Shine begins with some key aspects of snake anatomy. These include adaptations for a long and narrow body, dentition and poison delivery, vision and other senses, and coloration.
"The eyes of snakes apparently lack oil droplets, which are used for colour vision in other vertebrates. However, there is a wide variety of different types of eye structures among snakes, involving cones and rods in the retina, and at least some diurnal snakes are known to have red and green colour receptors. Vision is undoubtedly very important for some types of snakes, and relatively unimportant for others. For example, fast-moving diurnal hunters like whipsnakes have very large eyes and rely upon vision to chase and capture the fast-moving lizards that are their normal prey. At the other extreme, blindsnakes have the eyes reduced to darkly pigmented dots hidden beneath the scales of the head. These 'blindsnakes' undoubtedly can detect the difference between light and dark, but probably little else."
A general look at the taxonomy and evolutionary history of snakes worldwide is followed by detailed examination of the major Australian groups. We have two (of the world's three) species of filesnakes, thirty blindsnakes, ten colubrids and fifteen pythons, but no vipers. The dominant lineage is that of venomous elapids, totalling some 110 species, and the most momentous evolutionary event has been a radiation within this of live-bearing species. (This includes the viviparous seasnakes, but the common black snake has evolved live-bearing independently.) New species of snakes continue to be discovered.
Much-maligned in the popular eye, ectothermy ("cold-bloodedness") has advantages as well as disadvantages, most obviously in drastically reducing food requirements. The constraints of temperature control influence basking behaviour, variation in habitats and micro-habitats, and seasonal color changes; brooding female pythons even use shivering to maintain higher egg temperatures.
There is obvious latitudinal variation in the distribution of snake species, with elapids dominating to the south. Habitats also vary widely, though Australia has few arboreal (tree) snakes, and many snakes occupy different habitats in different seasons. Working out where snakes actually spend their time is difficult — people find them where people tend to go and where visibility is good — and studies using telemetry have often produced surprising results.
"It's not too hard to locate your transmitter-carrying water python Liasis fuscus, as Dave Slip shows (right), but how do you catch him again — to recover the expensive transmitter — once the study is finished? All you can do is to wade out in knee-deep water through the reedbeds, get a precise 'fix' on the signal and then try and jump on the python before it gets away. It's not easy to grab a snake by feeling in murky water, and there's always the worry that the python has been eaten by a croc, and hence that you're about to jump on the wrong reptile. When it all works out O.K., the relief is evident."
Many snakes give birth to live young (viviparity); this is more common in colder climates. And temperature has a major effect on reproductive cycles, with those of temperate snakes centred on summer. Some snakes gather in mating aggregations; in others male combat is common.
"Most untrained observers who see male combat in snakes interpret it as courtship or mating, rather than fighting."Clutch and litter sizes vary between species and among individuals. Investment in reproduction depends on habitat and conditions, with females not necessarily reproducing every year. Little is known about snake growth rates, but size is a critical factor in survival.
"The list of potential and actual predators of juvenile snakes includes almost any animal big enough to overpower the small serpent."
What do snakes eat? Australian environments offer "relatively little opportunity to feed on small mammals", which explains the small number of ambush hunters (the death adder is the best known). And no one knows why, but Australian snakes only prey occasionally on invertebrates, with only two species known to specialise on them (a blindsnake on ants and a colubrid on crabs). Most are active hunters and their most common prey are frogs and lizards, with many specialising on skinks. Some of the most dramatic photographs in Australian Snakes come in this chapter, with amazing multi-shot sequences of constriction and ingestion.
Finally, Shine covers human interaction with snakes. He touches on the place of snakes in Aboriginal diet and mythology, then surveys changing European attitudes: early fear and loathing has given way to increasing amateur and professional studies, but popular knowledge is still poor. While Australian snakes are the most venomous in the world, they are not actually a major danger to humans. Fewer than a dozen species are capable of killing an adult human and venom is not always injected — out of an estimated three thousand people who are bitten each year, perhaps two or three die. And Shine gives advice on how to avoid snakebite and what to do if bitten.
"If you are bitten, don't panic. Sheer terror causes many of the worst effects, and probably kills some people even if the snake is harmless."Snakes face many threats from humans: commercial exploitation, roads, feral animals, and above all habitat destruction.
So ends a fascinating tour of snake evolution, ecology, and behaviour.
December 2003
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- Harry W. Greene - Snakes: The Evolution of Mystery in Nature
- books about Australia + Australian history
- books about bushwalking
- more animals + zoology