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By Sarah Whittley, Peter D. Scott
St. Martin's PressCopyright © 2002 St. Martin's Press
All rights reserved.
THE ORIGINS AND EVOLUTION OF SNAKES
Snakes first appeared around 140 million years ago during the early Cretaceous period. In reptile terms, this is not very old. Lizards first appeared 250 million years ago and ruled the world for 235 million years. Mammals (which includes us) have been around for only some 65 million years. It is believed that snakes evolved from lizards. For some reason snakes chose a subterranean existence, therefore their bodies had to adapt to life underground. This meant the loss of all unnecessary appendages, like limbs and ear openings. As far as we know, Lapperentophis defrennei is the earliest known snake; its fossil remains were found in what is now North Africa. Two of the oldest known snake families are the Boidae (boas) and Pythonidae (pythons). These primitive snakes still have the remnants of their lizard ancestors. They have two tiny bones that were once hind limbs protruding on either side of the body. The bones are covered by a claw and are called spurs.
However, fossils dating from the late Cretaceous period show that snakes must have traveled far and wide as they are now found on almost every continent. In order to understand the distribution of snakes, it is necessary to understand how the seven continents we know today were formed. Around 250 million years ago, all the continents (or land masses) were grouped together forming one large landmass, called the Pangea. This was surrounded by one giant ocean called Panthalassa. About 200 million years ago, this land mass began to break up, gradually spreading across the globe. By around 65 million years ago, the continents had stopped shifting dramatically and had settled into pretty much the positions that we see today.CHAPTER 2
INSIDE A SNAKE
During the course of their evolution, snakes have adapted to a completely limbless life. As a consequence their skeletons are fantastically designed. Whereas we have 33 vertebrae in our spine or backbone, snakes have from 180 in smaller species and up to 400 in larger species. The snakes backbone runs the whole length of the body with hundreds of paired ribs attached to it. The backbone is remarkably flexible and strong, the trunk (body) muscles are also very powerful; they would have to be to enable a King Cobra to rear to 6 ft (2 m) or a python to constrict an antelope. The ribs, which run the length of the body, are paired and arch out from each vertebrae. There are many muscles; some connect specialized vertebrae to vertebrae and ribs to ribs. The skin muscles allow the ribs to pull scales forward. A snake's skeleton and skull is made up of many delicate bones, which can be easily damaged.
Snakes have internal organs very similar to ours. The major difference is that they have to fit their heart, lungs, kidneys, liver, and stomach into a long, thin body. Snakes, like us, have to breathe air; some of the more primitive snakes, like boas and pythons, have two lungs, although one is always bigger than the other. All other snakes have lost the use of their left lung. In some species of marine snakes, the right-hand lung runs almost the whole length of the snake's body. This useful development ensures that large amounts of air can be stored for prolonged dives. It can act as a buoyancy aid for marine snakes.
The digestive system of a snake starts at its mouth. Food then has some way to travel to reach its stomach, this journey is helped by the strong muscles in the esophagus and throat, which push the food through. Although snakes don't have bladders, they still need to excrete excess fluid. Once the fluid has passed through the kidneys it is expelled as uric acid, which looks like a lump of crystal.
TEETH AND FANGS
A hunter without an effective weapon would be pretty useless, so too would a venomous snake without its fangs. This is why all venomous snakes can re-grow or replace their fangs if broken or by natural shedding. If you look closely into their mouths you can sometimes see up to five standby fangs waiting to be used.
Many snakes have in excess of 200 teeth. Usually they are sharp and needle-like and curve towards the back of the mouth. This enables the snake to grab and keep hold of its prey. Some snakes have especially long, curved teeth, like the Wolf Snake, which needs to keep hold of the slippery, smooth-skinned skinks – its favorite food. The Neck-banded Snake has even developed large, flat, knifelike teeth for holding on to skinks.
Snakes can be front-fanged or back-fanged. As a rule, the more evolved the snake (Viperidae and Elapidae) the more likely that they will have front fangs. These snakes are normally venomous and need the front fangs to ensure prey is subdued quickly without causing damage to the snake itself. Most of the colubrids (this family makes up over half of all snake species) are back-fanged and harmless. Only two are dangerously venomous and they are the African Boomslang and Kirtland's Twig Snake.
Venom is basically a wonderful mixture of proteins and enzymes. It is important to bear in mind that this sometimes devastatingly lethal mixture is there for snakes' survival and defense, not for killing humans.
