Original subtitles

There was a time when myths and science were entwined,

when mermaids and unicorns could mysteriously appear.

Nature was weird.

When science revealed the truth behind these imaginary creatures,

it found real animals lay behind the legends.

Today, science still makes astonishing discoveries.

But nature seems just as weird.

It's just that fact has broken free from fiction.

BIRDSONG

In nature relationships are usually straightforward. It's either friend or foe.

But in domesticated animals, they sometimes take a bizarre turn.

BLEATING

Goslings treat a lamb as their mother if she's the first thing they see when they hatch.

And a lonely horse may choose a goat as a friend.

Our relationship with domesticated animals is equally strange.

Some we ride. Some are hunters.

Some we eat. Some we have as companions.

Others even control our pests.

Whatever their role, we share our lives with more creatures than any other animal.

Among them, other odd friendships sometimes develop.

MIAOWS

A kitten will get on with a mouse if it's too young to know any better.

Even wild animals can form accidental bonds.

A broody robin without a mate becomes nanny to a family of thrushes.

This odd couple even bicker over whose job it is to clean out the waste.

DOGS BARKING

Even a fox and hounds can be playmates. But only if they're reared together.

Such behaviour seems to go against nature,

but this weird journey will reveal wild partnerships just as strange.

We domesticated cows 6,000 years ago.

But these tiny farm animals have been kept for far longer.

As the aphids suck sap they create sugary produce for their equally diminutive farmers.

The ants milk their herds by stroking them with their antennae.

This sweet secretion is known as honeydew.

Like human farmers, the ants even move their livestock to richer pastures to increase production.

They also protect their herds.

On this scale a ladybird is more dangerous than a wolf.

But if the mini herdsmen work together, the cattle-raider hasn't a chance.

Our own livestock also attracts wild animals.

The North American screech owl hunts the pests of animal feed.

In a similar way the cat started out as a pest controller in our ancient grain stores.

Decorative breeds now bred as our companions have lost some of their original hunting skills.

But as old roles change, new ones begin even among the hunters and prey.

Owl chicks have poor table manners and they leave dropped food to rot, creating a serious health risk.

But there is a solution.

Although snakes are favoured prey...

..the blind snake's armoured skin is hard to break into.

Snakes that escape are ignored.

They soon slip into a new role. Like cats, they hunt vermin.

Their pest control may be accidental.

But nests with snakes have the healthiest chicks.

One day this fledgling partnership might develop like the one between the cat and us.

The narrow-mouthed toad has formed this kind of mature partnership.

But with a spider.

Tarantulas usually kill small toads, but these she welcomes with open arms.

Her guests provide pest control.

Spiders' egg sacs are often infested by insects.

But the toads have the problem licked.

In return for pest control, the tarantula protects her housemates.

Screech owls also eat toads.

It's payback time.

The tarantula unleashes a secret weapon. A barrage of barbed hairs.

These microscopic arrows aim for the eyes.

Here the landlord doubles as a bodyguard.

It's an idea that's caught on.

The Bedu tribesmen of the Arabian desert like a varied diet.

The thorny-tailed lizard is a popular local delicacy.

Its burrow shelters the lizard from the sun, but it's far from secure.

Caught alive, the lizard keeps fresh for days.

The black scorpion is also on the hunt for a lizard's burrow.

It likes to share in the cooling shade.

They've struck a deal. The scorpion never stings its landlord

and in turn, the lizard makes its venomous tenant feel at home.

The scorpion pays rent by doubling as a security guard.

It fends off foxes or other predators.

But their pact makes lizard hunting a risky business.

One with a sting in its tail.

Aaagh!

GROWLS

SHOUTING IN ARABIC

The pain lasts for days but it's rarely fatal.

Both gain from their living arrangement. The lizard is protected from predators,

the scorpion from the heat.

A different protection racket operates in the ocean.

Clown fish use sea anemones as fortified living quarters.

But even they get stung when they first move in.

The clown fish must first dance carefully amongst the tentacles.

This covers the fish with the anemone's mucus and stops the stings from firing.

Anemones are also used by some hermit crabs.

First the anemone is dislodged.

Then it is secured to the crab's shell.

This living security system is even transferred across when the crab moves house.

The boxing crab goes further.

It uses anemones for poisonous fisticuffs.

Modified pincers hold this fistful of stinging tentacles and keep predators at bay.

Ocean partnerships take many forms.

Here, personal hygiene is so important, some earn a living offering a wash and brush-up service.

Like a barber's pole, these shrimps' red and white stripes advertise their trade.

They wait for a customer like this yellow tag to call.

The cleaner shrimps are conscientious, removing parasites and cleaning infected areas.

They attend any large customer that visits.

The cleaner wrasse wears a different striped uniform but provides a similar service.

Their personal grooming attends the most intimate areas.

But a relationship based on trust is easily abused.

The fine-toothed blenny wears the cleaner wrasse's uniform but it's really a demon barber.

It's after flesh. Its teeth are as sharp as a cut-throat razor.

Partnerships need trust to survive. Fortunately most cleaners keep their side of the bargain.

Some land animals even provide a bus service for their personal attendants.

The deer mouse's passengers are rove beetles.

Up to 20 of these hangers-on may be attached to the mouse at any one time.

When the mouse returns to one of its many nests, the beetles disembark.

The nests are infested by ticks. The rove beetle's job is to hunt them down.

