Mandibular Liberation and Other Obscure Movements
- Snakes: The Evolution of Mystery in Nature by Harry Greene
California, 351 pp, $45.00, August 1997, ISBN 0 520 20014 4
The lengths to which people have gone to eradicate snakes are remarkable. A century ago, for example, a prolonged campaign was mounted against timber rattlesnakes in the north-eastern United States. Since this species tends to aggregate in large numbers in winter dens, eliminating them seemed quite feasible: the dens were variously dynamited and cemented over and timber rattlers have now disappeared from most of New England. Yet feelings run so high that in 1979 an employee of the US Office for Endangered Species was fired by the Secretary for the Interior for protesting against the presence of timber rattler meat on the menu of a Washington restaurant. Snake populations are almost everywhere in decline – which is why Harry Greene has set out to improve their image and enhance their appeal.
On a geological timescale, snakes are a modern – even post-modern – phenomenon, first appearing in the fossil record about 95 million years ago in the Mesozoic era, during which mammals, birds and flowering plants also originated. The major diversification of snakes took place much more recently, in the Miocene epoch (25 million years ago), and representatives of all the present-day groups were in place only five million years ago in the Pliocene period. Like most other reptilian groups, they are overwhelmingly tropical and terrestrial; very few species have made a good fist of living in seasonally cold regions and only fifty or so extant species are sea-dwellers, which is why most tropical islands – even quite big ones like Jamaica – are routinely snakeless or nearly so, and why, when the brown tree snake, Boiga irregularis, was introduced on Guam in the Forties or Fifties (probably by accident in a military cargo) it wreaked havoc on the island’s bird population. Compared with other major groups of terrestrial vertebrates, snakes are not enormously diverse – around 2700 species compared to 10,000 birds and 4300 mammals. Even so, it seems remarkable that there are so many ways of being a snake, given that the blueprint is so apparently simple (and superficially limiting): just a limbless cylinder covered in scales, all the vital organs behind the brain squeezed and staggered into a narrow, elongate tube, thus exposing ‘the profound role of peristalsis in everyday life’. As Greene points out, snakes are not the only vertebrates to have evolved, via the prerequisite of body elongation, to a limbless condition, but they are by far the most successful in terms of species diversity.
Whether evolution liberates or enslaves depends on your point of view, but snakes appear to have achieved some notable freedoms, which have contributed to their diversification. One of these is the ability to consume items of prey that would be impossibly large or awkward for any other predator – some snakes routinely devour animals heavier than themselves, and a few can make a satisfying meal out of whole porcupines. This is the result of a suite of anatomical features to do with the jaw and skull, including ‘mandibular liberation’ (by which the two lower jaw tips are connected by ligaments rather than fused into a chin), not jaw dislocation, as commonly imagined, as well as hinged fangs and a spectacular elasticity of the skin and gut wall. Greene’s book contains numerous photographs of snakes in the act of swallowing unlikely items. The best is a sequence tracking an egg-eating snake, with a God-do-I-really-have-to-do-this look in its eyes, as it swallows, intact, an egg several times its own circumference before eventually crushing and regurgitating the shell. Hardly any other vertebrate predators, aquatic or terrestrial, need to do this kind of thing. They can chew, rip and tear at the flesh of their prey, rendering it into chunks of a sensible size – something which very few snakes can manage (an exception is the white-bellied mangrove snake, which twists the legs off crabs for more convenient swallowing). The evolutionary loss of limbs means that the snake’s head cannot afford the heavy bones, teeth and musculature required for this sort of feeding: indeed, the head has to be as light as possible, to enable swift striking and movement. Geometrically, the most comfortable solution has been adopted by snakes that eat earthworms or smaller snakes – tubes within tubes. One of the more disturbing images in this book is of a coral snake mimic, devouring a northern cat-eyed snake tail first, the bright-eyed victim’s head protruding, with a disconcertingly serene expression, from the predator’s parted lips.
Oddly, it is easier to be anthropomorphic about snakes than it is about a lot of our closer relatives – or, more precisely, to attribute to them a whole variety of personalities, moods and emotions. I don’t suppose this is what Greene is getting at when he says, in his closing sentences, that we ‘can’t say what it’s like to actually be a black-tailed rattlesnake, much less a little ridgenose’ (why that ‘much less’? How can he tell?) But, the personalities of different species – another remarkable feature of their diversity – are nicely displayed. When he describes coral snakes as ‘wriggly, uncontrolled, wild and excitable’, cobras as ‘austere and confrontational, all speed and eyes ... downright belligerent’, big elapids as ‘among the scariest of vertebrates’, timber rattlesnakes as ‘elegant and velvety’ or a bushmaster as emanating a ‘powerful sensation of measured readiness’, it’s easy to see what he means, if you have encountered any of these creatures even once. One of the photographs shows an Indian long-nosed vine snake, a harlequin in diamond-patterned green, black and yellow, with gaping pink jaws, pointed snout and horizontal pupils, giving a faint but passable impression of Danny Kaye.
If you are not a herpetologist, you don’t see snakes that often – unless you enter silently into places where they are abundant. Since starting to read Snakes I have been to the biological station at La Selva in Costa Rica, the scene of Greene’s first encounter with a hush master.
