Swimming Snakes

Many snakes, such as the grass snake, Natrix natrix, can swim well.  They swim like eels and leeches, by making a wave pass backwards along the body.  Leeches differ in that the wave is in a vertical plane.  In fact many fish and whales swim in a similar manner, though others keep the body still and make waves in the fins.

In a very dense and slippery liquid the waves would be stationary with respect to the liquid.  In practice, with real liquids, there is some slippage.  To see how the system works, click here for a computer simulation, and choose "Run in the current location".

The same idea can be used on land.  On a smooth surface with a few scattered obstacles, a snake can bend a part of its body to touch each obstacle, and then move the waves along its body, pushing itself along.

Some surfaces are very difficult for snakes and slow worms.  Loose sand and shiny roads can leave an animal stranded.  Some species of snakes have adapted to sand by developing side-winding.  In this method, the snake creates a series of furrows and ridges in the sand, and uses these to create purchases for the waves.  The snake makes not only horizontal waves, but vertical ones, of small amplitude, just enough to push the body into the furrows, and to raise the rest of the body slightly off the  ground.  The two sets of waves are not in phase.  The snake is in a sense a flattened roller, and is not required to obtain purchase with it scales in order to progress.

Note that the wave is essential: if you draw a wiggly snake at forty-five degrees to the axis of a sheet of paper, make the paper into a cylinder and roll it, the snake moves at right angles to the axis of the cylinder.  There is nothing wrong with that, but the problem is that half of the snake is upside down at any time.  Even that is not in itself a problem: no, the real problem is that the head is rotating, and is upside down and sideways much of the time.

No doubt the vision of a snake could have evolved to cope with this, but the brain might have been bigger, and the difficulty of recognising threats would be great.  The sidewinding method undoubtedly wastes more energy than simply rolling, but the snake’s head takes no part in the wave motion, and points in the required direction all the time.  No part of the snake is ever upside down, or in fact more than slightly sloping.

To create a simple roll, the snake would have to continually distort the cylinder to create an imbalance, otherwise it would never move, so even rolling is not energy free.  Going up a slope would probably be very difficult by pure rolling.

Click here to see a simple computer simulation of a side-winding snake.

Here is a part of a frame from the simulation.

Do all snakes side-wind in the same way, or are some left-handed and some right-handed?

The motion of the side-winder is both elegant and fascinating.  Like the swimming snake and the sinuous snake, the side-winder produces forces that are not entirely in the direction of motion.  In this it resembles the wing of a bird or an aircraft, or the sail of a boat.  These aerofoils produce forces which are predominantly at right angles.  Indeed, a high performance glider may need to overcome drag which is as little as one sixtieth of the lift.

The sidewinder, the gliding eagle, and the sailing ship are using rotation, yet none itself rotates.  The snake is creating a rotating shape, while the wings of the bird and the sails of the ship are producing a rotation in the air which is analogous to that produced by a top-spun table-tennis ball..

The motion of the side-winder is both elegant and fascinating.  Like the swimming snake and the sinuous snake, the side-winder produces forces that are not entirely in the direction of motion.  In this it resembles the wing of a bird or an aircraft, or the sail of a boat.  These aerofoils produce forces which are predominantly at right angles to the motion.  Indeed, a high performance glider may need to overcome drag which is as little as one sixtieth of the lift.

Some snakes can even glide, albeit steeply, by flattening their bodies into a rather thick aerofoil, and adopting a sinuous shape which presents much of this aerofoil at right angles to the wind.  The snake slowly moves the wave along the body, probably to increase control and stability.

Not bad – sliding, side-winding, swimming and gliding, in a creature with no limbs.  In fact, no creature without wings or fins is able to glide at a shallow angle.  Human attempts to create aircraft without separate specialised wings and fuselage have been few, and most were experimental aircraft.

