0:00:05 > 0:00:10There was a time when myths and science were entwined,
0:00:10 > 0:00:14when mermaids and unicorns could mysteriously appear.
0:00:14 > 0:00:17Nature was weird.
0:00:19 > 0:00:23When science revealed the truth behind these imaginary creatures,
0:00:23 > 0:00:28it found real animals lay behind the legends.
0:00:28 > 0:00:33Today, science still makes astonishing discoveries.
0:00:33 > 0:00:36But nature seems just as weird.
0:00:39 > 0:00:43It's just that fact has broken free from fiction.
0:00:54 > 0:00:57BIRDSONG
0:01:04 > 0:01:11In nature relationships are usually straightforward. It's either friend or foe.
0:01:14 > 0:01:19But in domesticated animals, they sometimes take a bizarre turn.
0:01:19 > 0:01:22BLEATING
0:01:24 > 0:01:31Goslings treat a lamb as their mother if she's the first thing they see when they hatch.
0:01:31 > 0:01:36And a lonely horse may choose a goat as a friend.
0:01:36 > 0:01:41Our relationship with domesticated animals is equally strange.
0:01:42 > 0:01:45Some we ride. Some are hunters.
0:01:45 > 0:01:50Some we eat. Some we have as companions.
0:01:50 > 0:01:53Others even control our pests.
0:01:53 > 0:02:00Whatever their role, we share our lives with more creatures than any other animal.
0:02:00 > 0:02:04Among them, other odd friendships sometimes develop.
0:02:04 > 0:02:07MIAOWS
0:02:07 > 0:02:12A kitten will get on with a mouse if it's too young to know any better.
0:02:20 > 0:02:24Even wild animals can form accidental bonds.
0:02:29 > 0:02:34A broody robin without a mate becomes nanny to a family of thrushes.
0:02:37 > 0:02:44This odd couple even bicker over whose job it is to clean out the waste.
0:02:51 > 0:02:54DOGS BARKING
0:02:56 > 0:03:02Even a fox and hounds can be playmates. But only if they're reared together.
0:03:03 > 0:03:08Such behaviour seems to go against nature,
0:03:08 > 0:03:13but this weird journey will reveal wild partnerships just as strange.
0:03:24 > 0:03:27We domesticated cows 6,000 years ago.
0:03:27 > 0:03:32But these tiny farm animals have been kept for far longer.
0:03:32 > 0:03:39As the aphids suck sap they create sugary produce for their equally diminutive farmers.
0:03:40 > 0:03:45The ants milk their herds by stroking them with their antennae.
0:03:52 > 0:03:55This sweet secretion is known as honeydew.
0:03:57 > 0:04:05Like human farmers, the ants even move their livestock to richer pastures to increase production.
0:04:08 > 0:04:11They also protect their herds.
0:04:14 > 0:04:18On this scale a ladybird is more dangerous than a wolf.
0:04:19 > 0:04:26But if the mini herdsmen work together, the cattle-raider hasn't a chance.
0:04:36 > 0:04:40Our own livestock also attracts wild animals.
0:04:41 > 0:04:47The North American screech owl hunts the pests of animal feed.
0:04:51 > 0:04:58In a similar way the cat started out as a pest controller in our ancient grain stores.
0:05:00 > 0:05:07Decorative breeds now bred as our companions have lost some of their original hunting skills.
0:05:11 > 0:05:19But as old roles change, new ones begin even among the hunters and prey.
0:05:22 > 0:05:30Owl chicks have poor table manners and they leave dropped food to rot, creating a serious health risk.
0:05:32 > 0:05:35But there is a solution.
0:05:40 > 0:05:42Although snakes are favoured prey...
0:05:43 > 0:05:49..the blind snake's armoured skin is hard to break into.
0:05:49 > 0:05:52Snakes that escape are ignored.
0:05:54 > 0:06:00They soon slip into a new role. Like cats, they hunt vermin.
0:06:09 > 0:06:12Their pest control may be accidental.
0:06:15 > 0:06:19But nests with snakes have the healthiest chicks.
0:06:19 > 0:06:26One day this fledgling partnership might develop like the one between the cat and us.
0:06:26 > 0:06:32The narrow-mouthed toad has formed this kind of mature partnership.
0:06:32 > 0:06:35But with a spider.
0:06:35 > 0:06:42Tarantulas usually kill small toads, but these she welcomes with open arms.
0:06:48 > 0:06:51Her guests provide pest control.
