Original subtitles

The sun...

giver of light and life, shines most powerfully at the Equator.

Here, it powers an extraordinarily rich zone of life.

Brilliant and bizarre species from three continents... three oceans.

More than a line on a map... Equator is a powerful force of nature.

EQUATOR

Challenge of Change

The Equator meets the western edge of Africa

on the coast of Gabon.

It's one of few places in all of Africa

where forests grow down to the shore...

and where forest animals come in search of new opportunity...

To change or adapt can be a successful survival tactic.

And survival may depend on exploring unfamiliar frontiers.

Where are these Hippopotami going... and why?

Venturing into the unknown is

a recurring survival story here on a continent

that has undergone huge upheaval.

Animals have had to meet the 'challenge of change'... or perish.

In Equatorial East Africa is the arid savanna...

such landscape is rare at the Equator.

Equator lands around the world are mostly forest covered... and wet.

Here, on the savanna, it hasn't rained for months...

The unrelenting heat of the sun has sucked the earth dry...

none but a few animals can endure it.

Riding powerful heat waves are vultures.

They are winged harbingers of death.

...In the dry season, survival is all that matters.

In the dry season, lions are forced to steal from vultures.

Lions remain on the savannah throughout the long drought.

Hunger is a constant companion

and they can only gaze out on depleted hunting grounds.

A Thomson's gazelle will not satisfy their needs...

but it will help.

Day after day, the pursuit ends in failure.

Each hunt saps energy, especially for females with young.

Mothers have little milk... and their cubs are weak.

An adult lion must eat 10 kilograms of meat a week...

any less, and they and their young will be in peril.

In the dry season, 90% of cubs may starve to death.

And death comes in other guises.

Hyenas are also desperately hungry, and they are cub Killers.

Lions remain on this arid plain during the dry season

because their fate is linked to protecting their hunting domain...

and their fate is linked to a drama that is about to unfold...

In November, cool trade winds begin to blow from the south...

Animals can sense a change long before it happens.

Great herds are on the move,

millions of animals begin a long journey toward the savanna.

As they approach, so do dark heavy clouds,

clouds that bring a much needed gift to the dry plain.

The change is sudden and dramatic... and so are showers.

It pours for less than an hour... and stops...

At least it's cooler after these brief downpours,

and animals can drink, there is water everywhere.

Every day grasslands flood, then slowly dry out...

this will be the pattern for several weeks...

The rains come as the region approaches it's hottest time of year...

as the sun advances to its highest point...

The sun that parched and cracked the land for months

now brings clouds and rain

to quench the deep thirst of the dusty plain.

The herds of the great migration

time their journey to coincide with this abundance.

As far as the eye can see,

new growth is feasted on by close to 2 million wildebeests,

other gazelles and zebras.

Animals not only time their migration to the coming of the rains...

it's quickly followed by a frenzy of births and babies.

All wildebeest calves are born within just one month

and they must quickly find their feet

to keep up with their mothers.

The birth bonanza further swells animal numbers.

Parts of Equatorial East Africa now contain

some of the greatest aggregations of animals on the planet...

Equatorial Africa hasn't always

hosted such spectacular animal numbers...

in the past it was a very different place.

For a long time, Africa was isolated

from other land masses and mostly covered in forest...

in these forests lived ancient animals whose descendants survive today...

Perhaps the most famous of these early Africans is the elephant.

It is descended from a much smaller creature thought

to be related to the hyrax...

is it possible the mighty elephant once looked like this?

Primates also look nothing like their African ancestors...

Where as the strange,

termite eating aardvark has changed little in 60 million years.

About 20 million years ago,

Africa underwent a dramatic transformation.

Land and sea level changes brought Africa and Eurasia closer together,

until they finally connected.

The joining of continents brought an influx of strange new animals.

Most were forest dwellers...

among them were hogs and strange horse like animals,

as well as ancient antelopes...

and with them came predators...

including cats of many kind...

...Some of these ancient species brought secrets

that they handed on to their descendants...

This is the Okapi, a name that means 'forest horse'.

Although it's the size and shape of a horse,

it shares a common ancestor... and a secret, with the giraffe.

