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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
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