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Natural History of Snakes

The following pages are intended to provide basic accounts of the evolution, classification and biology of snakes in general.

Snakes are reptiles, and the branch of zoology that deals with this class of animals is known as "herpetology." The word herpetology is derived from the Greek word herpeton, which means "a creeping thing". In the early days of zoology all lowly animals such as amphibians and reptiles were lumped together in this group of "creeping things," and the word herpetology was coined to encompass all of them. Although the science of zoological classification has advanced tremendously since those early days, we are still left with the word that includes the study of all amphibians and reptiles. This is an enormous subject in itself, and it would seem that it is time that individual researchers in the various branches of herpetology have their own name. A person who studies snakes, for example, could be called an "ophiologist" or a "serpentologist".

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Evolution of Reptiles Snake Classification Snake Biology



Evolution of Reptiles - Back to top
The reptiles form a class of vertebrate animals intermediate between the fishes and amphibians on one hand and the higher vertebrates (the birds and the mammals) on the other. Modern reptiles include the crocodilians, the turtles, the lizards and the snakes - not forgetting the lizard-like Tuatara, a single species that has an order all to itself. Snakes, lizards and Amphisbaenia are closely related and belong to a single order, Squamata, with three suborders. The ancestors of our modern snakes and lizards appeared along with the first dinosaurs during the late Triassic period, almost 200 million years ago, although fossil records of these reptiles are sparse. The modern lizards (suborder Lacertilia) are likely to have branched off from the primitive order Eosuchia during the Triassic period, but the oldest definite fossil links between modern lizards and their ancestors originated in the Upper Jurassic period, about 140 million years ago. During the same period, the first bird ancestors arose.

It is generally accepted that modern snakes (suborder Serpentes) arose from the lizards in the early Cretaceous period, about 130 million years ago, but there is no hard and fast fossil evidence to link the two suborders.
Unfortunately, small lizards and snakes do not make good fossils, as the small, delicate bones tend to break down or become scattered. Due to this incomplete fossil evidence, snake evolution is based largely on theory.
The earliest known fossil creatures resembling snakes are from the Cretaceous period some 130 million years ago. These were short and heavy and had a mixture of lizard and snake characteristics. Unfortunately, there is no intermediary evidence to link these creatures with modern snakes.

One of the most widely accepted theories is that all snakes evolved from burrowing lizards. Certain primitive lizards would have taken to burrowing into the substrate in order to escape predators and perhaps to hunt other subterranean creatures (as some modern species still do). This subterranean existence, over countless generations, would mean that certain modifications to the body would be necessary for the animal to remain successful. In the dark, subterranean world, eyes were of little use, so they gradually became absorbed until only vestiges remained, these being suitable for detecting the difference between light and darkness. The method of burrowing meant that limbs also became dispensable, and all snakes have lost their external limbs (although some have retained limb girdles). External ear openings would have also been an encumbrance in a subterranean environment, so these were also lost.

At a further stage of their evolution, some of these burrowing creatures found it convenient to return to the surface. In a lighter situation, eyes again became important so they were redeveloped (although the eyelids were completely lost, the eyes being under a transparent protective scale called a brille). External ear openings were also lost, but modern snakes have developed an efficient mechanism for detecting vibrations through solid surfaces. Additional adaptations that probably developed during the burrowing period include a sophisticated method of scenting odours and a highly developed sense of touch. Most modern snakes still have these attributes.

In spite of the theory regarding the evolution of modern snakes from burrowing lizards, there is no modern lizard family that could be construed to be a link between lizards and snakes. Even the legless amphisbaenians (suborder Amphisbaenia), which were at one time thought to be lizards “turning into snakes,” have now been classified into their own suborder. Although they have some characteristics similar to some found in both snakes and lizards, they have further unique characteristics that suggest separate evolution. If the burrowing theory is true, it can only be proved if and when complete fossils of the intermediate forms are found.

