Chapter-i origins Why are snakes called reptiles? What is a reptile?
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- Bu səhifədəki naviqasiya:
- How good are snakes at camouflage
- What is sexual dimorphism
- What are the important skeletal changes that took place during the evolution of the snakes
- What major skeletal features distinguish snakes from mammals
- Which snake has the largest number of vertebrae
- Which snakes have the smallest number of vertebrae
- What are the hypophyses
- Are all snakes cylindrical in cross-section
- Which aquatic snake has twice as much blood as a land snake of the same size
- How many teeth do snakes have
- Why do snakes often move their throat up and down as frogs do
- What is special about the kidneys of snakes
- Which are the only snakes that can produce body warmth to any significant extent
- Are sea snakes found in the deep sea or in shallow waters
- Sea snakes are likely to swallow a lot of sea water. Does this not result in heavy concentration of salt in the blood
- What is meant by the ‘bends’ and how do sea snakes avoid this
- Are all sea snakes helpless on land
- In measuring a snake, how can one distinguish between the body and the tail
- Are there snakes that can ‘drop’ their tails like some lizards do to escape adversaries
- What is peculiar about the tails of sea snakes
How are some snakes iridescent? Some snakes are iridescent, that is, they show constantly changing luminous colours when seen from different angles. Also called ‘interference colour’. But this is not a colour. The outer layer of a snake’s scale is thin and transparent. When light strikes at it from an angle, the light is split into its spectral components showing different colours and these colours seem to change when the observer’s line of vision changes. All snakes with smooth scales have this property to minor and different extents. But the outstanding examples are the two species of the genus Xenopeltis found in South and Southeast Asia. The Indian species is the sunbeam snake (Xenopeltis unicolor) found only in the Nicobar islands. Another example is the rainbow snake (Farancia erytrogramma) found in S. Carolina, U.S.
Animals have to conceal themselves, atleast on occasions, if they have to effectively escape their predators or successfully capture their prey. If they cannot conceal themselves by the overt act of hiding in their surroundings, they have to make themselves difficult to be noticed. For this, their characteristic form or the outline of the body, and the colour and markings on the body have to be made as indistinguishable as possible from the surroundings. This is camouflage. Also called cryptic coloration / markings. Most snakes, like many other animal species, are experts at camouflage. In fact, snakes are more so than most other animals because of their not having a static form or body outline. When a predator searches for a prey or when a prey is on the look-out for a predator, it usually has a visual image in its brain of what it is looking for or trying to avoid (just as we, humans, have when we are searching for a missing article). But a snake assumes different forms depending on whether it is lying coiled up or partly curled up or moving about or lying partly concealed and so on and the predator or prey, as the case may be, fails, therefore, to notice it easily since it does not conform to any particular visual image in the brain. In some, the colouration itself is such that they merge with the background. They are said to ‘hide in full view’. This is particularly true of tree dwelling snakes like the green vine snake, green mamba, etc. whose predominant colour is green. These are also long and slender which, apart from helping their movement in the foliage, help to hide their body, making them resemble branches and vines. In ground-dwelling species, the markings easily merge with the leaf litter around. e.g. Russell’s viper and other carpet vipers. Many snakes have disruptive markings on their body ---- lines, spots, blotches, etc. which break up the body outline so that they become inconspicuous. e.g. Russell’s viper. One of the interesting aspects of camouflage is that, sometimes, an animal may be gorgeously coloured and may be prominently visible against a plain background but will become inconspicuous in its natural surroundings. For instance, the rhinoceros viper (Bitis nascicornis) is a flamboyantly coloured snake but becomes almost invisible when lying among the leaf-litter of its natural habitat in the African rainforests. Some snakes may employ the “psychological trick” (not an apt term because of its anthropomorphic connotation) known as the ‘harlequin principle’. In a snake covered by patches and blotches and bands and lines, if some of these have very bright colours, they will stand out from the background skin colour. The intruding animal or the prey will notice the very bright colours readily and, while staring perplexed at such spots, blotches, lines, etc. may fail to notice the rest of the body or the general outline of the snake. Truly, a case of missing the wood for the trees!. The harlequin snake (Homeroselaps spp.) of South Africa and the copperhead (Agkistrodon contortrix) of eastern U.S. fall in this category. Among Indian species, this type of camouflage may be seen in the coral snakes. Another feature that some snakes have, just as is the case with some fishes and frogs, is the “eye-mask”. The eye is a most prominent feature of an animal and is easily noticed by prey and predator alike. But, if the eye is made inconspicuous by disruptive markings around or across, it helps to escape detection. Among Indian snakes, many species of trinket snakes (Coelognathus, Elaphe and Orthriophis spp., Gunther’s stripe-necked snake (Liopeltis frenata), species of bronzeback tree snakes (Dendrelaphis spp.) etc. have this feature. Then there is camouflage by “counter-shading”. In many snakes as in many other animal species, the top of the body (dorsal side) is dark or coloured while the underside (or ventral side) is of a light shade. When the snake is in the open, the bright sunlight which shines on the top but not on the underside minimises the colour differentiation of the body as a whole making the three-dimensional effect into a two-dimensional effect and thus the snake becomes less conspicuous.