African Bush Bucks retain

the primitive shape of their European forbears...

but they also inherited a very special gift

from their forest dwelling ancestors.

The first antelopes had multi chambered stomachs.

They contained bacteria,

that allowed them to digest low-nutrient plants

that other herbivores could not eat.

This 'secret weapon' stomach would

one day transform life near the Equator...

but while forests covered the continent,

there was little opportunity for these animals.

However their fate would be determined by other events in Africa

and one was particularly earth shattering.

About 30 millions years ago, East Africa was torn apart.

Slowly, a great valley opened and highlands

were thrust up by enormous volcanic forces.

Over millions of years The Great Rift Valley spread north

and south for over four thousand five hundred kilometers.

The Ethiopian Highlands rose up to 3,000 meters...

blocking rain bearing clouds from the west,

creating a rain shadow area in East Africa.

As the land became drier, rain forests slowly vanished...

and grasslands appeared.

Grasses slowly covered the drier parts of East Africa...

These new grasslands would profoundly change life at the Equator.

And first change was to the plants themselves...

Being close to the Equator,

the intense power of the sun favoured a group of grasses

called "Carbon 4" or C4 plants.

These grasses possess

a revolutionary pathway for changing sunlight

and carbon dioxide gas into sugars.

In most grasses,

the cells that carry out photosynthesis,

are near the leaf's upper surface.

In C4 plants, they are packed tightly around sheath cells.

...and as CO2 enters the leaf,

the outer cells of this bundle, concentrate the carbon dioxide.

This is the CA plant's secret weapon...

the leaf's like an engine with a turbocharger...

It rapidly manufactures sugars

and growth rates are up to 8 times faster than other grasses.

...and as the power of Equatorial sunlight increases,

so photosynthesis can increase almost indefinitely...

The whole system is encased in a tough outer core,

and this made these grasses very difficult to digest.

CA grasses out competed

and replaced other grasses in the hottest parts of East Africa...

But these new "grasslands” were tough,

inedible wastelands for many grass eaters...

however, the new plants proved to be

highly desirable for one group of animals...

For millions of years,

these animals had been forest dwellers,

but as the new grasslands spread,

they emerged to graze on these tough,

nutrient poor plants.

The animals are called ungulates, a name that means 'hoof'...

but being able to digest grasses...

gave rise to another name... ruminants.

They're called ruminants, in honor of their 'super stomachs'...

They'd inherited a 3 chambered stomach from their ancestors...

but even this was not adequate for the job of digesting CA grasses.

These tough plants required more powerful digestion...

It's probable that it was on the dry grasslands

that grazers evolved a new model of stomach...

It had a large extra chamber called the rumen,

where the grass is first broken down.

The agents of digestion are microbes.

They attack the grass's tough cell walls,

releasing sugars and vitamins...

but this is only the first stage.

Partly digested grasses are pumped from the second stomach

back to the mouth and chewed again to speed up the next stage

that takes place in the 3rd stomach.

Here, the grassy broth feeds bacteria that multiply quickly and dramatically.

Finally, the contents move to

the last chamber which are the true stomach...

This is where final digestion takes place...

By now, the animal is not only digesting grass,

it's also digesting a high protein diet of bacteria.

The ruminant 'super stomach"

has allowed some herbivores to grow large... and powerful...

Modern ruminants arose in Africa about 2 million years ago.

The buffalo has grown to be one of the biggest

and most lumbering of ruminants...

but others differ hugely in size and speed.

Thomson's gazelles are lightly built sprinters.

Wildebeests are marathon runners,

but there are many other swift athletes of the savanna.

The biggest of all is the giraffe...

Ruminants now occupy many different habitats and lifestyles.

Klippspringers keep to steep rocky outcrops...

Areas of low bush and shrubs are favoured by Gerunuks.

Tiny Dik Dik feed deep inside the thickest bush.

Fast, agile Impala are one of many open plain gazelles.

Reedbucks are secretive wetland dwellers...

as are waterbucks

The preferred range of Nyara is semi desert...

whereas, Oryx inhabit the very driest desert areas.

There are 125 species of antelope in Africa...

almost 90% of the world total.