Snake Classification - Back to top
Classification is the means by which we categorize the infinite numbers of animal and plant species found on this planet. Without a logical system of classification, our biological knowledge of almost countless organisms would be in a most untidy state to say the least. Our present method of classification is based on the work of the Swedish botanist Carl von Linne, generally know as Linnaeus (1707-1778). He devised a system called the "binomial system of nomenclature" in which every kind of animal and plant is given a double name, the first part being that of the genus, the second that of the species. For international communication purposes Latin was the major language used by scientists at the time, and it is therefore not surprising that Linnaeus used this language as a basis for his system, although a fair amount of Greek and a smattering of other languages have also found their way into it.

As an example, the Indian Python has the scientific name of Python molurus, Python being the genus and molurus being the specific part of the name. As there are other types of Python in the genus, these are given different specific names (for example: Python reticulatus, Python sebae, etc.). In some cases a subspecific name may be added to the binomial, making it a trinomial. This is the case when two geographical races of a species are different, but not different enough to warrant separate species classification. For example, the Indian Python comes in two different geographical races sometimes referred to as the "light phase" and the "dark phase." The former is the true Indian Python and the typical or nominate subspecies, so it is named Python molurus molurus. The latter (sometimes referred to as the Burmese Python) is an eastern race or subspecies that has been given a subspecific name of Python molurus bivittatus.

For further classification, species are arranged in genera, genera in families, families in orders, orders in classes and so on, in ascending order, based on differences and similarities between them. One of the most convenient ways of illustrating the classification of of a particular species is to refer to a table. The following table shows as an example the classification of the Indian Python.

SNAKE CLASSIFICATION

(example: Indian Python)

Class:

Order:

Suborder:

Family:

Subfamily:

Genus:

Species:

Subspecies:

Subspecies:

Reptilia

Squamata

Serpentes

Boidae

Pythoninae

Python

Python molurus

P. m. molurus

P. m. bivittatus

All Reptiles

Lizards and Snakes

All Snakes

Pythons and Boas

All Pythons

Typical Pythons

Indian Python

Light Phase Indian Python

Dark Phase Indian or Burmese Python

In normal text, scientific names (genus, species, and subspecies only) are usually printed in italic script. When no italics are available, the names are customarily underlined. It is quite in order to abbreviate a binomial (or subspecific trinomial) once it has been mentioned in full in the text. For example: Python molurus bivittatus can be abbreviated to P. m. bivittatus.
There are four orders in the modern class Reptilia: Chelonia (tortoises and turtles), Crocodylia (crocodiles and alligators), Rhynchocephalia (the Tuatara), and Squamata (lizards, amphisbaenians, and snakes). Snakes therefor form a suborder (Serpentes) of the reptiles, sharing the order Squamata with the lizards (Lacertilia) and the amphisbaenians (Amphisbaenia). There are about 3000 species of modern snakes arranged in some 11 families and 354 genera.

In the following table, a simplified classification of the snake families is given. In the table, the snake families have been placed in their theoretical order of evolution. The blind snakes are considered to be the most primitive while the venomous snakes are the most highly developed. It can be seen that the family Boidae is somewhere near the middle of the scale, so pythons and boas, although not as primitive as the blind snakes are not as highly developed as the colubrids and the venomous snakes.

Snake Families

Approx. Number of Genera

Approx. Number of Species

1)

Typhlopidae
(Blind Snakes)

5

200

2)

Leptotyphlopidae
(Thread Snakes)

2

50

3)

Aniliidae
(Cylinder Snakes)

3

9

4)

Uropeltidae
(Shield-tails)

8

40

5)

Boidae
(Pythons and Boas)

22

90

6)

Xenopeltidae
(Sunbeam Snake)

1

1

7)

Acrochordidae
(Wart Snakes)

2

3

8)

Colubridae
(Typical Snakes)

250

2500

9)

Elapidae
(Cobras, Mambas etc.)