Venom can contain neurotoxins, which cause damage to the nervous system, cytotoxins, which destroy cell structure, cardiotoxins, which damage the heart, and hematoxins, which affect blood cells. The enzymes found in venom can cause serious damage to humans, but actually help the snake break down and digest its food. For example, proteinases specialize in destroying proteins and DNase for destroying DNA. Some snakes produce their own specific lethal mixtures. Kraits from the Bungarus genus produce Bungarotoxin, Crotoxin is produced by the genus Crotalus, which includes rattlesnakes. Snakes generally have one or two predominant toxins in their venom, elapsids tend to be more neurotoxic (attacks nerves) while pit vipers' is more cytoxic (attacks cells).
In the Western world snake bites are not common and fatalities even rarer. In the USA, the most common venomous bites are from pit vipers – usually rattlesnakes and Copperheads.
There is nothing sinister about the familiar sight of a snake flicking its tongue in and out of its mouth. It is just simply "tasting" the air. Whereas we only use our nose to smell, snakes use their tongues and a sensory device called a Jacobson's organ. As soon as the tongue is withdrawn, the tip is placed in the Jacobson's organ which is found in a recess in the top of the mouth; information is then processed by the brain and the snake can decide if an object should be avoided or pursued. The Jacobson's organ is not unique to snakes – all reptiles have one. As well as this organ, snakes have nostrils that connect to the olfactory part of the brain, thus enhancing their sense of smell.
Not all snakes have good eyesight. This is probably due to their burrowing habits, where the use of other senses would have been more useful. The position of the eyes on the head and the shape of the pupils are both important; snakes that hunt by lying in wait in leaf litter need eyes that face upwards, while those that hunt by chasing through trees need good forward- facing, binocular vision. The pupils of snakes come in a variety of shapes, from round to horizontal. Some snakes even have camouflaged pupils – the black stripes on Mandarin Ratsnake's pupils, give it a very distinctive look.
Unlike other reptiles, snakes don't have external ear openings. This means snakes are unlikely to hear the same airborne sounds that we do. Instead, they have a sophisticated inner ear which enables them to balance and pick up on the smallest of vibrations. However, to do this they must have their lower jaw in contact with the ground or a tree. The vibrations are then transmitted to the inner ear via the small bones in its head. Our approaching footsteps would usually be enough to make sure of a quick exit by most snakes.
If you look closely at the head of boas, pythons and pit vipers you can see small holes or pits between the eyes and nostrils. These facial pits are thermal receptors lined with nerves that enable the snake to seek out its warm-blooded prey with remarkable accuracy.
Boas and pythons have many pits which run along the edge of the mouth. Pit vipers have the most highly developed senses of any snake. The warmth emitted by their prey is picked up by the two pits found on either side of their nose. These act in unison, enabling them to gauge the size of their prey and then pinpoint it with stunning accuracy. Heat pits are so sensitive they can also pick up the smallest of temperature change.
Besides facial pits, it is thought that many snakes have thermoreceptors in their scales. This would help the snake recognize and react to touch.CHAPTER 3
MOLTING OR SLOUGHING
When the snake's outer layer of skin gets worn out, it grows a new one. A few days before the molt the snake's colors dull, the eyes go cloudy and it gets sluggish and bad-tempered. It usually only takes a few hours for the old skin to be shed. The old skin is normally intact, and can be found discarded on the ground. Freshly molted snakes always look pristine and brightly colored.CHAPTER 4
Scales grow from the top layer of skin called the epidermis. When the skin is stretched, the skin between the scales can be seen. This expansion is very useful when swallowing large prey. Some snakes, like the Rough-scaled Tree Viper, have keeled or ridged scales, which help them to hang on to their prey or break up their outline when hiding in leaves. File or Wart Snakes have rough, granular bumps covering their body, which help them hang onto their slippery fish prey. Other snakes have very smooth scales, especially the burrowing snakes, which need to slip with ease through soil.CHAPTER 5
Most snakes are egg-layers or oviparous but some bear live young. They are called viviparous. Some species don't even need a mate to reproduce!
Viviparous snakes do produce eggs but instead of being laid and left in the nest, like oviparous species, the mother keeps them inside her body until they are ready to experience the outside world. As a general rule, snakes from cooler climates tend to be viviparous; keeping the eggs warm during their gestation enhances their chances of survival. The downside of this is that carrying sometimes as many as 50 or more babies around makes the mother more sluggish and vulnerable to being attacked.
The Brahminy Blind Snake is an example of a parthenogenetic (reproduction without fertilization) species. When the eggs hatch, the offspring are genetically identical to their mother and usually all female. This is the only snake to reproduce this way.CHAPTER 6
FOOD AND FEEDING
There is no such thing as a vegetarian snake. All snakes, without exception eat other animals. Snakes can be either dietary generalists or specialists. Generalists take a huge variety of prey, e.g. the Black Racer will eat reptiles, birds and birds' eggs, small mammals, amphibians and insects. Some snakes, however, are very specialized eaters; the Thirst Snake from South America eats nothing but snails and Queen Snakes from North America only eat soft-shell crayfish.
