However, in the last four or five weeks there have been no calls and no sightings. We do not know how to interpret that – we thought the dry spell might be ideal for the reptiles that should now be starting to “feed up” for the colder period ahead. Perhaps the same dry conditions meant fewer eggs and nestlings to be predated. We have certainly noticed less nesting by the migratory birds, including Drongos, Forest Kingfishers and several Honeyeaters.

Monitor lizards are found in Africa, North and South Asia and Australia. They include a number of the largest lizard species in existence. The name Monitor comes from the habit of some species of these reptiles standing on their hind legs to check out (monitor) the surroundings. Here in Australia we usually call them Goannas, derived from Iguana, the name given to a genus of large American and Pacific Islands lizards.

Up to around 40,000 years ago, a large and savage reptile roamed Australia. This was Megalania priscus (the name actually means “great ancient roamer”) the largest terrestrial true lizard that ever lived. From fossil evidence, it is thought to have reached 7 metres in length; that is around twice the length of a Komodo Dragon. Its weight was estimated at 600 kilos or more. Armed with serrated blade-like teeth and an oral venom gland, this lizard was the top predator, easily able to prey on our ancient megafauna including Diprotodons, Giant Short-faced Kangaroos and other large marsupials existing at that time. Its speed was about three metres per second (not exactly Usain Bolt at just over 10 metres per second) so it captured its prey mainly by ambush. Whether its prey included humans – and or vice versa  – is an interesting question given the coinciding timeline of human arrival and the lizard’s disappearance.

It has now been established that Megalania had a very similar anatomy to the extant monitor lizards (Goannas) of Australia as well as the Komodo Dragon of Indonesia. In fact, it has been placed in the genus Varanus with all the others. (Varanus comes from the Arabic “waran” meaning lizard.)

Lace Monitor

Luckily, we don’t have to worry about these giant killers any more however, here in Australia there are still a few large extant monitors. The second largest, usually common in this area and any timbered area in a wide belt along the East Coast is the Lace Monitor, Varanus varius. This goanna is generally responsible for raiding hen-houses for chicks and eggs and seems to turn up at many local picnic spots. They can smell a good barbecue a mile away! In fact their olfactory sense is greatly enhanced through possession of a vomeronasal organ (VMO), also called Jacobson’s organ located on the palate in the roof of their mouths. This is a feature they share with snakes. The long, narrow, deeply cleft tongue is continuously flicked in and out, picking up particles of scent for analysis in the VMO as part of their chemosensory system. When retracted the forked tongue fits into two sites on the palate, depositing the scent particles. This way goannas can find and track prey by both smelling and using their long tongues.

The tongues are actually soft and cool and the claws are very sharp, this I know. Once at a barbecue a big goanna approached me and started licking my knee. I held pretty still! A dog appeared out of nowhere and the panicked goanna ran up the nearest vertical object. That was me, and I confess I freaked a bit.

At our place, we regularly used to see a few small ones (about a metre) and there is one huge, approximately two metre long resident. It has a good turn of speed to get to the nearest tree and despite its size is an excellent climber. Goannas are clever in that once they are on a tree trunk they always keep the trunk between themselves and an observer. We have also seen one leap a good two metres horizontally to reach another tree. One of the smaller Goannas climbed up to take Rosella chicks out of a nest-box despite the parent birds calling and flapping.

Like other reptiles, they are competent swimmers. We have watched one swim across the Mooloolaba River.

Goannas typically lay six to twelve eggs and either dig a burrow or lay them in a termite nest. Often the termites seal them in as they repair their nest. Despite this, after six weeks or so the mum goanna knows exactly where and when to dig to free the hatchlings.

Top predators

Goannas are still one of our top predators; they eat any mammals, birds and reptiles they can catch and overpower and they are known to dig out turkey mounds to scavenge the eggs. They also feed on carrion when they find it. This latter habit probably led to the belief that a bite from a goanna will take forever to heal. Whilst it is very possible that a bite could be infected, it is also likely that the symptoms are caused by mild venom injected with the bite. In turn, small goannas are preyed upon by raptors, snakes and larger goannas.

Some time ago at the Noosa National Park picnic area we watched an interesting event: a large goanna was approaching a barbecue when a brush turkey raced out of the bush, grabbed the lizard by the tail and dragged it away four or five metres. The turkey did this repeatedly until the goanna gave up and retreated.

In Australia all reptiles are protected by law. Native wildlife is protected for very good reasons. It is disappointing that so many of our amazing native goannas are killed every year for stealing a few eggs and the odd chicken or two. We do not want them to join Megalania in extinction.

Article by Kon Hepers.