Snakes The Wonder of Animals

Planet Earth.

Millions of different species.

But a few are special.

Born to thrive.

The key to their success lies in their opportunism.

For others it's down to their ability to collaborate.

And for some, it's all about surviving, where others can't.

So, what is their secret?

By exploring the details deep beneath the skin,

we'll discover the unique features that set some species apart.

In this series, new behaviour

and the very latest scientific discoveries

will throw fresh insight

into the wonder of animals.

Whilst many species were striving to develop limbs,

wings and fins,

one group of animals opted for a completely different strategy.

They lost their limbs.

And yet, they've still become some of the most

successful predators on earth.

The snakes -

now, I know you might think that they all look the same,

but that simple body plan hides some remarkable adaptations.

In this episode, we'll see how they move, sense

and hunt in a huge variety of habitats

all across the planet.

Revealing how they rival their limbed counterparts at every turn -

starting with movement.

How do you get around without limbs?

Without feet to lift its body, the entire weight

of a snake rests on the ground.

To spread the load evenly,

a snake's internal organs are not clustered in one place like ours,

but arranged throughout the body in a linear fashion.

The liver and heart are elongated.

Kidneys are not paired side by side

but sit one behind the other.

And a long, thin lung spans almost half the length of the body,

accompanied by a tiny vestigial second lung.

Protecting these delicate organs,

a snake can have up to 400 pairs of ribs.

And as many as 24 muscles are

involved with the movement of each pair.

It's this that gives snakes their unrivalled strength

and flexibility.

And they need it, to overcome the problem of resistance.

To slide gracefully and effortlessly over rough ground, snakes have

actually turned friction to their advantage.

The puff adder can weigh 6kg.

That's a big bulk to drag across the African grasslands.

By contracting the muscles between its ribs and skin,

it's able to pull itself forward in a straight line.

And it has a helping hand...

specialised scales.

The belly scales of a puff adder are large and oblong.

When laid flat, they're so smooth they offer very little friction.

But when contracted, the bottom edges of the scales

stick out and provide grip.

This precise, crawling movement known as rectilinear motion,

is common in large, heavy snakes,

such as vipers, like the puff adder,

boas and pythons.

Allowing these stealth hunters to travel almost undetected.

Other snakes can move much more rapidly

and they do it using a completely different form of locomotion.

The mamba is perhaps the fastest snake in the world.

Now, this footage may look sped up but it's not, this is real-time.

It's thought a black mamba can move at up to five metres a second.

And it does this by using a unique method of locomotion that

exploits the obstacles in its path.

It may look like it's travelling roughly in a straight line.

But slow down the footage and you can see

it actually moves from side to side,

pushing against the grasses and sticks that are in its way.

This S-shaped movement, called lateral undulation

is not only fast but efficient.

The mamba may be quick,

but it still has to push its body over the ground.

One of the most efficient movers in the snake world is the sidewinder.

The shifting, steep-sided terrain of the desert poses an even

greater challenge to movement.

But THEY have a solution.

Instead of pulling themselves along,

or pushing from one point to another,

sidewinders place only two parts of their body on the

ground at any time.

The rest of the body lifts off the sand as it moves.

Alternating these points of contact allows them to shift forwards.

The acute angles formed as it winds are possible

because sidewinders have fewer vertebrae than many snakes.

In fact, only half of those found in a mamba.

Having successfully overcome the challenge of moving on land...

..snakes' versatility doesn't stop there.

They can also climb.

As on the ground,

snakes use a combination of serpentine motion and

specialised scales to master the vertical trunks of trees.

Incredibly, some snakes can even

adjust the angle of individual scales

to fine-tune their grip and effectively defy gravity.

But once up in the tree canopy, snakes are faced with the challenge

of crossing from one branch to another.

Again, their bodies provide a sophisticated solution.

This Madagascan leaf-nosed snake mimics a vine as it

lurks amongst the jungle foliage.

Tree snakes are able to extend their body by more than a third

to bridge gaps in the canopy.

They maintain such poised cantilevers thanks to

locking vertebrae that reinforce their spine.

There are some gaps, however, which can't be bridged.

Moving from one tree to another requires something rather special.

For this, snakes have taken to the skies.

With no wings to control their descent,

flying snakes transform their whole body into an aerofoil.

Splaying their ribs, their body flattens, doubling in width.

They then arch this frame into a concave shape.

This simple change creates aerodynamic forces

comparable to a wing.

The lift generated,

combined with the snakes' characteristic sideways movement,

allows them to glide for distances of up to 30 metres.

So, with the land and skies colonised,

there remains just one realm to master.

All snakes can swim, to a degree.

But one group has modified their already streamlined bodies

to become high-performance swimmers.

Most species of sea snake spend their entire lives in the ocean.

They can swim to depths of more than 90 metres.

To move in this marine environment,

sea snakes have undergone a change in shape.

Unlike the flying snakes,

they've flattened their bodies in the opposite plane.

Narrowing themselves vertically

to become tall, thin blades.

This adaptation, working in tandem with their paddle-like tails,

increases their surface area...

..enabling them to push against a larger volume of water,

creating more thrust as they swim.

Sea snakes have also developed an internal physiology that

allows them to move more efficiently through water.

Their lung extends almost the full length of their bodies.

And whilst providing oxygen when the snake is submerged,

in addition, it acts as a buoyancy control,

helping the snake move up and down in the water column.

Sea snakes are able to supplement their air supply

by absorbing 25% of the oxygen they require through their skin.

And this means they can remain underwater for up to

two hours on a single breath.