0:06:51 > 0:06:55Spiders' egg sacs are often infested by insects.
0:06:58 > 0:07:02But the toads have the problem licked.
0:07:11 > 0:07:17In return for pest control, the tarantula protects her housemates.
0:07:19 > 0:07:22Screech owls also eat toads.
0:07:34 > 0:07:37It's payback time.
0:07:37 > 0:07:44The tarantula unleashes a secret weapon. A barrage of barbed hairs.
0:07:46 > 0:07:51These microscopic arrows aim for the eyes.
0:07:57 > 0:08:01Here the landlord doubles as a bodyguard.
0:08:01 > 0:08:04It's an idea that's caught on.
0:08:09 > 0:08:15The Bedu tribesmen of the Arabian desert like a varied diet.
0:08:16 > 0:08:21The thorny-tailed lizard is a popular local delicacy.
0:08:29 > 0:08:34Its burrow shelters the lizard from the sun, but it's far from secure.
0:08:44 > 0:08:48Caught alive, the lizard keeps fresh for days.
0:09:00 > 0:09:05The black scorpion is also on the hunt for a lizard's burrow.
0:09:09 > 0:09:13It likes to share in the cooling shade.
0:09:15 > 0:09:20They've struck a deal. The scorpion never stings its landlord
0:09:20 > 0:09:25and in turn, the lizard makes its venomous tenant feel at home.
0:09:25 > 0:09:29The scorpion pays rent by doubling as a security guard.
0:09:29 > 0:09:33It fends off foxes or other predators.
0:09:36 > 0:09:40But their pact makes lizard hunting a risky business.
0:09:40 > 0:09:43One with a sting in its tail.
0:09:47 > 0:09:50Aaagh!
0:09:53 > 0:09:56GROWLS
0:10:03 > 0:10:06SHOUTING IN ARABIC
0:10:11 > 0:10:15The pain lasts for days but it's rarely fatal.
0:10:19 > 0:10:24Both gain from their living arrangement. The lizard is protected from predators,
0:10:24 > 0:10:26the scorpion from the heat.
0:10:37 > 0:10:40A different protection racket operates in the ocean.
0:10:56 > 0:11:01Clown fish use sea anemones as fortified living quarters.
0:11:01 > 0:11:04But even they get stung when they first move in.
0:11:07 > 0:11:12The clown fish must first dance carefully amongst the tentacles.
0:11:12 > 0:11:18This covers the fish with the anemone's mucus and stops the stings from firing.
0:11:25 > 0:11:29Anemones are also used by some hermit crabs.
0:11:29 > 0:11:32First the anemone is dislodged.
0:11:34 > 0:11:38Then it is secured to the crab's shell.
0:11:38 > 0:11:44This living security system is even transferred across when the crab moves house.
0:11:47 > 0:11:50The boxing crab goes further.
0:11:50 > 0:11:53It uses anemones for poisonous fisticuffs.
0:11:55 > 0:12:01Modified pincers hold this fistful of stinging tentacles and keep predators at bay.
0:12:04 > 0:12:07Ocean partnerships take many forms.
0:12:07 > 0:12:14Here, personal hygiene is so important, some earn a living offering a wash and brush-up service.
0:12:14 > 0:12:20Like a barber's pole, these shrimps' red and white stripes advertise their trade.
0:12:20 > 0:12:23They wait for a customer like this yellow tag to call.
0:12:29 > 0:12:35The cleaner shrimps are conscientious, removing parasites and cleaning infected areas.
0:12:39 > 0:12:43They attend any large customer that visits.
0:12:56 > 0:13:02The cleaner wrasse wears a different striped uniform but provides a similar service.
0:13:06 > 0:13:11Their personal grooming attends the most intimate areas.
0:13:11 > 0:13:15But a relationship based on trust is easily abused.
0:13:15 > 0:13:22The fine-toothed blenny wears the cleaner wrasse's uniform but it's really a demon barber.
0:13:22 > 0:13:29It's after flesh. Its teeth are as sharp as a cut-throat razor.
0:13:31 > 0:13:37Partnerships need trust to survive. Fortunately most cleaners keep their side of the bargain.
0:13:42 > 0:13:47Some land animals even provide a bus service for their personal attendants.
0:13:56 > 0:13:59The deer mouse's passengers are rove beetles.
0:14:01 > 0:14:05Up to 20 of these hangers-on may be attached to the mouse at any one time.