This denotes occurrence of two or more forms in a population of the same species. This may be in colour or in patterns or in size or shape. Examples among Indian snakes: White-barred kukri snake (Oligodon albocinctus) Whitaker & Captain (Snakes of India : The Field Guide, 2004) say: “Two colour forms (i) Typical form reddish or pinkish – brown back with white, yellow or pale-brown cross-bands outlined in black… (ii) Brown with black or darker brown black-edged cross bands or big, rounded spots… Adults of this form may almost entirely be patternless”. Russell’s kukri snake (Oligoden taeniolatus) has atleast four forms with different markings. Gunther’s vine snake (Ahaetulla dispar) occurs in two distinct colour phases: bright green and brownish-green. It is believed that this could be a defence stratagem especially where colour or patterns are involved. Many predators have a mental picture of their prey and when an individual occurs with colours and / or patterns different from such ‘search-image’ it fails to be noticed and thus escapes predation (see Q & A 53).
This denotes cases where the male and female of a species have different colours or patterns or shape or size, etc. Only in very few snakes does this occur (see Q & A 97). In animal species, this generally plays a role in sexual selection between males and females. Snakes show this phenomenon less than in lizards and much lesser than in birds. Atleast in case of colour this could be because snakes have very limited colour vision (See Q & A 20) and, therefore, colour differentiation may have only a very limited role to play in interaction between the sexes.
The forelimbs, the shoulder (or pectoral) girdle and the breast bone (or sternum) were lost completely. Most snakes also lost the pelvic girdle and the hind-limbs. The primitive snake groups (boas, pythons, wormsnakes) still have remnants of the pelvis and the hind-limbs. In the boas and pythons, the remnants of the hind-limbs are seen externally as two spurs, one on each side of the body in front of the base of the tail. These do not serve any purpose in locomotion (See Q & A 60).
Unlike mammals, snakes have no sternum (or breastbone), no pectoral (or shoulder) girdle, no collar bone and no forelimbs. The majority of snakes have no pelvic girdles or hind limbs but these remain as vestiges in a few primitive species (boas, pythons). The absence of the breastbone, collarbone and shoulder girdle help in the passage of large-sized prey through the digestive tract (See Q & A 148). The absence of limbs, paradoxically, helps in the quick and smooth locomotion over different kinds of terrains, through dense vegetation etc. and through holes and tunnels and other narrow spaces.
The Oenpelli python (Morelia oenpelliensis). Australia. This has 585 vertebrae. Generally, in snakes, the number does not exceed 400. In contrast, the human body has a mere 26 vertebrae. Don’t we have reason to rejoice, what with the high incidence of slipped discs, cervical spondylitis, lordosis, kyphosis and so on even with the such meagre vertebrae we possess?
In snakes, the ‘smallest number’ of vertebrae may mean as many as about 180 as in short snakes like the blind snakes.
No present day snake has limbs though the ancestors of the snakes are believed to have been limbed. The limblessness is an adaptation to suit their life-style. But the pythons even now have a ‘spur’ (longer in males) on each side of the anal scale and these are believed to be the vestiges of hind limbs. These are of no use in locomotion. During copulation, the male uses the spurs to align its cloaca with the female’s and, perhaps, to stimulate the female as well.
These are the spike-like projections pointing downwards from the vertebrae in the snakes. The presence or absence of these, especially in the lumbar region of the vertebral column, is an important diagnostic aid in the classification of snakes. The egg-eating snakes of India and Africa (See Q & A 249) are peculiar in that their hypophyses have been modified as cutting tools to slice the egg as it passes down the gullet. For this reason, their hypophyses are also known as ‘gular teeth’. ‘Gular’ means relating to the throat. But the hapophyses are not exactly in the throat but lower down, in the gullet or oesophagus. These occur from the 17th through the 38th vertebrae. The Japanese rat snake (Elaphe climacophora) has several anterior facing vertebral hypophyses projecting ventrally into the oesophagus. However, unlike the egg-eaters, this snake ingests the entire egg, the shell and all.