When to see adders

When can you see adders?   The season is rather variable, because it depends on the weather.  You can be lucky enough to see an adder as early as St Valentine’s day, and you may see one eight months later in mid-October.  And these are not the extremes – during very mild winters people may, rarely, see adders in November, December or January.  They are not fond of high temperatures,  and are in fact found further north than any other reptile – as far north as the Arctic circle, and at altitudes up to about 3000 m in the Alps.

If the warm sun forces you to discard too many clothes, most of the adders will have stopped basking because it is too hot.  But if you are wearing a pullover and an anorak because it is cold, they probably won’t have come out.  As a rough guide, 10 degrees to 16 degrees is about the right range.  But adders, like people, differ considerably in their behaviour, and you can sometimes find one adder that is out when all the others are in.  You can sometimes discover the presence of adders by finding the sloughed skins – inside out.

Behaviour of adders

The behaviour of adders depends on other factors beside the temperature.  They can use shade from the sun and they can shelter from the wind.  Once out, they may be quite reluctant to go in.  Sometimes an adder will even stay out during a shower of rain.  Don’t forget that some heat and light can get through thin cloud.  If the sun is not warm enough, an adder can flatten its body to maximise the exposed area, as in the middle of the next photograph.

Much more rarely, an adder will turn a part of its body on its side to expose the large dark scales on its underside, to absorb more heat.  These large scales are used by the adder in moving.  The adder can go down surprisingly steep slopes on rocks, but if decides to go over the edge of a very steep slope, when about half the adder has gone over, it will lose control and fall off, landing in a heap on the grass below.  If you try to follow an adder, crawling on your knees with a camera over rocks, thistles or brambles, you will find out how much better the adder is than you are at getting around in this terrain.

Underside of adders – scales and skins

The pictures below show the large dark under-scales, two of contesting males, and one of an adder exploring a cavity in some rocks.  In the second picture the shiny scales have reflected the flash-light.

If a snake cannot progress further forward, for example in a blind hole, it can use these wide scales to go backward, though not as well as it goes forward.

The next two pictures show portions of cast skins, under side and upper side respectively, flattened out, while the remainder show pictures of skins as sloughed.

The sloughed skin shows no sign of the normal dark colour of the under side, and in fact is almost transparent there, but the upper side has retained the pattern well.  Do you think that the pattern of the new skin will be the same as the pattern of the old one?

Here above are pictures of some adder scales.  Each one has a ridge from front to back, like the scales of the grass snake Natrix natrix, but unlike those of the smooth snake, Coronella austriaca.  Note, in the second and third pictures, how the flexible skin between the scales allows expansion on the outside of the curve.  This ability to expand is essential when large prey is swallowed.  Actually, some of the expansion is probably achieved by smoothing out folds in the skin.

The ridges can be seen in this picture of part of a sloughed skin.  When the time approaches for an adder to shed its skin, the old layer separates from the new one underneath.  The eye often seems to be occluded by a bluish colour.  Eventually the skin splits at the head, and the adder uses friction against plants to help the skin to peel back inside out in one piece.  Every detail is visible in the old skin, including the round scales that covered the eyes.

Looking at the pattern of scales on a snake, a lizard, or a pangolin, you might be reminded of the pattern of a long pine cone.  But there is a fundamental difference.  The animal patterns have left-right symmetry: the plant ones do not.  The numbers of left spirals and right spirals in a plant are different, as a result of a physical difference between the plant and the animal.  The asymmetry is clearly visible in the picture.  In a plant, all the parts grow from a small region – the meristem: in a reptile, all the scales are present from the start.  To find out how this difference affects plant growth, click here.

Feeding

You might expect sometimes to see a snake with a swelling after it has swallowed a small animal.  You might wait a long time.  By the time that a small mammal has had the air squeezed from its lungs, had its rib-cage squashed, and had its fur pressed down to its body, it doesn’t occupy anything like the volume that the live animal seems to occupy.  If the prey is a small lizard, you certainly won’t see any sign of it.  We must remember that a great many animals take smaller prey than we might imagine.  We may think of foxes in terms of chickens and lambs, but much of the time they have to make do with things like grubbed up beetles.  In the case of a snake, small invertebrates are taken.  Adders do not need to eat many times during the summer, because they live rather inactive lives.

