Original subtitles

In the withering heat of the Australian desert

the Pierce snake hunts for prey

It carries in its armament less than two-thousandth of an ounce of venom

with a potency that could kill a quarter of a million mice

Why this snake needs the animal equivalent

of an atom bomb in its mouth no one really knows

But without the limbs and paws of other predators

snakes have had to find other ways to be sure of a kill

Snakes have been around for more than a hundred million years longer

than modern humans

Except for the polar regions and a few islands

they have spread throughout the world

Religion has endowed snakes with mystical powers

invoking human emotions from reverence to outright fear

Snakes live among us

Yet we find it difficult to accept their presence near our homes

whether we see them or not

This corn snake, however, is totally harmless

Although all snakes have the same fundamental body plan

there are wide variations on the theme

Measuring up to twenty-nine feet in length

the green anaconda is almost ten times longer than the typical corn snake

And at four hundred forty pounds

it is seven thousand times heavier than the blind snake

which belongs to the family that includes the smallest snake in the world

There are around two thousand seven hundred different species of snakes

Seventy percent of them hatch from eggs

Others are born live

after struggling to break free from a membranous egg sac

Venom and fangs are found in only a quarter of all known snakes

But all species have methods of attack

and defense that are adapted to habitats ranging from rain forest to desert

This is the tail of the cantle snake

Or if you are a frog, it is a worm

But not for long

Once tempted, the frog is turned from predator to prey in a single pounce

Snakes live by stealth, speed and sheer power

From the tops of trees to the depths of oceans

snakes have turned an apparent disadvantage into an evolutionary success

Creatures without limbs

they are the surprise and sometimes fearsome survivors

In the realm of the senses, snakes seem particularly ill-equipped

This cobra is not actually hypnotized by the snake-charmer music

In fact, it cannot hear it like us at all

Lacking an external ear and with a poorly-developed middle ear

it picks up vibrations only if its lower jaw is in contact with the ground

And with relatively poor eyesight, the cobra fixes its gaze on movement

In pit vipers, like the rattlesnake

vision is supplemented by two concave heat sensors just below the eyes

These so-called pits are lined with tiny receptor cells...

...sensitive to changes in temperature of two thousandth of a degree Fahrenheit

In its desert habitat

a rattlesnake can tell the difference between a warm breeze

and the heat given off by its potential prey

The brain takes this information and combines it

with visual images to precisely locate its prey

It is so accurate

it will have a near-one hundred percent success rate when it strikes

The importance of sight varies enormously between snake species

The blind snake has a degenerated eye

just a light-receptive cell

The eyes of many nocturnal snakes are so sensitive

that they can track the iris to a slit during the day

Most snakes are unable to focus their eyes the way humans do

Instead, they move the lens in and out like a camera

The long-nosed tree snake is the only snake

that can focus by changing the shape of its lens

It can also see in 3-D

The uniquely-grooved skull allows the snake's eyes to converge forward

Snakes can taste air, and the odors it carries

A specialized apparatus called the acobson's organ is found in all reptiles

In snakes, the twin tips of the tongue

pull scent molecules from the air

and deposit them into the organ, passing information to the brain

So despite its relatively poor vision

a corn snake's Jacobson's organ allows it to track a mouse

with rapid flicks of the tongue

For the green anaconda, the hunting tools are the same

It is the prey that varies

A mouse would hardly satisiy a twenty-nine foot anaconda

But a dwarf caiman might

This relative of the alligator is an imposing challenge

The anaconda is not deterred

Relying on its immense strength

the anaconda squeezes the life out of its helpless prey

But how is it possible for a snake to devour an animal

that is larger than its mouth?

