Home Reptiles and Amphibians Identifying snakes by scalation and other details
Identifying snakes by scalation and other details

Of all the major reptile groups, snakes are perhaps the hardest to identify at the species level. There are many different species, and unlike lizards which have a wide range of body shapes, snakes appear to be very similar in shape and often in size (most snakes in Europe at least fall within the 3'-6'/ 1-2m length).

Most detailed field guides, when describing snake species, will discuss details of scalation. This method is used to distinguish snakes at species and genus level from one another. Unfortunately this can be bewildering for the layman as the names of the scales tend to be Latinised, much as doctors and surgeons tend to use the Latin names for muscles or bones. Talk of species being distinguished by the occipital being absent or the 4th or 5th supralabial touching the eye is probably as much help to the ordinary nature enthusiast as the original Latin descriptions made in the eighteenth century.

What follows is therefore an attempt to introduce the reader to how we identify snakes in layman's terms. You can't get away from using scalation details, but hopefully this article will throw some light on the subject and remove the mystery and panic.

A brief word of caution is in order. Although Europe's vipers and rear-fanged snakes are mostly non-lethal to an adult, bites can still inflict a great deal of pain, and their effect on a child or person in lesser health can be considerable.  If you are uncertain whether a snake is venomous or otherwise dangerous, don't take chances but observe it from a safe distance. Binoculars or the zoom lens on a good camera can help.  Also do not attempt to molest the snake – this will only make matters worse.

Telling snakes apart from other legless reptiles
There are other legless reptiles apart from snakes, but they are fairly straightforward to tell apart.  Snakes have no eyelids and no external ear opening, whereas legless lizards in Iberia have eyelids and usually a visible hole to the rear of the eye which is the ear opening.  Some apparently legless creatures do actually have small limbs if one looks closer, e.g. Chalcides striatus the Western Three-Toed Skink, whereas no snake has externally visible limbs.  The amphisbaenian Blanus cinereus also lacks limbs or external ears, but its eyes are so tiny as to be almost invisible (they lay beneath the skin), whereas in Iberia all snakes have normal well-developed eyes.

General identification details
The differences between snake species are more subtle than those between different lizards, but they are helpful nevertheless.

Shape is obviously trickier than in most animals, but there are still things to watch out for.  If a European snake is relatively stocky with a short, fairly blunt tail, it may well be a viper, or in Greece, the harmless Sand Boa. Conversely if it is very long and relatively thin, then it could well be one of the whipsnakes (the clue's in the name!), highly strung but otherwise harmless serpents.  The shape of the head is especially important – a somewhat triangular or “spade-like” (as in the card symbol) head that is wider than the body is usually that of a viper.  Another aspect of the head is the appearance of the eyes.  In some snakes the scales over the eyes give an impression of scowling, a feature that distinguishes both vipers and the Montpellier Snake (Malpolon monspessulanus).  The eye itself is also a distinguishing feature – vipers have slit-like pupils in sunlight rather as a cat does.

Colour is not always helpful, since some species may be found in several different colours, including all black (melanistic) individuals.  However some snakes can be distinguished by their patterning, for example the Ladder Snake (Elaphe scalaris), which has two dark longitudinal lines running down its back from head to tail and whose young have the very obvious “ladder” pattern down the back. Likewise the Horseshoe Whipsnake (Coluber hippocrepis) has the distinctive horseshoe-shaped marking on the top of the head.  An Iberian snake with a distinctive zigzag pattern down its back is usually either a viper or the Viperine Snake (Natrix maura).

Size is somewhat subjective and hard to judge in snakes, since the theoretical length of the snake is really what one would find if the snake were stretched out perfectly straight, whereas in life most snakes when not moving do not adopt this posture but are either coiled or laying in an undulating line, thus appearing shorter.  Nevertheless there are size differences: Macroprotodon cucullatus, the False Smooth Snake, is never more than 65cm long and usually less than 45cm, whereas most Iberian snakes are at least twice this length as adults.

Habitat and geographical location may also be a guide.  Field guides will normally show the range of a species, so by process of elimination you may be able to arrive at a conclusion.  For example in Spain the range of a fair number of species stops short of the Pyrenees.  Habitat can also help to eliminate some possibilities, though maybe less so than in lizards since all snakes are good climbers as a rule. A snake found near or in a river or stream may well be one of the Natrix (water snakes).

Behaviour may also be a key. Whipsnakes are known for being highly strung and nervous, whereas some rat snakes are fairly calm. However this is rather subjective and depends not least on how the snake perceives the person watching it, i.e. as a threat or not.

Vipers – in Spain, a snake with keeled scales and a vertical pupil (when seen in daylight) is a viper.    Natrix maura Viperine Snake has keeled scales and similar-shaped head but a look at the eye will show a round pupil even in sunlight.