Having solved the problem of getting around,

the next step is to find food and for this,

snakes have an unrivalled collection of senses.

Perhaps the snake's most familiar feature is its forked tongue,

which it uses to smell.

Its two tips or "tines"

spread out as much as twice the width of its head...

..significantly widening its field of detection.

As the tongue swipes through the air,

each tip simultaneously samples different points...

..picking up scent particles as it goes.

But a snake's tongue doesn't taste as ours does.

It's merely a transport device,

carrying odour molecules

from the air or ground, back to the snake's

olfactory centre, the Jacobson's Organ.

Situated above the roof of the mouth,

THIS is where the sample is analysed.

The molecules from each tine are processed separately,

allowing the snake to assess its surroundings in stereo.

But a snake's tongue is capable of more than just

sensing their environment.

These garter snakes use them in a completely different way altogether.

In North America,

adult garter snakes hunt prey such as salamanders, frogs and toads.

But juveniles need to start small.

Having identified a good spot...

..they then fully extend their tongues to delicately touch

the surface of the water...

..mimicking worms or insects...

..to lure in their prey.

It's the snake equivalent of fly fishing.

Whilst garters use their tongues to smell and attract

their food, snakes have another sense...

..one that enables them to detect

prey before they can even see it.

Puff adders use their proximity to the ground to their advantage.

By resting their lower jaw directly on the substrate,

they can identify the slightest of movements...

..from as much as a metre away.

The footsteps of this striped mouse create tiny ground

and surface vibrations.

And when they reach the adder,

they're transmitted through the jawbones to its inner ear.

Whilst snakes don't have any external ear openings,

the inner ear is extremely well developed to sense vibrations

and low frequency sounds.

In fact, the puff adder is even able

to calculate the direction the mouse is travelling.

You see, their jaws,

like all snakes, are separated into two halves.

By comparing the timing of vibrations that each jawbone receives

the adder is able to detect the

precise location of their prey.

But what if there are no vibrations to detect?

What if your prey isn't even on the ground?

Some snakes have an additional sixth sense for hunting.

Deep inside this cave in Borneo, bats are returning to roost.

Flying fast and in the pitch darkness.

When all we could sense is chaos and flapping...

..a snake sees an opportunity.

Its senses cut straight through the commotion

to hone in on a victim.

It can see heat.

Pit vipers, boas and pythons

all have specialised heat-sensitive pits.

Located on the face, these organs are packed with

exceptionally receptive nerve fibres.

They can detect a temperature change

of as little as one-thousandth of a degree.

The nerves feed directly into the optic area of the snake's brain...

..creating a combined thermal and visual map.

With this unique ability,

snakes can sense each individual bat as they fly past...

..and select a precise target.

With their super senses,

snakes can see things that most animals can't.

Combined with a remarkable body, they have risen to the challenge

of locating and striking at prey.

But next, they have to kill it.

And, for this, our limbless serpents

have come up with some ingenious solutions.

SNAKE HISSES

Snakes hunt a huge variety of prey.

But the python takes on the largest.

They use brute force to overpower animals

often much larger than they are.

Squeezing so tightly, they even cut off the blood flow

to their own tissues.

But for short periods, their muscles can survive without oxygen.

To conserve energy, pythons only squeeze for as long as necessary.

Each time its prey exhales,

the python applies a burst of pressure.

Only when they no longer sense a heartbeat

do they stop squeezing.

A python's next challenge is to swallow and then

digest such a substantial kill.

Connected by an elastic ligament, the two halves of a snake's

lower jaw move independently and stretch around the prey.

It then uses a series of powerful, concertina-like muscle contractions,

to swallow the meal...whole.

And the skin around the mouth can stretch by 20%

to accommodate the load.

But inside, the change is even greater.

To fuel digestion, a python's organs

dramatically increase in size.

The heart, liver and intestines grow as much as 150%

to take on the task.

It can take weeks for a python to digest a kill of this size.

But then, following such a feast, it won't need to feed for months.

Possibly, as much as a year.

But what about smaller species?

They don't have as much muscle power.

So how do they disable their prey before it fights back?

Well, to hunt, these snakes have developed possibly

the most deadly weapon in the animal kingdom.

Venom.

A highly-modified,

sticky saliva containing a cocktail

of powerful toxins and enzymes.

It's administered in a split second through the snake's fangs.

And these modified teeth can take on a number of forms.

Recent research into their structure has revealed that most fangs

are, in fact, solid and not hollow.

The highly viscous venom lies in a groove on the front edge

of the fang, until the snake strikes.

It's then pulled from the groove into the prey's tissues,

like liquid into blotting paper.

However, the most effective delivery of venom is via hollow,

needle-like fangs.

The Pallas' viper has exactly such weaponry.

And it uses them to full effect

during the seasonal bird migration in China.

A tenth of a second...

is all it takes to strike.

A viper's fangs can even rotate to manoeuvre around obstacles

like bone and ensure that venom is

injected deep into the tissues.

It's not just one compound, it's a deadly concoction.

In the Pallas' viper, the venom includes haemotoxins

that attack the circulatory system, inducing massive

internal bleeding.

For other species it contains cytotoxins

that break down cell walls

and neurotoxins that disable the nervous system.

And in some snakes venom is even thought to include enzymes

which aid the digestive process.

Snakes have the ability to use their body as

no other animal can.

With a unique means of movement, acute senses

and remarkable hunting techniques,

these adaptable predators have

turned a lack of limbs to their ultimate advantage.

And THAT is the wonder of snakes!