An early development in the evolution of snakes

was the reconstruction of their heads

The jawbones and skull became held together

by elastic ligaments

rather than fixed tightly to one another

This enabled snakes to widen their jaws

and expand the capacity of their mouths

They also divided the jaw at the chin

allowing it to separate and open further

They use their backward-pointing teeth to grip their prey

rather than chew or tear like other predators

A more recent advance in the evolutionary story of snakes

is the development of fangs

Just a quarter of all snake species devised enlarged teeth

with a groove or hollow tube through the middle similar to a hypodermic needle

They could now deliver venom

Venom is produced by modified salivary glands

found in the top of the snake's mouth

A mixture of proteins

venom can destroy tissue or attack nerve and muscle function

As this Gaboon viper strikes

the dying rats heart pumps venom around its body

The snake can now wait

and track down the dead rat using its Jacobson's organ

The king cobra is the world's largest venomous snake

growing to over sixteen feet

It preys mostly on other snakes, even venomous ones

The mangrove snake delivers a mild venom

through a set of small fangs toward the back of its mouth

The cobra, however

injects its potent venom with fangs toward the front of its mouth

Although small in comparison to the Gaboon viper's inch-long fangs

the king cobra are no less effective

As it strikes, the cobra holds on tight

ensuring the venom finds its mark

Within minutes, the mangrove is dead

Around twenty-five thousand people a year die from snake bites

mostly in third world countries, where anti-venom is rare

Dean Ripa is lucky

He survived an attack by one of the most venomous snakes in the Americas

A bite from a bushmaster has an eighty percent fatality rate

even with anti-venom

"You feel a great deal of immediate pain

It is like a pair of hot needles

um, com... combined with vise..."

"...ah, and a burning sensation.

Ah, this pain has been compared to burn by fire and is one of the

the most, ah, ah, terrible pains

you can endure as it begins to spread

because you feel actually the, the poisons moving up your arm

You can feel this, this, the swelling increase."

"If it struck a vein or an artery

you may be unconscious within a few minutes

I was unconscious in seven."

"Rapid loss of blood, blood pressure

No heartbeat, ah, to speak of, ah, or a pulse that could be detected

And it was just a gradual blackening, darkening after that."

"Um, nada! Nada, nada, nada."

Snakes are rarely aggressive

and bites to humans are either accidental or defensive

Most snakes would rather warn large mammals of their presence

But some heavy-limbed creatures fail to heed the signals

A strike is the last resort

Snakes with venom that mostly destroys tissue can

cause devastating injuries on human limbs

But deaths from what are considered dangerous snakes are still quite rare

"Want to have a look at it?"

"Wow. Now, this is the, what, which type did you say it was?"

Herpetologist Harry Greene is working

to dispel the myth of snakes as murderous monsters

"Snake bite is wildly exaggerated

as a public health problem in the United States."

"We think probably a thousand people

, perhaps more, are bitten every year in the United States

of which probably fewer than ten people per year actually die

In the, in the US

snake bite is mainly a problem of people handling snakes intentionally

or of people just not being careful outdoors."

"And it is kind of amazing how hysterical

we can get about this perceived public health problem

and, to the point of, of wanting to kill snakes everywhere and

and, and that is the thing with snakes

If you just remember to not stick your hands in places

where you cannot see

you will never get hurt."

Snakes are not just predators

They are prey, and have inventive ways of defense

This red-spitting cobra projects venom towards its attacker's eyes

A cape cobra cannot spit, but it can play a game of illusion

Sometimes, however, it turns into a game of double bluff

In South Africa, meerkats prey on small snakes

but will take on a cobra to defend their young

The cape cobra spreads its hood to

make itself appear larger than it really is

The meerkats group together

to appear as one large animal to the snake

This time, the confrontation ends in a stalemate

The venomous saw-scaled viper ranges over Africa and parts of Asia

in areas of dense human population

It warns of its deadly presence

by rubbing together deeply-ridged scales on the side of its body

But how do you defend yourself if you are a harmless snake?

You appear to be what you are not

"One of the neat things about snakes is

that with such a simplified body form

it is easier for them to

to look like something else with relatively minor changes."

"This is a diadem rat snake from the Middle East that looks

and sounds a lot like saw-scale viper."

"Now this is an absolutely harmless snake."

"You just saw him bite my knuckle there

and nothing bad happened to me

It is making the sound by hissing

And the saw-scale viper makes the same sound by rubbing its scales together

It is a really nice example of mimicry

If I were in the Middle East and I heard this thing hissing,

I would probably jump back

because there would be the possibility that it is a saw-scale viper

an extremely dangerous animal."

