About Me

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Canberra-based naturalist, conservationist, educator since 1980. I’m passionate about the natural world (especially the southern hemisphere), and trying to understand it and to share such understandings. To that aim I’ve written several books (most recently 'Birds in Their Habitats' and 'Australian Bird Names; origins and meanings'), run tours all over Australia, and for the last decade to South America, done a lot of ABC radio work, chaired a government environmental advisory committee and taught many adult education classes – and of course presented this blog, since 2012. I am the recipient of the Australian Natural History Medallion, the Australian Plants Award and most recently a Medal of the Order of Australia for ‘services to conservation and the environment’. I live happily in suburban Duffy with my partner Louise surrounded by a dense native garden and lots of birds.

Friday 31 May 2013

Goanna, Go!

Australia has truly been described as the land of lizards, with central Australian spinifex hummock grasslands containing over 400 lizards per hectare, with over 40 species co-existing. They are both predators and prey in an ecosystem where the spinifex supports a vast biomass of grazing termites, which fill the role of antelopes in the African plains. The top predators - the lions if you like - in this system are goannas of the family Varanidae. They don't generally stoop to eating termites, but they eat many smaller animals which do. 
Sand Goanna Varanus gouldii north of Bourke, New South Wales.
This familiar species is found across virtually the whole continent, missing only from the far south-east and north-east.
Elsewhere in their wide range - across Africa and southern Asia - they are generally known in English as monitors, allegedly for their supposed habit of 'warning' of the presence of crocodiles. In Australia however (where varanids have reached the peak of their diversity) the British colonists, who certainly weren't herpetologists, confused them with New World iguanas; the quintessentially Australian 'goanna' is a corruption of that. However, we're a perverse lot, and some local species are still called 'monitors'...
Lace Monitor V. varius, Finch Hatton Gorge, near Mackay, tropical Queensland.
This is probably the best-known Australian goanna, being found right along the east coast where most people also live.
It's a big animal, up to two metres long, and can be a tad intimidating around camps and picnic areas.
It seems that the goannas are more closely related to snakes than to other lizards, though this is not universally accepted. One thing that goannas and snakes have in common, only recently recognised, is the presence of venom in the mouths of varanids (and iguana relatives), and other work proposes that this supports the idea of a common ancestor for all three groups. To date most of the work on this relates to the huge Komodo Dragon V. komodoensis of Indonesia; venom flows from glands between the front teeth into wounds made by the animal's ferocious bite. There have long been stories in Australia that goanna bites "never heal"; it was supposed that this relates to a diet that includes carcasses and poor dental hygiene, but there may be more to it than that. These venoms are relatively low-potency and slow acting; it seems that the large victim is simply patiently followed, perhaps for days, until it weakens.

Goannas and snakes (and at least some other lizards) make use of Jacobson's Organs, a pair of sensory organs located above the roof of the mouth; while not unique to goannas, they are particularly well-developed in them and are doubtless very significant. Chemicals in the air are transferred by the tongue to the organ via ducts; this sense (perhaps best thought of by those of us without them as 'smell-taste') appears to supplement scent.
Yellow-spotted Goanna Varanus panoptes, Bladensburg National Park, Queensland.
The forked tongue - another goanna characteristic reminiscent of snakes - flicks constantly in and out, 'tasting' the air.
This species is found across much of north-east and north-west Australia, but where the introduced poisonous Cane Toads have invaded, numbers have crashed. Fortunately for this individual (investigating our camp) the toads
haven't entered this drier environment.
The largest goanna that lived only left the scene some 40,000 years ago, well within the time of human occupation of Australia. Megalania prisca (sometimes called Varanus priscus) may have been up to seven metres long and weighed well over a tonne; I would not have argued the occupation of a campsite with that one! Moreover it is within the venom-bearing group, and lived long after that group split up, so would have probably been the largest venomous animal ever to live.
Yellow-spotted Goanna in camp, Bladensburg NP.

Goannas are now the top predators in most of Australia (leaving aside the recently-introduced Dingo); indeed it has been suggested - I hope in jest! - that marsupials don't need a very big brain to stay a jump ahead of a reptile... Indeed these reptiles show intelligence in locating and accessing prey; they dig out burrows, either of mammals or scorpions, and climb readily. For both activities, their enormous claws are extremely efficient.
Yellow-spotted Goanna, Bladensburg NP; note huge claws.

They are the bane of the lives of tree hollow-nesting birds, which will attack them fiercely, but often with little effect. Many species will also take to the trees when threatened; plenty of stories also tell of them running up the nearest tall object in such a situation, including a person, though first-hand accounts are harder to find...

Lace Monitor on ironbark, Pilliga State Forest, New South Wales.
They effortlessly descend head-first.

All lay leathery eggs; some seal them in termite mounds where the controlled temperature and humidity is perfect for them. There is now evidence that the female of these species returns at just the right time to break open the mound again and release the hatchlings. 

Goannas are an integral part of Australia, through every part of which they still swagger with an inimitable insouciance, and I for one am glad of it.
Sand Goanna, above, and Yellow-spotted Goanna, below.



Susan said...

I love goannas -- they are magnificent creatures (if a little intimidating).

Flabmeister said...

As always, very informative.

I was taken by the sentence "It seems that the goannas are more closely related to snakes than to other lizards, though this is not universally accepted." Is there any aspect of species development which is universally accepted (ignoring the Darwinism/Intelligent Design rift).