They've conquered the grasslands in spectacular fashion...

animals range in size from half a meter to 5 meter giants;

from loners to herds of millions...

The ruminant stomach has other advantages...

grass is digested more slowly and thoroughly,

and because it is more efficient,

Antelopes don't have to graze constantly...

When an approaching enemy is detected...

escape is swift and sudden.

The herd has many eyes, ears and noses to sense danger...

and this has shaped the evolution of their main predators, the cats.

As antelopes evolved,

so cats changed to become more deadly predators.

A leopards spies its prey from the treetops.

Its ancestors were probably tree dwellers too.

In the grass, it's silent as a shadow...

slowly working its way into range.

The leopard doesn't kill immediately...

there may be others that would steal it...

the home tree is the safest place.

The leopard has developed stealth

to outwit the antelope's finely tuned sense of danger.

The cheetah is a cat of the open savanna...

out here, there's nowhere to hide...

it uses a different tactic to hunt prey... it uses speed.

If it can begin the chase before its prey sees it,

the cheetah has the advantage.

In Africa, the cheetah's body shape

became adapted to run down small, fast antelopes.

Exploding to reach speeds of 110 kilometers per hour,

it's the fastest land animal in the world.

It's body became streamlined,

its claws work as spikes, it is light, flexible and powerful.

The grasslands have boosted the number of small fast gazelles...

and cheetahs use blinding speed to outrun them.

Of all Africa's cats,

the lion has been most shaped by the Equatorial sun.

Lions live in family groups called prides...

they are the only truly social cats in the world.

They are also the biggest cats on the savanna.

Males stand over a meter high and weigh 260 kilograms...

yet, despite their size,

males remain on the margins of lion society.

A lion pride is an extended family of several

to dozen females and their young.

It is a society of females...

all are equal, all breed at around the same time...

The oldest female in this pride wears a radio tracking collar.

She's the leader; she has much useful experience and knowledge...

The male's role is to defend

the pride's territory and father the cubs.

The lion's imposing size

and unique social structure exists for a reason...

the biggest cats in Africa,

work in groups to hunt some of the largest plant eaters...

Yet, even working together does not guarantee success...

prey animals often see lions from a long distance...

so most hunting is done after dark.

Night is the killing time... when many animals are active.

Darkness restricts the vision of prey animals

allowing lions to get closer...

but they hunt at night mainly because it is cooler.

Hunting uses a lot of energy...

lions easily overheat under the blazing sun...

night kills are preferred.

A lion hunting party is highly organized.

Once a group of prey animals is identified,

they are slowly and quietly surrounded...

When all lions are in position,

one or two will make themselves visible.

The gazelles move quickly away...

but the trap is set...

they run into the waiting jaws of the group.

The Kill is shared among members of the pride...

Each pride has a hunting territory,

and the constant mission for the male is to protect both pride

and territory against other males that would overthrow him.

The territory can extend over 100 sq kilometers...

And his loud roar can be heard over most of it.

As he patrols, he leaves scent marks along the borders.

A successful hunt does not always mean enjoying the Kill...

As they feed, the females are surrounded by a band of hyenas.

For their size, hyenas have the strongest jaws in Africa...

they're not to be taken lightly... and they outnumber the lions.

The females have no choice but to give up their hard won meal.

But now the male returns...

The Hyenas victory is short lived...

superior numbers are no match for an angry male lion.

And when he returns, his mood has not improved...

The price of protection is that he takes over the Kill.

One of the hunting females now departs.

She returns to the Kill with the nursing females

who are caring for all the cubs of the pride.

The male allows the young cubs to feed alongside him...

It's a privilege he is not prepared to extend to the female.

For the male, the safety and survival of the cubs is most important

as they carry his genes.

The male may bully his way to first helping of the Kkill,

but the females control the pride...

Their major imperative is a strong society...

and to retain their hunting territory.

The lions hunting territory is passed from females to their daughters...

Young males must leave the pride once they reach maturity...

this keeps the pride strong by removing the risk of inbreeding.

Male blood moves from pride to pride.