These are the colors of danger.

They belong to the poisonous coral snake

But this snake, the tantilla, has similar color markings

The coral snake, however, sees through the disguise

Other snakes are better mimics

These colors belong to a group of snakes known as the false corals

As near-perfect replicas

they are much more likely to fool predators, some of the time

As the old American saying goes, red to black, venom lack

Red to yellow, kills a fellow."

All twenty-seven hundred or so snake species of today are descended

from one common ancestor

Unfortunately, the fossil record is so poor

that little is known about the details of snake evolution

Some of the best clues lies in the evidence of the present-day

rather than the fragmented remains of the past

Consider the bushmaster

Although he barely survived a bushmaster's bite

Dean Ripa continues to collect and study this rarest of snakes

Until recently

the scarcity of the bushmaster meant that it was poorly understood

"One problem that the zoos have had is trying to obtain good specimens and

ah, that is kind of where..."

"...I came in and how I fin... found myself collecting bushmasters

and able to support myself while I was doing it."

"So I, I spent months and months in rain forests just looking, ah

for bushmasters and catching them in a way that

that would not hurt them

The bushmaster is basically found over most of, ah

South America and, and parts of Central America."

"It requires a, a specific environment and, and that is

ah, best met with, in a, a cool, ah

dark, humid area like, like a rain forest."

The bushmaster certainly is the ultimate snake in that it is so unique

and so strange to most of the other snakes, and its rarity

of course, makes it, ah

the ultimate thing for herpetologists to go out and find

"It is the largest of all vipers

perhaps the heaviest of all poisonous snakes

reaching a weight of, in excess of forty pounds

It is the only New World, ah

pit viper that, that lays eggs

It has a spine in its tail

which is completely unique to that genus

which it vibrates in the sticks or leaves to produce a

a warning sound like a rattlesnake."

"I think I must have been about twenty-seven

when I caught my first bushmaster

I was alone in Surinam in

in a rain forest area, and, ah, in, at night

I had no gear, equipment

And I simply had to free-hand it

and throw it into my suitcase

One, one case I was chased

a bushmaster left a safe burrow and

and actually pursued me a distance of about ten meters through the forest

This snake sometimes is a, is a very dangerous customer."

Basically, the, the idea is to capture with

with the least-amount of stress onto the snake

so that it will survive, ah

transport and it will survive

ah, ah, long enough to, to breed for you."

"Okay."

"You, you, of course

bear in mind that it is all still a matter of luck."

"Good job. Ha, ha. Good job."

"Success."

"You may walk ah, for two days and find one

or you may walk for three months and not see one."

Back in the United States

Dean Ripa has successfully bred the black-head bushmaster

and now has over forty specimens in captivity

Dean's experience with these snakes led him to believe

that there may be more than one species

by studying his own snakes

he began to suspect that the South American

and the Central American bushmasters were entirely different

based on variations in body structure

Harry Greene and Kelly Zamudio at Berkeley were testing a similar idea

By studying bushmaster DNA

they came up with some startling results.

What was once considered a single species

they actually found to be three.

This gives us a key to understanding

how present-day snakes evolved.

"What we have on the screen are,

are the sequences of three snakes.

And on the bottom we have the two Central American forms,

and on the top we have the South American form."

"And I have put these three up just to contrast,

because if we move through the DNA here,

move through the sequences,

we can see that there is..."

"...a very high similarity between the two Central American forms,

but large differences between the two Central American forms

and the South American form.

Um, the two Central American..."

"...forms are not identical, though."

"If you scroll through the DNA here

you can see that there are also some differences

not quite as many as, differences from the South American forms

But there are also some differences between the two Central American forms

Okay. So from these..."

"...raw data, the sequences and the differences

between the different subspecies

we can reconstruct an evolutionary tree

that clearly indicates the relationship between the subspecies

and the evolutionary history of the species as a whole."