41

180

10)

Hydrophiidae
(Sea Snakes)

16

50

11)

Viperidae
(Vipers and Pit Vipers)

14

150


Snake Biology - Back to top
It is generally recognized that there are six classes of vertebrate animals, the most advanced (in evolutionary terms) being the Mammalia (mammals), followed by Aves (birds), Reptilia (reptiles), Amphibia (amphibians), Osteichthyes (bony fishes) and Chondrichthyes (cartilaginous fishes). Snakes, being reptiles, are therefore contained somewhere near the middle of the evolutionary scale in the subphylum Craniata (vertebrates). All reptiles posses certain combinations of anatomical, morphological, and behavioral characteristics unique to themselves, making them different from all the other vertebrate groups. The simplest recognizable characteristics of reptiles is that they have a scaly skin, they respire by means of lungs, and they are poikilothermic ("cold-blooded," relying on environmental conditions such as sunlight in order to control their temperatures). No other class of vertebrates posses the combination of all of these three attributes. Fishes may have a scaly skin and be poikilothermic, but they do not possess conventional lungs; birds may have scales on their legs and possess lungs, but they are homoiothermic ("warm-blooded," with a constant normal body temperature controlled by internal metabolism); amphibians may have lungs but they do not have a scaly skin; mammals are neither scaly skinned or poikilothermic.

In spite of the apparent disadvantage of being poikilothermic, reptiles are able to control their body temperature to a certain extent by a process known as "thermoregulation." They do this by moving in or out of the sun or other warm places, by flattening their bodies, and by using various appendages to gain extra warmth from the sun's rays. In general, it can be said that reptiles have a great dependence on thermoregulation, and this is a major factor when designing captive housing.

Reptiles increase in size and range of species as one approaches the tropics, because the climates in these regions allow activity for most of the year. In contrast, the temperate regions contain fewer species that are relatively small and are forced to hibernate during the winter months. Most species of pythons and boas come from the tropics, however, and do not require long periods in hibernation.
In spite of the limits of environmental preferences, snakes have managed to colonize a remarkably high percentage of the earth's surface; they are only completely absent from areas of high latitude and altitude, where the presence of permafrost precludes the opportunity of frost-free hibernation. Even so, a few species manage to survive near, if not within, the Artic Circle (for example Vipera berus, the European Common Adder, and Thamnophis sirtalis, the American Common Garter Snake), while in the Southern Hemisphere Bothrops ammodytoides, an Argentine pit viper, is found as far south as latitude 50. Such cold-climate snakes are active for only 3 or 4 months of the year, and it is remakable that such species are able to complete ongoing life cycles in these unlikely environments. The winter months are spent in a state of torpor in a frost-free refuge, usually well below the surface of the ground in a burrow or crevice.

In spite of its long cylindrical shape, a snake possesses all of the essential internal organs that mammals have, but of course they are modified to fit into the narrower space. In the Boids, for example, the left lung usually is reduced or absent, only the right lung being fully functional. Looking at the exterior of a typical, snake we will see that there is a head that usually is fairly distinct from the body, a narrower neck running into the body, and, finally, a tail. The body is covered with dry over-lapping scales arranged somewhat in the manner of tiles on a roof. Most Boids have smooth, glossy scales, but in some species these may have a dull finish. Colors vary from species to species, and in most cases, including all of the Boids, the function is camouflage to help the snake hide from predators and to confuse its own prey.

Perhaps the most obvious point about all snakes is that they are limbless, although some of the more primitive families posses a vestigial pelvic girdle, an indication of evolution from limbed reptiles. The pythons and boas possess such a pelvic girdle and, in addition, the vestiges of the hind limbs appear as "spurs" on either side of the cloaca. In most non-burrowing and non-aquatic species the ventral scales are many times larger and broader than those on the rest of the body. The belly often has a single row of these broad scales that play an important part in the locomotion of terrestrial and arboreal species.

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