Contrast this with the life of the shrew, which must eat voraciously just to remain alive.  So although there are great benefits from being a mammal, there can be disadvantages too.  In a sense no living animal is more "advanced" than any other, as we have all survived this far.  Adders must have many genes in common with us, because they have many of the same organs, and they have descended, like us, from four-legged animals.  We must in fact share a common ancestor with them. We have some concerns in common – we have to survive, eat and reproduce.  Nevertheless the adder is an alien creature.  We cannot share its world.

No mammals, even those who live underground, or seek prey which live underground, have become as long and thin as snakes.  One penalty paid by snakes for this shape is that the organs are long and thin, and undoubtedly less efficient than those of mammals.  For a mammal, such a shape would result in a much greater rate of loss of heat for a given mass, and the animal would need to carry more fat, carry more fur, or eat more, in order to maintain temperature, at least in a cold climate.  The first two methods would tend to negate the thinness, and the last is an obvious penalty.  Mustelids such as the stoat and the weasel seem to be about as far as the mammals have gone in this direction.  In fact, on average, the smaller a mammal, the more spherical it needs to be to conserve heat.  Is this borne out by the facts?

A snake swallows its prey whole, because it is not equipped with cutting teeth or chewing teeth, though it may possess small backward pointing ones.  By advancing different parts of its mouth in turn, the snake slowly engulfs the prey.  By taking the prey head first, the snake uses the the ratchet effect of the fur, as well as the ratchet effect of the teeth, to help in moving the animal back.  The teeth are not permanent: the snake drops them and grows new ones throughout its life.

Some snakes can find a mammal by detecting the infra-red radiation emitted by the warm body.  Of course, all objects emit this radiation all the time: the snake detects the difference between the animal’s emission and that from the surroundings.  Although the temperature difference is small, the rate of emission is a strong function of temperature, varying as the fourth power of the temperature.  A difference in temperature of 5 Celsius represents about 1.7 per cent on the absolute scale.  The difference in radiation emission might be four times as much as the temperature difference, about 7 per cent in the  example given.

When the adder is close enough to the prey, it strikes with the fangs, which are at the front of the mouth, injecting a lethal dose of venom down a canal in each fang.  The venom does not kill immediately, but the animal soon weakens and dies.  The adder has only to follow the scent and the warmth to find the dead animal.  Snake venoms are quite  complicated liquids, containing digestive enzymes as well as the venom.  The enzymes can greatly hasten decomposition of tissue.  Many of the venomous constituents work by interfering with nervous impulses; some degree of paralysis may result, as well as difficulty in breathing.  The venom may impede coagulation of the blood, and may affect the cardio-vascular system.

The fangs of some snakes have grooves to carry the venom, but in vipers the grooves have closed over to form a canal or tube.

Habitats

Adders can live in a wide range of habitats, being found further north than any other snake, even as far as the Arctic circle.  The further south they live the higher the ground they choose, so that they are always found at about the same temperature range.

Adders breeding

A good time to see adders is the breeding season.  The males become less timid, and can be followed for quite long distances as they seek the females. 

An adder leaves its hole to look for a female.

The journey can be arduous.

A female is basking          A male arrives    

How did he find her?  The male adder uses its tongue to test the air for the scent of a female.  The mouth is shaped so that the adder can put out its tongue though a little gap without opening its mouth.

In the next two pictures you can just about see the adder’s tongue. 

In the right hand picture the female’s head shows the slot for the tongue. 

If you decide to watch a male adder searching for a female, you may need great patience – they can seem very slow in finding a female which you yourself can already see.  But then, you aren’t half an inch from the ground.  When a male adder comes within sight of a female, what happens next depends on whether another male is already in attendance.