0:14:14 > 0:14:20When the mouse returns to one of its many nests, the beetles disembark.
0:14:23 > 0:14:31The nests are infested by ticks. The rove beetle's job is to hunt them down.
0:14:31 > 0:14:36The healthiest mice are those with the most tick-killing passengers.
0:14:45 > 0:14:48The three-toed sloth carries even more hitch-hikers.
0:14:51 > 0:14:58The outer coat hairs are covered with algae, making the sloth the only green mammal in the world.
0:15:05 > 0:15:09Her baby has yet to acquire this jungle camouflage.
0:15:12 > 0:15:19Sharing the ride is a menagerie of up to 900 different species that eat the algae.
0:15:19 > 0:15:27Sloth moths are the most conspicuous passengers. They wait for a rare moment before they disembark.
0:15:27 > 0:15:33Once a week nature calls and the sloth leaves the tree.
0:15:33 > 0:15:38Strangely, it does its business at special latrines on the ground.
0:15:42 > 0:15:47The moths are patient commuters. The whole process takes over an hour.
0:15:47 > 0:15:50A seemingly pointless and dangerous journey.
0:16:00 > 0:16:06This is the moment the moths are waiting for. They only lay their eggs in fresh dung.
0:16:08 > 0:16:11They race to be first.
0:16:19 > 0:16:22The moths' caterpillars feed on the dung and pupate here.
0:16:31 > 0:16:36When the adult moths emerge, they find the sloths somewhere in the trees.
0:16:44 > 0:16:48A different passenger service is provided by hummingbirds.
0:16:48 > 0:16:56Their speciality is air charter. This rufous-tailed hummingbird feeds from heliconia flowers,
0:16:56 > 0:17:03which also provide food for flower mites. When a bird arrives, the mites race on board.
0:17:03 > 0:17:07Size for size, they're as quick as a sprinting cheetah.
0:17:13 > 0:17:17They rely on the hummer to take them to fresh blooms.
0:17:17 > 0:17:20The mites stow away in the birds' nostrils.
0:17:23 > 0:17:26Up to a dozen mites may cram into these flight compartments.
0:17:32 > 0:17:38Like human passengers, they only disembark when their transport stops at their preferred destination.
0:17:42 > 0:17:46Each type of mite alights at his own favourite flower.
0:17:49 > 0:17:55These mites do little harm but by blocking the nostrils they reduce flight efficiency.
0:17:55 > 0:17:59They also compete for pollen and nectar.
0:17:59 > 0:18:02They have started to act like parasites.
0:18:07 > 0:18:14Parasites come in many forms. One of the weirdest inhabits the remote reaches of the Amazon.
0:18:16 > 0:18:19Its grizzly story sometimes involves people.
0:18:25 > 0:18:29One of the Amazon's commonest fish is the catfish.
0:18:31 > 0:18:36It often plays host to another, far smaller fish.
0:18:36 > 0:18:40This is the candiru, a parasite of fish gills.
0:18:43 > 0:18:51For the catfish there is nowhere to hide. A trail of urea and ammonia streams from its gills.
0:18:51 > 0:18:56The candiru simply swims in circles until it picks up the scent.
0:19:04 > 0:19:07The signal becomes stronger as the fish approaches.
0:19:21 > 0:19:24It slips inside unnoticed.
0:19:27 > 0:19:33Spines on the candiru's head gaff the fish, securing it as it sucks blood from the gills.
0:19:39 > 0:19:45The worst that can happen to a parasite is to end up in the wrong host.
0:19:45 > 0:19:50The worst that can happen to a human is to become the wrong host to a candiru.
0:19:53 > 0:19:56Urinating in the water is unwise.
0:19:56 > 0:20:01The same cues of current and urea lure the candiru deep inside.
0:20:05 > 0:20:07Argh!
0:20:07 > 0:20:09Oh!
0:20:14 > 0:20:16In the southern states of America,
0:20:16 > 0:20:20an even stranger parasitic story recently emerged.
0:20:22 > 0:20:27Frogs began to appear with extra limbs and other deformities.
0:20:27 > 0:20:32These mysterious mutants were believed to be the result of chemical or radioactive pollution.
0:20:38 > 0:20:40The full story is stranger still.
0:20:43 > 0:20:46Herons that eat the frogs also acquire their parasites.
0:20:53 > 0:20:58The birds pass the eggs of parasitic flatworms in their droppings.
0:21:00 > 0:21:07Snails eat the eggs, which soon hatch and emerge as mini parasites.
