Chapter-i origins Why are snakes called reptiles? What is a reptile?
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- Bu səhifədəki naviqasiya:
- How many species of reptiles are there worldwide
- How many species of reptiles are found in India
- Which is the nearest relative of snakes among lizards
- When did the first snake appear and where
- Which is the next oldest species known and from where
- Which are the earliest known snakes still found
- Which are the most primitive families of snakes that now survive
- Why is there no dependable fossil record of snakes
- What is the latest fossil evidence for snakes with legs
- How many families, genera and species of snakes are there worldwide
- Families : 20 Genera : 465 Species : 2968
- Distribution Genus Species
- How many families, genera and species of snakes are there in India
- Could the rattlesnakes of the Americas have evolved from the pit vipers of south and south-east Asia
- What is special about the snake family colubridae
- Which family of snakes is the most advanced in evolution
- Chapter-II Form, Structure Function Why is a snake called ‘cold blooded’
- What are the main external physical characteristics of snakes
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Chapter-I Origins
Classification of animals is essential for their scientific study. Land-dwelling animal groups with backbone have been split into four classes: amphibians, reptiles, birds and mammals. Generally speaking, reptiles differ from amphibians by their dry, scaly skin; from birds by absence of feathers; and from mammals by absence of fur or hair. The class ‘reptiles’ consists of lizards, crocodiles, turtles and tortoises and snakes. There are about 6032 living species of reptiles, almost double the number of living species of mammals.
Crocodiles : 22 Turtles and Tortoises : 242 Lizards : 2800 Snakes : 2968 -------- : 6032
-------- (The numbers are approximate).
Crocodiles : 3 Turtles and Tortoises : 32 Lizards : 155 Snakes : 276 (It is not unlikely that a few more may be discovered). -------- : 466
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The evolution of snakes has been difficult to study because of the rarity of fossil evidence and the highly damaged condition of these fossils which are very fragile (See Q & A 10). Scientists, generally, believe that snakes evolved from lizards. But some scientists believe that snakes and lizards evolved independently of each other from a common ancestor.
According to current scientific opinion, the monitor lizards of the family varanidae are the nearest relatives of snakes. There are many species world-wide. Indian species: common Indian monitor (Varanus bengalensis), desert monitor (V. griseus), yellow monitor (V. flavescens), water monitor (V. salvator). The tongue of the monitor is distinctive in that it is forked as in snakes. The monitor flickers its tongue just like a snake does to smell (See Q & A 32). The progressive stages in evolution from monitors to snakes are not known.
A land-dwelling snake named Lapparentophis deferenni evolved most probably about 130 million years ago, during the early cretaceous period, in what is now north Africa. (The first humanoids, our ancestors, evolved less than six million years ago).
A marine species, Simoliophis, found in parts of Europe and north Africa that were once under the sea. This is believed to have evolved at the beginning of the late cretaceous period, 100 million years ago.
The two families to which Lapparentophis and Simoliophis (See Q & A 6 & 7) belonged became extinct by the end of the cretaceous period, 65 million years ago. Among the snakes that evolved soon thereafter, we still have with us the boas (family: Boidae) and the South American pipe snake (family : Aniliidae).
Aniliidae, South American pipe snake. Only a single species. Boidae: In this are included the boa constrictors, the sand boas and, till sometime back, the pythons. Pythons are now classified as another family, the Pythonidae. Another of the oldest groups of snakes consists of the families Typhlopidae, Leptotyphlopidae and Anomalephidae under which come the worm snakes. Interestingly, sizewise, the boas and pythons are at one end of the scale and the typhlops and their relatives at the other end. The longest snake in the world is a python; the smallest is a typhlop. See Q & A 101 & 102.
The simplified and fragile body skeleton, the slender teeth and the delicate skull with many of its bones only loosely joined together have been responsible for a clear fossil not emerging. It has been said that the evolutionary tree of the extinct groups of snakes will never be reconstructed in detail.
According to a 2006 report in Nature, a 90-million year old fossil snake has been discovered in the Patagonia region of Argentina. This is the most primitive snake fossil known so far. This snake had two legs. It has been named Najash rionegrina. Najash is Hebrew for ‘snake’; Rio Negro is the province of Argentina where the discovery was made.
Taxonomy of snakes has been an uncertain field and there are frequent revisions in the classifications into families, genera and species. Snakes, hitherto unknown or undescribed, are also discovered and are described from time to time. An authoritative account is thus difficult to obtain. There is little possibility for any finality in the matter at any time. The following table has been prepared based on Chris Mattison’s The New Encyclopedia of Snakes, 2007. Even in this book, there are discrepancies between the figures in the tabular statement and in the narrative portions. The details in the narrative portions have been adopted in preparing the table below: Families : 20 Genera : 465 Species : 2968
Families:11 Genera:82 Species:276
Yes, in all probability. In the distant past, during the Ice Age, the Asian and the American continents were connected by an ice bridge which stretched from the eastern tip of Siberia to Alaska. Animals and plants had a passage from the Asian continent to the American continent through this land bridge (This accounted also for the earliest human arrivals in the Americas 30,000 years ago. Their descendents, the Inuit and Aleut peoples, still inhabit the remoter parts of Alaska). Many snake species including the boas and the pit vipers are believed to have taken this route from Asia to America. Some 10 million years ago, when the last Ice Age ended, the ice bridge disappeared and the waters took over. This stretch of ocean is now called the Bering Strait. (After Vitus Bering (1681-1741), Danish Russian navigator, who led the Russian expedition to determine whether Asia and N. America were at any time connected by land). The pit vipers from Asia which so migrated to the Americas are believed to have evolved into the rattlesnakes. Interestingly, the habit of vibrating the tail as the rattlesnakes do is found in many of the pit vipers of Asia though they do not have any rattles. E.g. The bamboo pit viper (Trimeresurus gramineus), the hump-nosed pit viper (Hypnale hypnale), the Himalayan pit viper (Gloydius himalayanus) – all found in India and elsewhere in South and South-east Asia. This habit must have marked the beginning of the evolution of the rattle. The habit of vibrating the tail is found in some species other than pit vipers also. Examples from India: Himalayan keelback (Rhabdophis himlayanus), cat snakes (Boiga spp.), striped trinket snake (Orthriophis taeniurus), sunbeam snake (Xenopeltis unicolor).