Young lone males are called Nomads...

this nomad has spent the previous month

wandering and hunting by himself...

He's just 2 years of age, with just the beginnings of a mane.

He's still far too young to challenge a full grown male.

Nomads often lurk on the fringes of prides...

They mount frequent challenges,

to test the strength and speed of the dominant males.

One day, the pride male will not be fast or strong enough to defend himself...

but that day is not today.

This nomad will return, he will keep trying...

if he can survive out there on his own.

The "Nomad"” strategy ensures that strength is bred into all lions...

strength to face the hardships of the dry season...

and strength to hunt large herbivores on the savanna.

There's one plant eater that's taller than all the rest...

Giraffe are so big; they're rarely attacked by Lions.

The giraffe has a ruminant stomach but it does not graze the savanna...

Its long neck gives it access to acacia trees,

one of very few trees that can survive these conditions.

Acacia leaves are rich in nutrients...

but they're well protected... eating is a delicate art.

Giraffes can easily reach high into the trees...

but they experience some difficulty bending to graze or drink.

One problem from having such a long neck is controlling blood pressure...

specifically, the pressure of blood to the brain.

When standing upright,

the giraffe's heart must raise blood pressure to

more than twice human pressure

in order to push the blood up the neck and into the head.

But when it bends down,

all that pressure should suddenly rush blood to the head and potentially

cause fatal hemorrhage...

but it doesn't.

That's because, a giraffe has a special blood pressure control organ

just beneath its brain.

The structure, called a "wondernet”, is a network of blood vessels.

As blood rushes to the head,

the wondernet absorbs the increased flow

and swells like a sponge.

As it swells, arteries between wondernet

and brain bend and crimp like a water hose,

further reducing blood flow.

The wondernet enabled the giraffe to become

the tallest animal of the Equator and reach for the richest food...

but how did it come about?

It is likely that as grasslands spread,

the giant giraffe evolved from

a short necked forest ancestor very quickly...

because of a secret it brought from the forest...

And that secret is still found

in the remotest parts of Eastern Congo...

it lives on in the giraffe's forest relative.

This relative stands only one third of the giraffe's height

and is the forest horse, the Okapi.

The short necked okapi

doesn't need to regulate it's blood pressure,

yet, it also has a wondernet...

and this could explain how the giraffe evolved.

Okapi and giraffe have a common ancestor... the Paleotragus.

If it could be proved that it too had a 'wondernet',

it would solve the mystery of how the Paleotragus of the forest

could so rapidly become the giraffe of the savanna...

It's possible that the Giraffe was pre adapted to life on the savanna...

but one of Africa's original inhabitants went down aquite different path

to survive life on this arid region of the Equator...

It changed its brain as well as its body.

Elephants don't have ruminant stomachs...

so, how do they beat the heat... and the dry?

They do it by being 'super sized'.

In dry seasons, smaller animals will starve whereas,

larger animals, like elephants,

eat less food per unit of body weight, and are more likely to survive.

But elephants also grew their brains.

They use their tusks as a tool to peel away tree bark for nutrients.

In the dry season,

they remember where the best feeding grounds are...

and pass this knowledge on to their offspring.

Elephants might be less efficient

than ruminants at turning plants into energy...

their survival is based on brain power and size.

Hippopotomi are also giants.

They spend much of the day in rivers and lakes,

to protect themselves from the sun's powerful rays...

the pink colour is not sunburn

but a form of sunblock that they exude onto their skins...

The water also saves energy,

it helps support the enormous weight of their thick skinned bodies...

Like elephants, they don't have ruminant stomachs.

They too have increased body size

to improve energy efficiency and their chances of survival.

Hippos have never shaken off their strong bond with water...

they cannot survive beyond rivers and lakes.

Yet unlike many animals of the savanna,

they are meeting the challenge of change...

far beyond the dry plains of East Africa.

Moving west on the Equator,

between the savanna and Africa's Atlantic sea coast,

there's a mosaic of forest, wetland and grassland...

This is what much of Equatorial Africa looked like before the savanna existed.

...And helping maintain this patchwork landscape

is an animal called the farmer of the forest.

Red river hogs regularly commute between forest and grassland.