If another male is already present, the incoming adder will assess the situation carefully from a safe distance.  If the incumbent adder looks too big and strong, the new adder will quietly withdraw and go elsewhere, if possible without being detected.  If you stand quietly behind such an adder, you can almost see it thinking.  But if the other adder doesn’t look too big, the new adder will approach the pair.  Then you can see something which is not at all uncommon, but is rarely seen because most people have not looked for it.  This is the contest between male adders for possession of the female.  It is a competition rather than a fight. The adders never seem to open their mouths, let alone bite or use venom.

Competing adders

The adders rear up and push each other around, often twining around each other, in a wrestling match which looks sometimes looks rather ineffectual, but it can be surprisingly athletic.  The adders seem to be completely oblivious of their surroundings, allowing you a really good chance to observe them closely.  The first picture below was taken after the adders had been struggling together for over fifteen minutes.  They were clearly tired, and were losing interest.  Shortly after this they went their way, without an obvious winner emerging, and neither of them seemed to remember the female that had been the cause of the battle.  Male adders can remain engaged in these contests for as much as thirty minutes, though there may be pauses in the action.

If the adders are not well matched, one will quickly give in and slip away, chased by the winner until he is well away from the female.

Large JPEGs of Adder Contests

The adders in the next five pictures were almost equal in length and diameter, and their contest last intermittently for over thirty minutes.  It fizzled, as long contests often do, much less dramatically than is usually the case after short ones.

In the last picture, the winning male surveys the scene after seeing off the other male for the second time.  Occasionally the losing male does try to return to the scene, presumably to look for the female, but the winner always chases him away very quickly.

If there is no clear winner, an adder contest may go on for many minutes.  It may carry the males up to several metres away from the female as they wrestle and pursue. 

After a contest, the losing adder seldom returns, but if he does dare to reappear, the successful adder will be after him like a flash, and he will have to turn and get away at top speed.  In his panic he will go over everything in his path without stopping to check, including your foot.  After a contest the winning adder may take some time to find the female again, perhaps because there may be adder scent over all the places where she has been, making a confusing pattern.

Courtship

If you watch the pair from this point on, you will then see how the male courts the female and excites her interest.  Sometimes she will glide along, with the male flowing around and over her in a way that is hard to describe, and impossible to forget.  Great coordination is necessary in the muscles, because some parts of the male are on the ground and some are on the moving female.  Snakes also cope very well when several are curled up together and they need to move.  They never seem to get in a muddle.

Here the male is trying to stimulate the female’s interest by gliding over her and making sudden little movements.  Eventually she may allow the male to couple with her. Once they have joined together, they may stay like that for an hour or more.  If you disturb them, the female will slide away, dragging the male backwards behind her. This must be uncomfortable if they go down a narrow hole.  So it is a good idea to keep your distance so as not to spoil their basking, as indeed it is at all times.

The next set of pictures was taken during a typical encounter between a male and a female.  The courtship had already begun when the adders were found.  After ten minutes or so, they were left to continue undisturbed.  In a small population, any disturbance in the reproductive cycle could be significant, especially as females breed only in alternate years at most.  Unfortunately, adder colonies in England tend to be small, and they are very vulnerable.  Killing a single animal, especially a pregnant female, could have a very bad effect.  Numbers vary noticeably from year to year, because the effects of predation and conditions.

After the mating the male may stay with the female for up to a week, but after that he will wander off to seek food.  So males may no longer be seen in the breeding area, but the females will go on basking.  Sometimes if you get too close during this time, the female will hiss rather than glide away when disturbed.

Females breed only in alternate years – sometimes even less frequently.  Perhaps this is because they carry their young until they are ready to live without help, which requires a lot of energy from the female.  She seldom catches prey during gestation.  Adders produce from four to ten young, the smaller numbers being more common.  Females reach sexual maturity at four to five years old, males a year younger.