The largest family of living snakes, the colubridae, has 1807 species in 309 genera i.e. more than 60 % of the total snake species of the world. Its members have evolved to occupy every possible ecological niche. They are found on trees, on the ground, under the ground and in water. Though there are no sea snakes in the family, some members of the sub family Hamalopsinae are found in coastal and estuarine waters around South East Asia and certain races of Nerodia fasciata are found in the coastal and estuarine waters in the Gulf of Mexico. Colubrids have a worldwide distribution and constitute the dominant family wherever they are found except in Australia where they are poorly represented, viperidae being the dominant family. Colubrids have slender, elongated bodies, large scales on the head and large eyes. They are mostly non-venomous, but some are mildly venomous. The mildly venomous secretion from the Duvernoy’s gland does no serious harm to humans (See Q & A 224). But, a few of them are dangerous e.g. Twig snake (Theletornis spp.) and boomslang (Dispholidus typus) found in Africa. Red-necked keelback (Rhabdophis subminiatus) in Northeast India and elsewhere). Some of the colubrids lay eggs, some bear live young. They are often referred to as “typical snakes”. The colubrids probably had their origin in Asia.
Viperidae. Pit vipers and pitless vipers. They have also the most highly developed venom delivery system. They probably had their origin in Asia.
We do not know the full story of the extinction of snake species. Some may have become extinct even before they were known to exist. (The same is the case with many other life forms, particularly the amphibians and insects). In The Encyclopaedia of Vanished Species, 1989, David Day says: “One of the most successful of reptilian orders is the Serpentes, but even here we find Man’s activities have resulted in five extinctions during the last few decades”. Four of these were West Indian colubrids and these are known to have become extinct atleast by 1975. Apart from direct human persecution, the ravages wrought by the introduced mongoose was the prime reason. The Jamaican tree snake (Alsophis ater) and the related St. Croix tree snake (A. santicrucis) of the American Virgin Islands became extinct sometime after the 1960s. The Martinique racer (Dromicus cursor) and the St. Lucia racer (D.ornatus) which occurred in different islands of the West Indies have not been seen after 1962 and 1973 respectively. The Round Island boa (Bolyeria multocarinata) that occurred in the Round Island, off Mauritius, had become ‘effectively extinct’ by 1974, leaving a single individual with no hope of reproduction (It is not known whether this still survives). The reason for the destruction of the many endemic species of flora and fauna on this tiny island formed of volcanic tuff (basalt) was the destruction of vegetation caused by the introduction of rabbits and goats in 1844 and the consequent massive erosion of soil. In some cases of presumed extinction, there is, sometimes, a possibility of the snake surfacing later. A recent example is the Indian egg-eating snake. See Q & A 249. Chapter-II Form, Structure & Function
Mammals, birds and reptiles have to maintain their body temperatures within an optimum range to sustain their biological functions and meet behavioural requirements. This is called thermo-regulation. Snakes, like other reptiles, are ‘cold blooded’. This does not mean that the blood is cold but that there is no internal mechanism for regulating the body temperature as in the case of birds and mammals including humans. Snakes and other reptiles regulate their body temperature by choosing their environment. The technical term for this is exo-thermic as opposed to endo-thermic. While the snake has been able to surmount almost all its handicaps and, indeed, turn some of them like its leglessness to its advantage (See Q & A 57), cold-bloodedness is one handicap that it would have preferred to do without. Its temperature tolerance limit is between 2º C and 44º C. But, because it is cold-blooded and cannot regulate its body temperature internally as birds and mammals do, it becomes necessary for it to frequently choose different temperature gradients in the environment, go into hibernation or aestivation for long periods and so on. Extremes of temperature at either end, i.e. less than 2º C or more than 44º C, may even prove fatal to it (See also Q & A 144). Cold-bloodedness is, however, not entirely without its advantages: it drastically reduces food requirements.
As mentioned in Q & A 335 there are some other animals that may appear like snakes. But the main external physical characteristics of snakes, as listed below, will help to avoid confusion.
spur-like vestiges of the hind limbs can be seen. These serve no purpose in locomotion).
vestiges of the pelvic girdle are present).
row. (Lizards too have ventrals but these are not in a single row. The scales of amphisbaenians are arranged in rings).
united at the snout, but can move apart sideways.
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