From the forest, they bring seeds that they spread on the grass...

Red River Hogs not only eat the seedlings that grow up...

but they also spread the forest.

Forest elephants are half the size of those on the savanna...

life in the forest is cooler, food is more abundant,

they do not need to be giants...

But they are always seeking new sources of food...

and during the rainy season,

their appetite draws them toward the sea.

In their path, is a tangled forest of mangrove trees?

These trees with long seed pods, fringe the edges of lagoons...

and in these lagoons, there are hippopotami.

These Hippos may look just the same as those on the savanna,

but their behavior is very different...

they too are drawn to the sea...

but for reasons that remain a mystery.

Animals come to the sea shore,

where the Equator meets the western coast of Africa, in Gabon...

Why do forest buffalo come here?

They come here for food.

The rain washes vegetation from the coastal forest

out into the ocean and back onto the beaches...

animals seek out Mangrove seed pods

that contain high concentrations of minerals.

Elephants also know of this special food source...

and they pass this knowledge onto their young.

But there is another surprise appearance at the beach.

Hippos come to the coast... but not to eat mangrove pods.

A Hippo mother and baby seem attracted to the ocean.

It's strange to see them away from rivers and lakes.

Yet, they seem at home here,

the baby is completely submerged by the surf.

What draws them to the sea?

Perhaps they swim in the ocean to wash off parasites.

...do they use the buoyancy of salt water to travel along the coast?

...or do they feed on sea plants that grow in the surf zone?

No one knows why they come here.

Only the hippos of Loango in Gabon swim in the ocean.

In ancient times,

hippopotamus like animals went down to the sea and became whales...

Perhaps swimming hippos, on this unfamiliar frontier,

are the beginning of what could become totally different animals,

tens of thousands years from now.

Are we are seeing evolution in action in the surf of Gabon?

And are there other evolutionary experiments

happening elsewhere on Africa's Equator.

The jungles of Gabon and Congo are dark and impenetrable...

but in places they open out into broad wetlands called "bai".

Succulent grasses and bulbs rich in minerals grow here...

and forest animals emerge to search out these plants...

among them, west lowland gorillas.

These gorillas don't like the water and will avoid it if possible...

but the drive to get these precious plants

leads some gorillas to overcome this dislike.

Not only is this gorilla immersing itself in the water

but it is walking upright...

The buoyancy of water helps it stay upright...

and it walks, arms raised,

like a reluctant bather, not wanting to get wet.

It's tempting to imagine that this is

how our African ancestors began walking on two legs...

...and if climate or other conditions change here in the future...

perhaps a walking gorilla will be better able to survive.

But change is already happening...

further east, animals are already changing to cope with extreme heat.

Tsavo National Park Kenya,

is at low altitude and has very high temperatures.

Here, lions live at their limit of survival.

The earth is cooler beneath the surface...

they try to avoid heat as temperatures soar.

In the shade it is nearly 40...

out in the open it's close to 50 degrees Celsius.

Lions here, usually hunt at night,

but if prey wanders too close, they cannot resist their instincts.

To bring down a buffalo in this heat is hugely taxing of energy...

Lions overheat very easily...

if their body temperatures rise they risk heat exhaustion and death.

Death comes quickly for the buffalo,

a throat bite constricts air and blood flow.

After the Kill,

the lions must seek shade and rest before they can begin to eat.

A daytime hunt can induce heat exhaustion among these animals...

but in Tsavo, male lions have another strategy for beating the heat.

Male lions here have greatly reduced manes.

A lion's mane raises body temperature...

So, they have adapted to be

without their traditional symbols of dominance.

Dominant males can still be identified their short manes are darker.

If it gets even hotter at the Equator,

lions with short manes could become a more familiar sight...

Dawn on the savanna...

A new day, after another night of killing.

Over millions of years,

Equatorial East Africa has undergone profound changes...

its animals have adapted... and survived...

and every day that struggle to survive continues

in battles between predator and prey.

East Africa is like no other place on the Equator...

The incredible diversity of life on the savanna has arisen

from merciless seasons of dryness and thirst...

inflicted by the intense heat of the Equatorial sun.