The healthiest mice are those with the most tick-killing passengers.

The three-toed sloth carries even more hitch-hikers.

The outer coat hairs are covered with algae, making the sloth the only green mammal in the world.

Her baby has yet to acquire this jungle camouflage.

Sharing the ride is a menagerie of up to 900 different species that eat the algae.

Sloth moths are the most conspicuous passengers. They wait for a rare moment before they disembark.

Once a week nature calls and the sloth leaves the tree.

Strangely, it does its business at special latrines on the ground.

The moths are patient commuters. The whole process takes over an hour.

A seemingly pointless and dangerous journey.

This is the moment the moths are waiting for. They only lay their eggs in fresh dung.

They race to be first.

The moths' caterpillars feed on the dung and pupate here.

When the adult moths emerge, they find the sloths somewhere in the trees.

A different passenger service is provided by hummingbirds.

Their speciality is air charter. This rufous-tailed hummingbird feeds from heliconia flowers,

which also provide food for flower mites. When a bird arrives, the mites race on board.

Size for size, they're as quick as a sprinting cheetah.

They rely on the hummer to take them to fresh blooms.

The mites stow away in the birds' nostrils.

Up to a dozen mites may cram into these flight compartments.

Like human passengers, they only disembark when their transport stops at their preferred destination.

Each type of mite alights at his own favourite flower.

These mites do little harm but by blocking the nostrils they reduce flight efficiency.

They also compete for pollen and nectar.

They have started to act like parasites.

Parasites come in many forms. One of the weirdest inhabits the remote reaches of the Amazon.

Its grizzly story sometimes involves people.

One of the Amazon's commonest fish is the catfish.

It often plays host to another, far smaller fish.

This is the candiru, a parasite of fish gills.

For the catfish there is nowhere to hide. A trail of urea and ammonia streams from its gills.

The candiru simply swims in circles until it picks up the scent.

The signal becomes stronger as the fish approaches.

It slips inside unnoticed.

Spines on the candiru's head gaff the fish, securing it as it sucks blood from the gills.

The worst that can happen to a parasite is to end up in the wrong host.

The worst that can happen to a human is to become the wrong host to a candiru.

Urinating in the water is unwise.

The same cues of current and urea lure the candiru deep inside.

Argh!

Oh!

In the southern states of America,

an even stranger parasitic story recently emerged.

Frogs began to appear with extra limbs and other deformities.

These mysterious mutants were believed to be the result of chemical or radioactive pollution.

The full story is stranger still.

Herons that eat the frogs also acquire their parasites.

The birds pass the eggs of parasitic flatworms in their droppings.

Snails eat the eggs, which soon hatch and emerge as mini parasites.

Each searches for the nearest tadpole.

With torpedo precision, it tracks its target.

It aims for the limb buds. As it burrows inside, cells explode.

The fragmented limb buds try to regrow but in the process they create several legs instead of one.

The parasite creates a mutant for one simple purpose.

Deformed frogs make easy prey.

If its gruesome life cycle is to continue, its host must be eaten by a heron.

CROAKING

We too harbour many parasites, but our relationship with one of them has changed over the last 3,000 years.

The medicinal leech was used to treat a range of different ills

until blood-letting fell from favour at the end of the 19th century.

But recently, the leech has staged a medical comeback.

Its blood-sucking skills can reduce the swellings known as haematomas,

making it a valuable medical instrument.

As it bites, an anaesthetic numbs the pain.

The patient doesn't feel a thing as the leech sucks up the accumulated blood.

The three jaws have 100 teeth each and act like circular saws.

As the teeth cut, the leech releases anti-coagulants to disperse blood clots.

It can balloon to ten times its original size.

Leeches are highly adapted parasites.

Even these aquatic leeches have no problem looping their way over land.

They seek out prey by sensing the carbon dioxide of its breath.

The leech shows that even the most gruesome creatures can become our partners.

But like the leech, most of the animals we use, we also keep under our control.

Or at least we think we do.

But some of our partners still live a wild existence.

One is said to have a similar relationship with this African animal.

The honey badger is reputed to have the greater honey guide as its partner.

The bird leads the honey badger to bees' nests by calling for it to follow.

It relies on the mammal's strong claws to break into hives it has previously found.

Oddly, this partnership has yet to be proven by science,

but these people have not only witnessed it, they too are regularly guided by the bird.

By flying to and fro and calling, it leads the honey gatherers to a bees' nest several kilometres away.

Studies show that the bird knows every hive within 250 square kilometres.

The honey gatherers rely on the bird for their livelihood and always leave some comb as a reward.

As for the badger, its ancestors were probably in this relationship before people even appeared on the scene.

In Laguna in South Brazil, people and wild animals cooperate in an almost magical partnership.

The bottlenose dolphin's arrival is eagerly anticipated.

The fishermen rely on the dolphins to herd fish to their nets.

As they approach, the net is cast.

Dolphins are highly successful predators but by cooperating with fishermen, the odds of both improve.

Their system is elegantly simple.

They drive the shoals towards the men and use a rolling dive to cue them to throw their nets.

While the men gain a bounty, in the chaos, the shoal fragments and the dolphins pick off stragglers.

This relationship has survived centuries.

Both fishermen and dolphins pass on the rules of the game to their offspring.

With so many different partnerships,

humans are among the weirdest animals on the planet.

Perhaps this is our most perfect alliance,

for the animals involved live totally free.