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Quintessential Australian snakes

Synonymous with summer, our venomous snakes are misunderstood Aussie icons.

Dwarf king brown

A dwarf king brown snake is known for its highly toxic venom. (Source: Heather Catchpole)

Australia is justifiably famous for its snakes. We harbour more venomous snakes than anywhere else in the world - of the 140 species of land snakes, 110 are venomous, 12 of which are deadly to humans.

A familiar sight in backyards and on bush tracks in summer, snakes possess an 'otherworldliness' that renders them fascinating to those who dedicate their lives to the study of these extraordinary creatures. Snakes can move rapidly without limbs, swallow prey much larger than themselves and possess an excellent sense of smell.

Australian ecologist Rick Shine from the University of Sydney has spent 30 years getting to know snakes.

"Snakes are one of the great success stories of modern times. They evolved recently, diversified rapidly and became very common. They really are quintessential Australian animals," says Shine.

On an island continent without large carnivorous mammals, these much-maligned and misunderstood creatures have evolved to become our top land predators, and occupy a wide variety of habitats from the desert to the sea.

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Australian snakes evolve

The largest group of venomous snakes in Australia are elapids. The family includes the broad-headed snakes, tiger snakes, Australian copperheads, red-bellied black snakes, king brown snakes and death adders, and varies in size from 20cm long up to the deadly taipan, which grows to three metres.

By looking at the structure of their DNA scientists estimate that venomous or elapid snakes first appeared in Australia around 20 million years ago.

"They may have come from the ocean, they may well have had an ancestor somewhat like the living sea kraits," says Shine.

Some sea krait species spend most of their lives in the water catching eels and fish but return to land to lay eggs. So maybe they swam to Australia from Asia.

What we do know is that Australia was the perfect home for elapids. They spread across Australia, dominating the southern half of the continent, and diversified rapidly, in particular taking to the trees.

"Australia went through phases when it was pretty cool and damp and had lovely big forests, and so we probably had all kinds of wonderful elapids, most of which we only have tiny remnants of now," says Shine.

About five million years ago, as Australia was going through another Ice Age, some elapids returned to the sea and evolved into new families of extremely venomous sea snakes. So not only does Australia have the highest number of venomous snakes on land, but also in the oceans to the north and east of the continent.

These colourful snakes from the families Hydrophiidae and Laticaudidae live mostly on fish and eels.

"Most of today's living sea snakes are in fact very recently evolved species that have come from the Australian tiger snakes," says Shine.

Fact file:

When: Most southern snakes produce young in the heat of summer. Snakes can be egg layers (oviparous) or live-bearers (oviviparous). Oviviparous snakes do not gestate like mammals but hatch their eggs inside their body.

Where: Snakes are found throughout Australia from the northern wet tropics, where pythons and tree snakes dominate, to the south, where venomous land snakes are more common. They inhabit all environments in Australia from the desert to the sea.

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Successful predators

With Australia's very low rainfall, poor soils and low productivity it's very hard for warm-blooded animals that need to eat constantly to make a living in droughts, so snakes are well adapted predators in many Australian ecosystems.

"Animals like snakes that can have one meal and then sit around for a year before the next one are beautifully suited to dealing with a very harsh, unpredictable environment that Australia provides," says Shine.

Snake diet staples include small mammals, reptiles, frogs, insects and fish.

But the way they hunt for this prey can vary quite widely between different species. Some snakes such as eastern brown snakes are active during the day, others prefer to hunt in the evening, while some species are more active at night during the hotter months.

Many snakes have excellent eyesight, but it's not usually the main sense they rely on when hunting. The earthworm-like blind snake, which feeds on ants and termites, has eyes which have devolved to mere eyespots. These snakes can sense light but not see in the same way as other species.

Snakes' most vital sense is their sense of smell. They smell by flicking out their forked tongue, which then presses back against an opening on their mouth leading to the Jacobson's organ, a sensory organ which can distinguish between different odour chemicals.

Their forked tongue helps the snake determine the direction of the scent; in the same way we use the arrival of sound vibration at either of our ears to detect the location of a sound. They can also pick up sounds through vibrations along the ground which travel through muscles to bones in their jaw.

Some snakes, such as the eastern brown snake, are active hunters. Once they've detected their prey, the race is on. Their lean muscular bodies enable them to move quickly after fast-moving prey. Once they are close enough they will bite the animal and then coil around their victim until it dies.

Other species like the death adder lie in wait for prey, using their diamond-shaped scales as camouflage and their thin worm-like tails as a lure for small animals who think they've found some food of their own. When the animal gets close the adder strikes and kills it almost instantly with its powerful venom.

Snake fangs, which may be at the front for elapids or at the back of their mouths for members of the Colubridae family such as green and northern tree snakes , vary from perfect hypodermic syringes to simple grooved needles.

Snake venom is a complex and highly evolved saliva with a number of characteristics peculiar to each species. Elapids have venom which is a powerful neurotoxin. Snake venom can also contain components poisonous to nerves, muscles, blood, cells and cardiotoxins.

The two halves of a snake's jaw are joined together by flexible ligaments, enabling them to swallow relatively large prey. Powerful digestive juices then break down the meal, which satisfies the snake for several days.

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A sign of summer?

These cold blooded creatures obtain body heat from the sun - their ideal body temperature is 30°C — which they can warm to quite quickly when the outside weather is just 10 to 12°C.

Some species, such as the inland taipan, change the colour of their scales to make better use of the sun, and have a lighter colouring in the summer so they can reflect more heat.

While snakes are the most active in spring and autumn, they lay eggs or give birth to live young — a feature which evolved in the Elapid family around five million years ago — in summer. Some species hibernate in southern winters, others become more sluggish as their metabolism slows but remain active through the colder months.

But while they are more active in warmer weather, snakes still spend 80 to 90 per cent of their time in warm burrows or under rocks — not only to regulate their temperature but to avoid predators such as birds, goannas, dingoes, cats, dogs and foxes.

"If there's no reason to be out there you're better off being hidden," says Shine. "Many of these animals live for 20 or 30 years and they're pretty careful to try to avoid things like predators."

Despite their evil reputation, most snakes will go to amazing lengths not to bite you, says Shine.

If you come across a snake in the bush, walk away, he advises.

"In my opinion there's no such thing as an aggressive snake."

Snakes will respond if you're a threat or if you're prey, if you're not in the area you can't be either.

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Unique Australians

Australia's snakes — both venomous and their non-venomous cousins such as pythons and file snakes — are extraordinarily mysterious, elegant and beautiful creatures, says Shine.

All native snakes are protected, and some are under threat from habitat loss, feral species, and road-kill. We still don't know enough about these exceptional creatures, Shine warns, with ecological research unfairly biased towards "cute and cuddlies".

"Snakes play an important role in the ecological system - remove the predators, and you could tip the ecological balance."

"Ecologically they're very important and because they're so different from many other kinds of animals they give us a fresh look at lots of interesting general questions in ecology and evolutionary biology," he says.

"It's sort of an unlimited candy store that biologists can keep going back to and keep finding new and wonderful things to work on."

They are, he says, an Australian success story.

"We should take that coat of arms and get rid of that rotten kangaroo and emu, and put up a goanna and a snake."

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Thanks to Rick Shine from the University of Sydney, Campbell Clarke from the Wet Tropics Management Authority, the Australian Reptile Park, Craig Adams, and Lyall Naylor.

Tags: reptiles

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Published 28 January 2009