No, there cannot be. One of the distinctive features of a snake just as of other reptiles is that it has no hair on its body. In a few rare cases, the scales, some or most, are modified to look like hairs. One good example is the African rough-scaled viper Atheris hispida which has long and pointed scales with tips raised. This makes the snake look as if it is covered with hair. For this reason, it is also called the ‘hairy viper’. It is worth retelling here how the cobra was once depicted in Italy as covered with hair. Cobra de capello was the name given to the snake by the Portuguese and means ‘snake with hood’. J.A. Da Gama in the Journal of the Bombay Natural History Society Vol.III Issue 3 (1888) says: “Cobra de capello had once been fabulously described and painted in Italy with more hairs on its body than a bear has. ‘Capello’ in the Italian language means hair and a hood as well. In giving the description of the cobra, the writer who had never seen a cobra de capello before, said that it had long, thick and grisly hair, and illustrated it so”.
Mostly, yes. This helps in movement especially through narrow spaces. But there are exceptions, the reasons for which are not always clear. Many heavy-bodied snakes like the larger vipers are dorsally (top to bottom) flattened. This, perhaps, helps to get a better grip on the surface. Many arboreal or tree-living snakes such as tree-boas, (Corallus spp.) found in S. America and the many cat snakes (Boiga spp.) found in India and also sea snakes are laterally (sideways) flattened. This streamlines the body and facilities quick motion. But, why are some like the African file snakes, the Mehelya spp. found in Sub-saharan Africa, the America indigo snake (Drymarchon corais), and the banded krait (Bungarus fasicatus) found in parts of India and neighbouring countries triangular in cross-section? There is no known explanation. An even more peculiar case is that of the tentacled snake (Erpeton tentaculum) found in Thailand and Indochina which is almost rectangular in cross-section.
The file snake (Acrochordus granulatus). It is seen all along the Indian coast and estuaries in India (also in South and Southeast Africa) is not a true sea snake. The large quantity of blood and the high count of red blood cells make it possible for this aquatic snake to store up enough oxygen in the blood and thus to remain submerged for more than two hours at a time. This is as against about half-an-hour at a time in the case of true sea snakes after which they have to come to the surface to breathe (See Q & A 35)
The number varies; but some species have more than 200 teeth.
In the course of evolution, three bones that form part of the hinge of the reptilian jaw, the articular, the quadrate and the columella auris got incorporated into the middle ear of the mammals as a three-bone link, the malleus, the incus and the stapes (the hammer, the anvil and the stirrup). These small bones of the middle ear give mammals a more acute sense of hearing than other vertebrates. (See Q & A 29)
Frogs do this to pump air into the lungs. Snakes do this to draw air into the mostrils to enhance their sense of smell (in addition to the use of the forked tongue – (See Q & A 32).
The kidneys in snakes are generally elongated. They are not side by side as in other vertebrates but one below the other (i.e. length-wise).
No, snakes do not excrete their nitrogenous waste in the form of urine. The nitrogenous waste is voided along with the rest of the excreta in the form of uric acid, a semi-solid waste material.
Snakes are cold-blooded creatures. (See Q & A 18). They are generally incapable of producing body heat as warm-blooded animals can. The only confirmed exceptions are the Indian python (Python molurus molurus) and the Burmese python (Python molurus bivittatus) and, that too, only when they are incubating their eggs. (See Q & A 189). The muscular contractions which produce the heat are easily visible to us. Some authors say that snakes of the Farancia and Leptophis species also incubate their eggs (See Q & A 189).
Generally in the shallow waters. But they can dive down to a depth of 300 ft. A man, unequipped, can dive down to a depth of 500 ft. by holding his breath.
Among Indian sea snakes, the yellow-lipped sea krait (Laticauda colubrina) has sometimes been seen in tidal rivers. The hook-nosed sea snake (Enhydrina schistosa) and the short sea snake (Lapemis curtus) occur in estuaries also. The annulated sea snake (Hydrophis cyanocinctus) and the Malacca sea snake (Hydrophis caerulescens) can be found in mangrove swamps. Elsewhere, there are even instances of sea snakes being found in fresh water or brackish water lakes if the lakes are open to the sea at any time for varying durations. Hydrophis samperi, a sea snake endemic to the Philippines, is confined to a fresh water lake in Luzon near the coast. Laticauda crockeri is confined to a brackish water lake on Renault island in the Solomons. A species of sea snake is found permanently residing in the fresh water Lake Taal in the Philippines even though the lake is land-locked and the sea is some 10 k.m. away. The ancestors of the snake must have entered the lake when, sometime in the distant past, it was linked to the sea and later adapted themselves to a fresh water regime when the link was cut off permanently. .
Sea snakes have ‘salt glands’ in the mouth which absorb excess salt from the body fluids. The concentrated salt solution so formed is ejected from the mouth. Sea snakes are not alone in this. Sea birds get rid of the excess salt through special nasal glands at the base of the bill, dribbling the saline solution. The giant petrel has a prominent ‘tubenose’ for this purpose. Marine iguanas, a species of large lizards, have special glands in the nasal cavity which excrete excess salt; the lizard vigorously shaking its head from time to time to help this process.