The South American bushmasters were separated from those in Central America

by the uplifting of the Andes over twelve million years ago

Then a mountain chain running down the middle of

what is now Costa Rica separated the Central American bushmasters...

...resulting in the evolution of three separate species of snake

Geographic isolation forces all animals to evolve on their own separate paths

Scientists can start to follow those paths back...

...into snakes' evolutionary past by studying DNA

the building blocks of life

"The origin of snakes is a little bit muddied

Ah, it certainly is something

that happened over a hundred million years ago."

"We feel pretty confident now

that snakes arose from within a group of lizards."

Whatever those lizards were, at some critical point,

they are thought to have become burrowing creatures...

...prompting a fundamental change in body plan

"If you look among these groups of lizards

the loss of limbs and elongation of the body

is something that happened evolutionarily over and over again,

probably dozens of times."

"And it is as if certain groups of lizards have experimented

with losing their legs

and becoming elongate, crawling creatures."

"Most of those experiments do not seem to go very far.

We only have a few species remaining today

One of them really took off, and that is snakes."

The remains of that successful experiment

can be seen near the tail of this python skeleton

Tiny spurs that were once hind legs actually protrude

through the skin of living pythons

But to get around

snakes have come to rely on a complex system of muscle fibers

and tendons wrapped around as many as three hundred ribs

Flexing particular muscles allows snakes to move in several ways

Large, bulky snakes move similarly to caterpillars

as a wave of muscle activity travels from head to tail

Called rectilinear locomotion

it involves gripping the ground with the edges of their belly scales

and dragging themselves forward

While hunting for prey in a barn

the corn snake finds itself between the tire

and metal rim of an old wheel

Unable to move from side to side

the snake is forced to move more like an accordion

But this concertina movement requires a lot of energy

Once free of the wheel, it resumes its usual motion

Snakes have adapted their limited methods

of locomotion to work in a wide variety of habitats

Through their unique evolution

snakes have proven that you do not need limbs to run

to climb, to jump, or to swim

Living in the oceans presented the sea snake

with a host of tricky adaptive challenges

A paddle-like tail and a keel that runs along

its belly allow the sea snake to glide effortlessly through the water

A single lung that extends almost

the full length of its body gives it buoyancy

and allows it to dive up to three hundred feet

and on one breath remain under water for up to five hours

To guarantee a rapid kill when hunting fish and eels

the sea snakes packs a powerful venom that makes it more toxic

than its closest family member, the cobra

Having adapted so well to the ocean

dry land is not an option for many sea snakes

Most sea snakes no longer have fully-formed ventral scales

that grip the ground

and allow snakes to move and survive on land

The ventral scales are wide, flat-belly scales

They are linked to the ribs by a series of muscles

Acting in concert with other belly scales

they perform like the treads on a tire

making their dragging movement possible

So effective are these scales that some snakes

like the corn snake, can climb vertically

Snake scales are actually part of the skin

and not just a covering

They protect the snake

while allowing for considerable elasticity and flexibility

But their uses may be as varied

as their remarkable patterns and colors

Snakes constantly change their skin

Live cells beneath the surface separate

and add to the cells above

As the old cells break down

the snake's outer skin thickens

and is ready to be discarded

A transparent scale known as the brill

and covering the eye takes on a milky hue

the clearest sign of the transformation to come

With the help of a log or rock to rub against

the snake literally crawls out of its skin in a process known as sloughing

Only a ghost-like trace of the snake's pattern

and coloration is left in the sloughed skin

The pigment cells that give snakes their often-brilliant colors remain

with the snake itself

Snake skin can do many things

including act as a brilliant disguise

But it can do little about one particular enemy, the cold

Snakes must rely on their external surroundings to warm up or cool down

unlike mammals and other creatures

that can regular their body temperatures

That is why snakes cannot survive north

or south of sixty-nine degrees latitude

The red-sided garter snakes of North America deal

with winter temperatures of minus forty degrees

by hiding underground in the thousands

In the warmth of spring, they emerge en masse

"To a geologist, this is a simple limestone sinkhole

But to a biologist, this is a gold mine."