Retaining the eggs until the hatch is an adaptation to cooler climates where there are not enough warm places to incubate them outside a body.  The extra bulk and weight makes life difficult for the pregnant female, who will have to feed on small prey because of the limited space available in her body, and will experience problems with mobility.

Contented Couples

After mating, male and female may stay together for a long time, possibly up to a week.  After this period, the males tend to disappear, so that during the summer you are much more likely to see females than males.

Later in the summer you may be lucky enough to see the young adders as they bask in the sun, before they leave the place where they were born.  They are hard to see, being very small – no thicker than a pencil – and they tend to hide in little sun-traps. Just after their emergence, they may be seen in some numbers.  A few days later, when they have dispersed, they will be even harder to find.  They are quite difficult to photograph, for the same reasons.  Their colours do not seem to vary as much as those of the adults.  Many young adders are sandy to brownish in colour.

A few days after their birth, when they have dispersed, they will be even harder to find.   Here are adders which were less than a week old.

When the days become too cold, the adders will disappear below ground, and will probably not be seen again until the warm spring days, unless the winter includes some exceptionally warm days.

When you see a very young adder, and you compare it with an adult, you may find it difficult to believe that an adder can produce from four to ten young.  Let’s see whether this is reasonable.  Here are diagrams of an adult snake and two young ones.

The larger young one is a third the length and width of the big one: the ratio for the smaller one is a quarter.  The ratio in volumes between adult and young in the two cases is (1/3)³ and (1/4)³, that is 1/27 and 1/64 respectively.  If we assume ten young in each case, which is exceptionally high for an adder, the total volume of young will be 1/2.7 or 1/6.4 of the volume of the adult.  Let’s assume that the young are spread out along about a third of the length of the adult, remembering that all the organs of a snake are elongated.  

The ratios of the volumes of the young to this third of the adult will be 3/2.7 and 3/6.4, which are 1.11 and 0.47.  The new volume of that part of the adult will be increased in the ratio 1+1.11 and 1+0.47, that is, 2.11 and 1.47. To accommodate these volume increases, the radius of the adult will have to increase by the square root of these ratios, that is by the ratio 1.45 and 1.21.  These are not excessive values.  A pregnant adder does indeed look more plump than before, but not by a huge amount.  Nevertheless, you can easily recognize a pregnant female, and even by early July, a female adder can experience difficulty in moving, especially up a slope, and is unlikely to catch any prey.

The disadvantage of having to nourish little adders all the way through to the point where they can fend for themselves is a disadvantage to be weighed against the advantage to the young of being well protected during that time.  This burden is probably one reason that female adders do not breed in consecutive years.  The grass snake takes a very different route, laying a large number of eggs and leaving them to develop in a warm place, while the female is free to feed.

Adders, like many other animals, choose places that do not necessarily look attractive to us.  Tidying up the countryside to make it look more like a park is so inimical to the interests of many plants and animals that it should not be done without a great deal of circumspection.  Never move anything in order to find an adder. You may injure an adder, or you may inadvertently block an entrance if you replace a stone inexactly.  And of course you may be bitten.

All the adders in these pictures lived in a rectangle of no more than a square km.  In fact they were in several strips of ground no more than a few hundred metres long by 5 m to 10 m wide, a very small area.  Clearly they are vulnerable to changes in their small habitats.

If one segment of a spider web is removed, all the others will move slightly to a new equilibrium.  If one species is removed, all the others will adjust.  The loss of snakes from Great Britain might affect very few people and very few things; after all, they manage perfectly well in Ireland without any.  They would be missed only the people who care about them and enjoy watching them.

Adder’s Relations

The adder has relations in many parts of the world.  Here is a tree viper of the genus Trimeresurus, found by a helpful guide in Sarawak.  Unlike the adder, it’s camouflage depends on a single patternless colour.  At the other extreme, the beautiful Gaboon viper illustrates the tendency for nature’s most intricate patterns to be unseen.