When an air-breathing animal dives deep, the water pressure compresses the air in the lungs. This forces more air from the lungs to dissolve in the blood. While this assists more of oxygen-intake into the blood, it also results in a greater nitrogen build-up in the blood since air is four-fifths nitrogen. Nitrogen in the blood has no beneficial function. The level of nitrogen accumulation may get very high, depending on the duration of submersion. When the animal surfaces and the pressure is released, the nitrogen comes out of the blood and forms bubbles in the blood stream that can cause severe distress, crippling or even death. This is called ‘bends’. This can happen to human divers also when it is also called ‘caisson disease’. There are many explanations as to how sea snakes avoid this. The sea snakes may remain at the surface only long enough to take a quick breath and then submerge again before bubbles are formed in the blood. Secondly, since the blood absorbs nitrogen only from the air in the lungs and since sea snakes also breathe through the skin to some extent, the nitrogen intake into the blood may get reduced. Thirdly, some of the excess nitrogen in the blood may pass into the water through the skin by a process of osmosis.
Most, yes. But a few e.g. the annulated sea snake (Hydrophis cyanocinctus) and the short sea snake (Lapemis curtus) can crawl slowly on land; the yellow-lipped sea krait (Laticauda colubrina) can crawl well on land.
Most land snakes, including tree-living and burrowing forms, can swim quite well.
The cloaca or the vent on the underside marks the division between the body and the tail.
The snake being structured economically, it has to use its body parts to maximum advantage. In most snakes, the tail helps in locomotion whether on land or in the water. In the arboreal species, it is curled around branches to get a good grip. This facilitates both climbing and also reaching out to the prey which may be at a considerable distance or even flying. In some snakes, the tail is used for what is known as agonistic (=conflict–related) actions such as the following: In some, the tail is blunt and shaped like the head which distracts the predator’s attention from the real head (e.g. red sand boa, See Q & A 113 & 296). Some vibrate the tail rapidly as a threat display, this action being perfected in the rattlesnakes (See Q & A 115). Some raise the tail as distraction display (e.g.slender coral snake – Calliophis melanurus) and this is very effective when the tail is brightly coloured (See Q & A 113). Some snakes, when handled, thrash and whip the tail wildly and this may be enough to put off the intruder. Some may jab the tail into the perceived enemy (e.g. Farancia, Typhlops. See, however, Q & A 128). Some use the tail to lure their prey (See Q & A 158). The uropelts use the tail to plug their burrow (See Q & A 257).
When seized by an enemy, some lizard species ‘drop’ the tail, the severed tail twisting and turning as if having a life of its own, leaving the predator totally confused and allowing the lizard to escape. ‘Lose your tail, but save your head’ seems to be the motto. This ability, known as autotomy, is found, for instance, in the all-too familiar geckos or the house lizards, the skinks and the anguids (which include the glass snakes (See Q & A 370)). A lizard that indulges in such self-mutilation may grow a new tail but it will be an apology for the original. Though a regrown tail is not a thing of beauty, it more or less performs the functions of the original. But the main drawback is that a lizard with a re-grown tail cannot repeat the trick. Some snakes are capable of losing their tail-tips under duress but they are incapable of regrowth. (See Desmond Morris: Animal Watching, 1990). This has been reported in a few species belonging to the genus Scaphiodontophis (small snakes from Central America) and also some species of sand snakes (genus: Psammophis). Inida has four species of Psammophis but there is no report of this behaviour in the Indian species.. No Indian species of snakes is known for certain to resort to autotomy. However, Whitaker & Captain (Snakes of India : The Field Guide, 2004) say about the Khasi hills keelback (Amphiesma khasiena), a common, non-venomous snake of the Northeast: “curiously, most adults and juveniles have missing tail tips”. Whether this is because of tail-dropping or for some other reason is not known.
Most sea snakes have paddle-like laterally flattened tails. This helps movement in water.
Harold Heatwole and Kenneth Zimmerman found in experiments with olive sea snakes (Aipysurus laevis) that the tip of the tail of this snake, which frequents coral reefs, is photo–sensitive, that is, sensitive to light. See Heatwole: Sea Snakes, very little research has been done on this. This is a very challenging area for research since the complex structure of eyes in the animal kingdom is supposed to have had its origin in two light-sensitive spots in certain primitive organisms. In the beginning, the spots could distinguish between light and darkness only, but over millions of years, they evolved into complex structures that could distinguish movement, then form and then colour. So, why should this particular snake which has regular eyes on its head also have these light-sensitive spots on the tail? Will it evolve into a snake with both foresight and hindsight, in a manner of speaking?
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