Doctor Robert Mason has been studying these snakes for over fifteen years

and has solved many of the puzzles of their curious life cycle

"Well, here we are in a limestone pit approximately the size

of an average living room

and I am surrounded by twelve

fifteen thousand red-sided garter snakes."

"These snakes afford an outstanding opportunity

to study some very difficult questions

because these numbers here are just unprecedented

You will never see numbers of snakes like this anywhere else in the world."

"There are several things that are involved

with the over-wintering of these garter snakes

Most animals when they come out in the spring

they have the luxury of time

These garter snakes do not have that luxury of time

and so they kind of cheat the system in a little way and in

by that I mean the male garter snakes produce their sperm in the fall

and they store it over the winter

And then when they come out of hibernation in the spring

they are ready to go immediately

And they immediately can court and fertilize a female."

"Females, they are cheating the system as well

These snakes, ah

not only can they store sperm for six weeks

these females are noted for storing sperm for literally years

So years from now

this female could produce viable offspring

from a mating several years earlier."

As the female emerges from the den

she is approached by dozens of potential suitors

Attracted by her unique scent, the males rub their chin

against the female in the hope of stimulating her

They create what is known

as a mating ball around a single female

a mass competition in which there can be few winners

This one female may be their only chance to mate this year

"How does the female, for instance

choose that lucky male among the hundreds of suitors

Currently we just do not have a good feeling for that

In many other species of mammals, for instance

the big males are the ones that mate with the females

In the case of the garter snakes, that is just not so."

"Sometimes we find large females mating with small males and vice versa

So there are some other cues that are important to garter snakes..."

"...that enable females to choose the proper mate

The females, after they leave this den

will disperse out into the fields and marshes

where they will feed over the summer

And that is where they are going to give birth

at the end of August."

"The babies are born live.

And there is a mystery with these babies

There is a lost year, we call it

Because in the fall, immediately after giving birth

the females and the males..."

"...the adult males, will return to these same dens

But the babies are a myster

We do not know where the babies are

There are no first-year..."

"...snakes in this den. So, where are they?

Where do they over-winter that first year?

That is a great mystery to us

and we want to try to figure that out."

In most other species, like these Australian black snakes

rival males will fight each other one-on-one for the right to mate

But this combat can be a remarkably civilized affair

more a ritualized dance than deadly battle

In the pine barrens of North Carolina

the corn snake emerges from its winter burrow

It is spring, and he is ready to mate

With snakes, mating is an act of sinuous grace and beauty

Around a month after mating

the female corn snake lays her eggs in a secure hollow

Parental care ends here

The corn snake embryos develop inside the protective, leathery shell

After about two months

the young corn snakes break through their shells

Some emerge immediately

Others may wait several days before heading out alone

Throughout human history

snakes have been the object of myths and legend, often as demons

In the Garden of Eden, evil was embodied in the form of a snake

condemned forever by God to crawl on its belly

Yet there are many cultures where snakes are revered

and endowed with magical powers

In the northern Transvaal of South Africa

a human fertility rite is about to be performed

As the young women of the Venda tribe dance around the fire

they link arms to symbolize the movement of a python

The python is at the heart of many Venda legends

One story tells of a python marrying two women

Another says that a small python lives inside the women's wombs

and plays a vital role in conception

To the Venda, the snake is a potent life force, not an evil spirit

The ancient Mexicans believe the snake to be a god

linking not just the underworld with the heavens

but the earth with the universe

In the language of these lost civilizations

the constellation of Scorpio was called Amaru, meaning snake

The constellation had a spiritual influence over all of Central

and South American cultures

It has always been tempting for humans to mythologize snakes

It is not hard to see why

The ancestors of snakes emerged from an evolutionary experiment

which left them without limbs

Yet they are one of the great success stories of the natural world

"Snakes have been around on the earth for millions of years..."

"...and as far as I can tell

they should be around for millions of more years

As long as they are not interfered with..."

"...there is no reason to believe they would not

They are exquisitely well-adapted to the environment they live in.

"Given the opportunities

they should be going on for millions more years."

The end