Scalation details on European snakes can be quite helpful in discerning a species, particularly with colubrid snakes (i.e. the “normal” snakes) which have regular head scales rather like those of lacertid lizards.  Viper scalation is a bit more irregular, but then vipers in Europe can usually be distinguished by some of the other features mentioned above.

There are several main areas to be considered – dorsal (essentially the back, from head to the join with the tail), ventral (the belly), the head or cephalic scales and the tail or subcaudal.

Dorsal area
The scales on the back may be in regular rows (in which case the number of longitudinal and transverse rows can help identify the snake) or very irregular in their arrangement, and may be flat or else overlapping one another (imbricate). In addition the individual scales may be smooth or with one or more keels, usually at the rear edge.

To count scale rows, start at a point about halfway along the body (i.e. between the snout and the anal vent). From the first row of dorsal scales adjoining the flat belly scales, count each row diagonally forward until you get to the middle of the back and then proceed backwards down the other side until you reach the other side of the belly scale. Each species has its own range of dorsal scale rows, e.g. the Smooth Snake (Coronella austriaca) invariably has 19 rows, while the Ladder Snake (Elaphe scalaris) usually has 27 but may occasionally have 25 or 29.

Ventral area
In colubrid and viperine snakes the scales on the belly are more like flattened plates and arranged in a single regular row. The number of ventral scales can be a guide to the species.  The scale or plate that protects the anus is the anal shield and may be either undivided (i.e. a single unit) or divided (having the appearance of being divided in two down the middle lengthways).  Ventral scales are counted from the ventral scale closest to the head and adjoining the lowest row of dorsal scales up to the anal shield (see the diagram further down). As with dorsal scales, each species has its own count of ventral scales, e.g. 201-220 for the Ladder Snake or 153-199 for the Smooth Snake.

Tail area
Scales on the underneath of a snake's tail are the subcaudals.  They are counted from the anal shield to the tip of the tail, and again may help in identification.

Head area
This is tricky to describe if only because of the number of different scales.  However, it is not as hard as one might expect.  We will start with the easiest parts and work through the whole area.  The diagrams used in this guide were adapted from Alfredo Salvador's Guia de Campo de los Anfibios y Reptiles de la Peninsula Iberica, Islas Baleares y Canarias (1985).

Sketch of the dorsal view of a colubrid snake head

The above diagram is a view of the colubrid (i.e. non-viper) snake head from above.  The scales shown in the key are the most important for the identification of a snake using this method.

The scale right on the front of the snout on the upper jaw is the rostral. Above the rostral on top of the snout sits a pair of internasals

The internasals are followed by a pair of prefrontals.  A third scale may sit between the two prefrontals.  Behind the prefrontals, roughly between the eyes, is a large scale, the frontal.  On either side of the frontal is a single supraocular sitting above the eye socket.

Continuing backwards from the frontal there is a pair of parietals, each of which adjoins not only the frontal but also a supraocular.

Sketch of the lateral view of a colubrid snake head

Although snakes lack the fleshy lips of humans, their “lips” are formed from two rows of scales, one above the line of the jaw (the supralabials) and one below it (the sublabials). In the above diagram the supralabials, for example, run between the rostral and the corner of the mouth, giving a total of seven on each side. On each side of the head, below the internasal but above the first supralabial and behind the rostral, is one or a pair of nasals.  In colubrid snakes the nostril tends to sit between two nasals, whereas in vipers it is placed in the centre of a single nasal. Behind the nasal(s) and above the supralabials are usually 1-2 loreals.

Behind the prefrontal and below the supraocular is a scale in front of the eye, the preocular. The scales bordering the rear of the eye are the postoculars. Behind the postoculars and between the parietal above and the supralabials below are the temporals.

(awaiting sketch of snakes head ventral view)

In the above diagram the lower part of the upper jaw can be seen overlapping the lower jaw, so the supralabials (not all labelled due to space in the picture) and rostral can be seen.

The scale on the front of the lower jaw is the mental, followed on either side by a row of sublabials or infralabials that form the lower “lip”.  Behind the first sublabials are two pairs of submaxillaries.

Sketch of a snake's anal plate

The final diagram (above) shows the anal plate (shaded). That on the left is the anal plate of a viper and is undivided, that on the right, the divided anal plate of a colubrid snake.  The anal plate is the scale covering the anal vent, and also marks the end of the body (and hence ventral scales) and the beginning of the tail.  In the diagram above the scales beyond the vent are subcaudals, i.e. scales found on the underneath of the tail. The number of these scales and whether they are found in pairs (as in the above diagram) or singly in a row, rather like the ventrals, is another clue to identifying a particular snake.

In summary

The above guide is as much a guide to the conventions and terms used in describing snakes as a guide to their identification in Spain.  It can thus be used anywhere in the world, especially those countries where a multitude of different snakes can cause confusion.


Special  thanks for the writing of this article and for the detailed sketches go to Cyberlizard, visit this link for more information including care of lizards and amphibians in captivity.

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Last Updated ( Wednesday, 22 October 2008 22:28 )

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