Edwin stopped fifty metres or so up a broad ravine and peered over an enormous fallen tree. Then, motioning caution with one hand, he pointed for Manuel Santana and me to lean over the chest-high log. Coiled in a mound on the forest floor, its calligraphic black and tan colours blending with the surrounding debris, was the most magnificent snake I’d ever seen in nature ... As we scrambled over to it, the bushmaster’s only response were slight elevation and retraction of its head, then a slow, vertical sweep of the long black tongue, aimed directly at us. The snake’s behaviour was not exaggerated – no lunging strikes, no frenzied efforts – but there was a powerful sense of measured readiness ... ‘Don’t come closer.’
I had no such luck; the only snakes I have seen recently have been in England – one a captive corn snake in the garden of a Cambridge pub, the other an annoyed grass snake with an impressive hiss, trodden on by my three-year-old son. Probably half of my previous chance encounters were packed into just a few hours. On my first day of solitary botanical fieldwork, 20 years ago in Guajira, the north-easternmost corner of Colombia, I variously stumbled on, warily skirted or was startled by six different snakes – a coral snake, furiously thrashing about in the leaves in a dry river bed, a small boa softly draped on a low branch and a pit viper coiled motionless in the middle of a warm sandy path; the other three rushed off too quickly to be recognisable. So, I earmarked a portion of my travel grant for paying alert local companions and encountered only one more snake in the rest of the six-month trip. They must have been all around, but the precaution paid off.
Despite the tendency of most snakes to make themselves scarce in the presence of noisy humans, several thousand people die from snakebites every year, the majority in the tropics. This puts snakes comfortably in first place in the league of homicidal vertebrates and explains the New Englanders’ response to the timber rattlesnake. Most tropical bites are ‘legitimate’ – the epidemiologists’ po-faced term for accidental, unanticipated bites – while most bites in the United States are ‘illegitimate’, suffered during the intentional handling of snakes; most illegitimate bites are to the arms and upper body (the victims are usually young men with a point to prove), while most accidental bites are to the legs and feet. The mortality rate has fallen considerably over the past half century, with the more widespread availability of anti-venoms and the decline in snake numbers as a result of persecution and the fragmentation of natural habitat by deforestation, agriculture and other human activities; fatalities result from the inadequacy or absence of hospital facilities, poor training of physicians and incorrect first-aid procedures.
The last of these is a big problem: all the things one is supposed to do on being bitten – keep calm, immobilise the affected limb (but what do you do if it’s not a limb?), don’t apply a tourniquet, and, according to a tropical first-aid guide I once carried, capture the snake and take it with you to hospital for identification, being careful not to damage its head – are for most of us the least likely instinctive options. It is easy to imagine that one would resist the temptation to hack away at the bitten area with a knife – though that is one of the most frequent responses – but the notion of walking or driving sedately to hospital, having calmly bagged the offending snake, is implausible. On the other hand, it is important to know what you have been bitten by, for there is a huge range of venoms with different effects (though the range of snake species in a given place is usually quite limited). Most species of venomous snake inject not one toxin but a cocktail of digestive enzymes and neurotoxins, which variously induce haemorrhage, blood clotting, paralysis, swelling and tissue destruction; swift and effective treatment depends on rapid identification.
Greene loves and is thrilled by snakes, and so he doesn’t really address fear; for that, try Redmond O’Hanlon’s imagined anaconda in Congo Journey: the snake, anchored to a submerged tree-root, slithers silently out of the river to seize him by the face as he sleeps by the water’s edge. But Greene succeeds brilliantly in conveying some of the pleasures of fieldwork – especially in the rattlesnake country of Arizona and New Mexico. He provides a marvellous account of every aspect of the life of snakes, from digestive physiology to parental care (mother pythons coil around their eggs and shiver to heat them up, cobras build nests, vipers remain corralled with their offspring until they’ve shed their first skin); and in doing so illustrates the practical, everyday side of evolutionary biology – quite differently from, and often more engagingly than, the grandiloquent sweeps of Dawkins and Gould.
Traditional literature for snake-people is not always easy to read. A new book for the professional herpetologist begins uncompromisingly: ‘Scolecophidians have serous as well as mucous supralabial glands, but do not otherwise foreshadow later developments.’ At the other end of the scale, there are the manuals for the amateur snake enthusiast, such as the Internet-based Care Sheet for Snakes – an amalgam of the ambiguous (‘If the box is too large for the snake, fill it with loosely wadded newspaper’), the unexpected (‘If a live adult mouse frightens your snake, try a freshly killed one’) and the blindingly obvious (snakes ‘never eat lettuce, carrots, bread and similar items’). Greene is one of the most accomplished contemporary herpetologists, and Snakes is a condensation of contemporary snake biology, much of it learned from three decades of his own research, mostly in the wet forests of Costa Rica and the south-western deserts of the United States.
He slightly spoils things with his subtitle, and by unnecessarily claiming his book as an attempt at an amalgam of science and art. Snakes is a book about natural history in an evolutionary context, motivated by Greene’s desire to impart his feeling for snakes to a wide audience. The text is copiously illustrated with colour photographs by Patricia and Michael Fogden, some of them – such as a shot of an eyelash pit viper striking at a hummingbird, or a pair of male black mambas entwined in ritual combat – quite breathtaking. The book is quite heavily weighed down in places by specialist terminology, although some herpetological words – opisthoglyphy, proteroglyphy – are so glorious that it hardly matters. Greene provides a massive bibliography of papers from learned journals, but doesn’t pin down his references, giving no individual footnotes – a serious handicap to anyone wanting to use the book to delve further into the topic. ‘I wish to speak of the bees very simply,’ Maeterlinck wrote at the start of his Life of the Bee, ‘as one speaks of a subject one knows and loves to those who know it not.’ That is still the most dependable way to go about it.