Snakes have had thousands of years to perfect their hunting strategies, which make them very efficient predators. There are three basic strategies: the sit and wait approach used by many heavy-bodied vipers; the chase and stab technique used by many fast-moving, active snakes, and the lure, where a snake waves the tip of its tail to entice animals to within striking distance. Once caught, prey is either subdued by venom or by constriction.CHAPTER 7
Snakes are currently grouped into 18 families. Each family is then divided into genera, which is further divided into species. However, new species are still being discovered and more information collected on recognized species.
Ongoing research may indicate that already known species may in fact be more than one species. In other cases, the opposite may occur, with two or more species being "lumped" as one.
This book looks at each family from the most primitive to the most advanced. After a brief introduction to each family, accounts of representative species follow.
The 18 families are as follows:
LEPTOTYPHLOPIDAE .. thread snakes
ANOMALEPIDAE ..... dawn blind snakes
TYPHLOPIDAE ....... blind snakes
ANOMOCHILIDAE ... dwarf pipe snakes
ANILIIDAE .......... South American pipe snake
CYLINDROPHEIDAE ... Asian pipe snakes
UROPELTIDAE ....... Shield-tailed snakes
LOXOCEMIDAE ...... New World sunbeam snake
XENOPELTIDAE ...... Asian sunbeam snakes
BOIDAE ........... boas
PYTHONIDAE ....... pythons
TROPIDOPHIIDAE .... wood snakes
BOLYERIIDAE ....... Round Island boas
ACROCHORDIDAE ... wart or file snakes
COLUBRIDAE ....... colubrids
ATRACTASPIDIDAE .... burrowing asps
ELAPIDAE .......... cobras, kraits, mambas, coral snakes, sea snakes
VIPERIDAE .......... vipers
THREAD (OR SLENDER BLIND) SNAKES
This family consists of 86 species in 2 genera: Leptotyphlops (85 species), and Rhinoleptus with one member, R. koniagui of western Africa. Thread snakes look very similar to blind snakes only they are much smaller, a pinky color and almost threadlike, as their name suggests. At a first glance you might mistake these snakes for worms, that's how small they are. In fact, some herpetologists claim that they are not snakes at all but legless lizards, on the basis that they have a well-developed pelvic girdle, vestigial hind limbs, very small scales and no belly scutes. They don't have a left lung, while females lack a left oviduct. Physiologically, they differ from blind snakes in that they have teeth in the lower jaw and non in their rigid upper jaw; normally blind snakes have teeth in their upper jaw
Thread snakes have a wide distribution, occurring in southwest Asia and the New World, preferring semi-arid regions. Basically they can be found wherever there are termites. Being small snakes, they have to be wary of a termite attack. Luckily, thread snakes can produce a pheromone (chemical substance) which fools the termites into believing they are one of them. As with all burrowing snakes, they rarely venture above ground, where they would be extremely vulnerable to predators due to their small size.
WESTERN BLIND SNAKE
This small snake, which is about as thick as a pencil, reaches a maximum length of 16 in (40 cm). It is usually pale brown, pink or purplish, with a silver sheen. The head and tail are blunt, with a spine at the tail tip. Rudimentary eyes appear as spots under the head scales. Teeth are lacking from the upper jaw. The Western Blind Snake occurs from southern California to western Texas and down to Mexico. It is found in all of Baja California and western and north-central Mexico. If disturbed, it will writhe and wiggle its tail to focus attention here instead of on the head. This small serpent shares a feature with the much larger boas and pythons – the remains of a pelvic girdle and femur, complete with a tiny spur! A secretive, nocturnal species, it lays up to 7 eggs in mid-summer. They prefer moist, loose soils suitable for burrowing. This may include the sandy washes or canyon bottoms of brushy mountain areas or desert grasslands.
SCHLEGEL'S BLIND SNAKE
This tiny pencil-thin snake from South Africa, reaches a maximum length of 16 in (40 cm). It has a blunt, rounded snout and a short blunt tail. The eyes are hidden under scales. Teeth are lacking from the upper jaw, and it has a vestigial pelvic girdle. If disturbed, it will writhe and wiggle its tail to focus attention here instead of on the head. It is preyed upon by a wide variety of animals, including birds, mammals, snakes, fish and even spiders.
It eats mainly ants and termites along with their eggs, pupae, and larvae. Millipedes and centipedes are sometimes eaten. It hunts by locating ant pheromone trails, following them back to the nest and then consuming the residents. The smooth, tightly-overlapping scales provide protection against ant bites. Lays up to 7 eggs in mid-summer.
Excerpted from Snakes by Sarah Whittley, Peter D. Scott. Copyright © 2002 St. Martin's Press. Excerpted by permission of St. Martin's Press.
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