Snakes Harmful and HarmlessTM


TIGER SNAKES

ON CARNAC ISLAND!

WHERE DID THEIR ANCESTORS COME FROM?

Brian Bush

Tiger snakes occur on many islands off the southern Australian coast. These insular populations were isolated about 6000 years ago during the rising sea levels caused when water trapped as ice at the north and south poles during the previous glacial period melted.

With Western Australia’s Carnac Island population there are a couple of different scenarios offered to explain its occurrence, although I am more than a little sceptical, but one of these is quite a romantic tale nevertheless.

Both stories involve the recent introduction by people. One states that, “… the university put the snakes on Carnac so that they may be readily accessed for venom milking.” The other is far more interesting.

Australia has had its fair share of characters, amongst which were the snake showmen of earlier years (pre-1950’s). Rocky Vane was one of these, however on 12 January 1928, his first wife Dorothy died in Perth of tiger snake bite. Around that time snake people were having a bad run with several deaths occurring in quick succession, causing the authorities to restrict snake shows. It is rumoured that after a disagreement with the WA authorities related to these restrictions, Rocky jumped in a rowboat and took 40 tiger snakes to Carnac Island and released them.

Great story! But I reckon it is just that. Carnac was an extension of Garden Island, which also has its population of tiger snakes, before being separated with the rising sea level. It has been suggested that Carnac tigers are larger than those on the adjacent island, but this is incorrect. Size is of little relevance anyway. A North American researcher (Dr Terry Schwaner) believes that the numerous isolated populations of tiger snakes are a single species because there is little genetic divergence between them. He also found size was related to prey, i.e. dwarf-sized population on island with only small prey adjacent to large-sized population with large prey. He also sampled Carnac Island during his study.

The primary food taken by adult Carnac tigers is silver gull chicks and although mainland snakes also take birds, frogs appear to be the preferred prey, but there are no frogs on Carnac. This shift in prey is not unusual, with many island tigers taking birds. Apparently there is a memo on file at CALM regarding a ‘phone call to Dr Andrew Burbidge in the 1980’s from a bloke stating that “back in the 1920’s he rowed with Rocky Vane to the island with about 40 snakes”. If Rocky Vane did liberate some snakes, he probably only strengthened the gene pool of the already existing population.

I suppose I will have to keep a little of my mind open to the possibility that this population has a unique and very recent origin, but the evidence I have seen to date is unconvincing. As with all insular snakes I have worked with, Carnac tiger snakes are quiet and inoffensive. In fact I have been unable to get one to flare a hood, an attitude quickly attained by a disturbed individual on the adjacent mainland. This inoffensive nature is the result of thousands of years of isolation with few predators to contend with; I do not believe it could result after only 70 years.

The islands were infrequently and poorly sampled in the early days (Brad Maryan, WAM), with the first WA Museum accession from Garden Island not collected until 1930 (R2997) and from Carnac (R4975 collected by Serventy) not until 15 October 1934. The next specimens from Carnac were not to be collected until 1958, twenty-four years later (R12818, R12827).

CALM biologist, Dr David Pearson advised me in a personal communication that Carnac Island had a settlement in the early days related to commercial whaling and a penal colony - I would suspect, if the tiger has always been there that some mention of it would occur in the journals of both facilities if they are available.

Too many questions requiring answers: the joys of a natural history interest - there is no time to get old!


NOTE: Many of the adult snakes on Carnac have lost one or both eyes. This is the result of damage caused by silver gulls protecting their chicks from foraging snakes. Some of the fattest, healthiest snakes on the island are totally blind - there are no predators to contend with, no subjective appreciation of pain, and now no detection of movement to be intimidated by!

Dr Xavier Bonnet is studying this tiger snake population. Eventually his work may confirm its origin.



Carnac Island here we come!


Captain Mark Ainscough - our skipper.


The end of a day's snake collecting - the orange vests were required to allow ready identification of the group on this A-class reserve.

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