It would be fair to say that when I set out on my voyage to Chappell Island I had a pretty good idea of what I would find. I did my dissertation on island tiger snakes in Western Australia and I was very familiar with the scientific literature describing the Chappell tigers. Some morphometric work had been done in the in the late 1950's and more intensive studies on feeding and body size were published in 1988 by Schwaner and Sarre.
I was also acutely aware of just how lucky I was to be granted permission to land on the beach. As you can imagine, the traditional owners are very protective of this mysterious island they call Hummocky. You can see it, way out in the distance (top left of frame).
In more recent times, a young, slightly cocky and politically incorrect scientist was denied access to the island after failing to acknowledge that Hummocky belongs to the aboriginal people! So he should have been denied access: Hummocky is a very special place indeed.
So what did I expect to see? To be honest, I expected to see an island full
of very large and very black tiger snakes that look like
they are not much enjoying life and that was exactly what I saw. Having not eaten for 10 months, they are merely long black sacks of bones being sucked dry by ticks from the outside and eaten from the inside out by internal parasitic worms.
However, as I am such a sensitive new age scientist, I was able
to look beyond their drab and beleaguered appearance and see their true beauty as it is reflected in the glint of their glassy black eyes. Never one to judge on looks alone, I fell in love with the tiger snakes of Hummocky. How can plain be so damn pretty?
The literature tells us that these tigers are big! In fact, they
have been measured to be the biggest tiger snakes in all of Australia. We also
know that they feed only once a year on hatchlings of the Mutton Bird or
Wedge-tailed Shearwater. As adults, this annual feeding frenzy, that spans only a few weeks, is the primary driver for their incredibly large size. The phenomenon of Island Gigantism is commonly observed and documented in many studies of island ecology and demographics. We know from a very recent study by Aubret and Shine (2009) that these island tiger snakes are born bigger and
grow quicker than any population of mainland tiger snakes.
But Hummocky (Chappell Island) is not unique in supporting a population of large tiger snakes. Many of the islands in the Furneaux Island group, including Flinders Island, support similar populations of tiger snakes and their mutton bird prey. So why are the
Hummocky snakes so big? No body actually knows the answer to this
question and until I have time to find out for certain I can only speculate.
In the first instance, Hummocky has no trees on it.
At present, it is believed that the island never had trees on it. There has
been a significant amount of historical clearing and agriculture, but trees
have never been reported as growing on the island. Could this influence the snakes? I am not sure?
Secondly, Hummocky is very small. Fundamental to the concepts of island biodiversity, smaller islands are less species rich. This means there are no other competitors to the tiger snakes other than humans that harvested mutton bird chicks for decades. Could this influence the snakes? Again, I am not sure. Mutton bird harvesting has taken place on many, many islands in the Furneaux Group.
The topography of the island is very unique. Though
many of the bigger islands, such as Cape Barron, have terribly steep hills across
their length and breadth, Hummocky is essentially flat all over with the
exception of a very large dome shaped granitic extrusion in the centre. This is very odd, to say the least.
Now, I am not a geologist, nor an archaeologist, nor a physical
scientist but I believe there to be a number of factors at play here that are
canalising the changes in this population. Based on my work on ecophysiology of
island vs. mainland tiger snakes in Western Australia I believe the primary
driver for gigantism is water availability.
I believe that the Devonian granite dome is holding a
permanent water source for the snakes that is available to them all year round.
That, coupled with short and long term refuges (mutton bird burrows and boxthorn
bushes) that enable the snakes to capitalise on energy conservation and a reduction of evaporative water loss, contribute favorably to mass and energy conservation throughout the year. This, in turn, allows these massive snakes to emerge from their 10 - 11 month fasting period in relatively good condition and ready to eat.
From a scientific perspective, something unique is happening on Hummocky and I am keen to return to find out exactly what.
Science aside, on an island with no natural predators a tiger snake's life is blissful; a symphony of eat, sleep, mate, repeat. Most scientists consider snakes to be primitive and poorly evolved? I would rather consider them as evolution honed to perfection. They are nothing they don't need to be. They've got it right; we have it wrong!
My sincere thanks to the traditional owners of Hummocky and the Tasmanian Aboriginal Centre Inc. for making a life-long dream a reality.
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