0:21:07 > 0:21:09Each searches for the nearest tadpole.
0:21:12 > 0:21:15With torpedo precision, it tracks its target.
0:21:19 > 0:21:25It aims for the limb buds. As it burrows inside, cells explode.
0:21:28 > 0:21:35The fragmented limb buds try to regrow but in the process they create several legs instead of one.
0:21:37 > 0:21:41The parasite creates a mutant for one simple purpose.
0:21:43 > 0:21:45Deformed frogs make easy prey.
0:21:49 > 0:21:54If its gruesome life cycle is to continue, its host must be eaten by a heron.
0:21:56 > 0:21:59CROAKING
0:22:06 > 0:22:14We too harbour many parasites, but our relationship with one of them has changed over the last 3,000 years.
0:22:18 > 0:22:23The medicinal leech was used to treat a range of different ills
0:22:23 > 0:22:28until blood-letting fell from favour at the end of the 19th century.
0:22:28 > 0:22:32But recently, the leech has staged a medical comeback.
0:22:38 > 0:22:42Its blood-sucking skills can reduce the swellings known as haematomas,
0:22:42 > 0:22:45making it a valuable medical instrument.
0:22:49 > 0:22:53As it bites, an anaesthetic numbs the pain.
0:22:53 > 0:22:57The patient doesn't feel a thing as the leech sucks up the accumulated blood.
0:23:02 > 0:23:08The three jaws have 100 teeth each and act like circular saws.
0:23:14 > 0:23:19As the teeth cut, the leech releases anti-coagulants to disperse blood clots.
0:23:22 > 0:23:25It can balloon to ten times its original size.
0:23:30 > 0:23:32Leeches are highly adapted parasites.
0:23:34 > 0:23:38Even these aquatic leeches have no problem looping their way over land.
0:23:44 > 0:23:48They seek out prey by sensing the carbon dioxide of its breath.
0:23:58 > 0:24:03The leech shows that even the most gruesome creatures can become our partners.
0:24:09 > 0:24:15But like the leech, most of the animals we use, we also keep under our control.
0:24:19 > 0:24:21Or at least we think we do.
0:24:26 > 0:24:30But some of our partners still live a wild existence.
0:24:30 > 0:24:35One is said to have a similar relationship with this African animal.
0:24:42 > 0:24:46The honey badger is reputed to have the greater honey guide as its partner.
0:24:50 > 0:24:55The bird leads the honey badger to bees' nests by calling for it to follow.
0:24:59 > 0:25:05It relies on the mammal's strong claws to break into hives it has previously found.
0:25:05 > 0:25:09Oddly, this partnership has yet to be proven by science,
0:25:09 > 0:25:14but these people have not only witnessed it, they too are regularly guided by the bird.
0:25:16 > 0:25:22By flying to and fro and calling, it leads the honey gatherers to a bees' nest several kilometres away.
0:25:24 > 0:25:29Studies show that the bird knows every hive within 250 square kilometres.
0:25:36 > 0:25:43The honey gatherers rely on the bird for their livelihood and always leave some comb as a reward.
0:25:57 > 0:26:04As for the badger, its ancestors were probably in this relationship before people even appeared on the scene.
0:26:17 > 0:26:24In Laguna in South Brazil, people and wild animals cooperate in an almost magical partnership.
0:26:34 > 0:26:39The bottlenose dolphin's arrival is eagerly anticipated.
0:26:41 > 0:26:46The fishermen rely on the dolphins to herd fish to their nets.
0:26:49 > 0:26:52As they approach, the net is cast.
0:27:07 > 0:27:15Dolphins are highly successful predators but by cooperating with fishermen, the odds of both improve.
0:27:19 > 0:27:22Their system is elegantly simple.
0:27:22 > 0:27:28They drive the shoals towards the men and use a rolling dive to cue them to throw their nets.
0:27:34 > 0:27:41While the men gain a bounty, in the chaos, the shoal fragments and the dolphins pick off stragglers.
0:27:45 > 0:27:49This relationship has survived centuries.
0:27:49 > 0:27:55Both fishermen and dolphins pass on the rules of the game to their offspring.
0:27:57 > 0:27:59With so many different partnerships,
0:27:59 > 0:28:02humans are among the weirdest animals on the planet.
0:28:05 > 0:28:08Perhaps this is our most perfect alliance,
0:28:08 > 0:28:12for the animals involved live totally free.