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There was a time when myths and science were entwined,

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when mermaids and unicorns could mysteriously appear...

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..and devilfish flew the oceans.

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Nature was weird.

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When science revealed the truth behind these imaginary creatures,

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it found REAL animals lay behind the legends.

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Today, science still makes astonishing discoveries.

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But nature seems just as weird.

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

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In Weird Nature, real animal behaviour is set against human backdrops.

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They give astonishing insights.

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The spiral tusk of the unicorn has been traced to a white whale, just as wonderful as the legend.

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The narwhal uses its twisted horns in jousts, like a medieval knight.

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The mermaid's forked tail belongs to the dugong.

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Half-seen, its body and face can appear surprisingly human.

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The devilfish is a manta ray.

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They leap to shed parasites and then briefly glide, like phantom craft.

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Weird nature takes you into a world where imaginary human settings

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showcase animal behaviour that is absolutely real.

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What is weird is best defined when compared to our own lives.

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Our first weird journey shows the wonderful ways that animals move.

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Life began in the oceans.

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This imaginary wreck brings together some early forms of motion.

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Jellyfish are the only animal to move by wafting water through a pulsating skirt,

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but such graceful jet propulsion is no match for the tides.

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A strange ancient mollusc improved the concept.

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The nautilus jets water through a siphon that can move to control direction.

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It controls depth, by regulating gas in a series of buoyancy chambers in its shell.

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Flame scallops jet-propel by clapping their shells like castanets.

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They jet water from either side of their hinge.

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They dance to escape predators or to find new places.

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The Spanish dancer's technique is more elegant.

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It flamencos, by rippling the ribbons along its body.

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The frills of its costume are actually gills,

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and, despite its romantic name, it's really a sea slug.

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The sea horse can't move its armoured body in sinuous waves, like other swimming fish.

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Instead, a fin that quivers 20 times a second whirrs it around.

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Shimmering side fins act as steering propellers.

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Jointed legs were an exciting new movement in evolution.

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Spiny lobsters used them to conga.

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Their procession provides safety in numbers, as they move to deeper water to avoid winter storms.

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The slipstream from the leader cuts drag for those behind.

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Legs helped animals make the next evolutionary step.

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Today, we prefer faster options.

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But legs have been in vogue

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since the first amphibians crawled from the sea.

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They dragged their bodies along, using four legs, splayed to the side.

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The Mount Lyell salamander uses its tail as a fifth leg

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to negotiate the Sierra Nevada mountainside.

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They are also special in more human terms.

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The wheel was hailed as one of our greatest inventions.

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But, as these creatures prove,

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we were simply reinventing it!

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Rolling is the quickest way down a hill,

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as this salamander has discovered.

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It's designed to flex like a rubber tyre, so it doesn't feel the bumps.

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The pearl moth caterpillar is common,

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but its talent for doing "wheelies" is a new discovery.

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It rolls with its caterpillar track on the inside.

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Salamanders and caterpillars were born to rock'n'roll,

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so this behaviour is second nature.

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The salamander's more normal motion

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can cause problems for heavier creatures that move in the same way.

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For the early reptiles, crawling along the ground was literally a drag.

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Crocodiles had an odd solution.

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Walking on stilts cuts friction on rough ground.

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But the splayed feet have to rotate forward to take a step.

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It may be slow, but it saves energy,

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and most crocs use this stiff-legged walk whenever the going gets rough.

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The Australian fresh-water croc shows an even stranger gait,

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when provoked by predators, such as the salt-water croc.

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It gallops...

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but not like any other animal.

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Its front feet work together in opposite motion to the back.

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As the front feet hit the ground, the back swing forward.

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When the back push away, the front reach out.

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Galloping horses were once painted like this.

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In truth, only fresh-water crocs run in this seesaw way.

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It can reach 15mph and easily leap to safety.

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Evolution has taken many weird twists and turns.

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Each year, 20 million leapers of a more playful kind

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arrive in the world's toy shops, from a tiny part of Mexico.

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The jumping bean is really the seed of a desert shrub.

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Only a few show this odd leaping.

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The seeds, imported as novelty toys,

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seem to have a mind of their own.

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There really IS a mind behind the jumping.

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A moth caterpillar lives and feeds inside the seed.

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The caterpillars flip their home to escape.

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They not only feel the heat,

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light makes them jumpy.

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They fuss and fidget into the shade.

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This behaviour is a life-saver in the hot Mexican desert.

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The caterpillar repairs the damage by weaving a silken wall.

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It grips onto the silk wall when it leaps.

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The shape of the bean helps it roll.

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For weirdness, the Mexican jumping bean is hard to beat.

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But Sifaka lemurs from Madagascar

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turn leaping into a surreal ballet.

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The secret of their graceful pogo-ing stems from a life in the trees.

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They can leap 30m with ease.

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These acrobatic skills have to be modified for the challenge of moving over the ground.

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The legs act as springs...

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..and feet curled for gripping become landing pads.

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With no branches to grab, the arms are free to act as stabilisers.

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Every move of the legs is complemented by a matching move of the arms.

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The balletic poses maintain perfect balance.

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Sifakas only show such bizarre and beautiful motion because they evolved for a life in the trees.

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The same is true of the bushbaby.

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This time-slice sequence

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shows the various poses a bushbaby adopts as it jumps.

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It leaps two-and-a-quarter metres in height -

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equivalent to us clearing two stacked double-decker buses.

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To achieve this, the legs and feet act like springs,

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converting the energy of impact back into propulsion.

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It really DOES have a spring in its step.

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It waits until the peak of its jump before reaching out.

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The tail acts as a counterbalance.

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It's like a rubber ball with a brain -

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almost impossible for a predator to catch.

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This sequence freezes a bushbaby as it becomes airborne.

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But gliding animals specialise in prolonging their time in the air.

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The golden tree frog of Malaysia is a treetop acrobat.

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Usually, it hops just a few metres.

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But if it meets a golden tree snake,

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it happily makes a leap into the unknown.

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As it plummets, spread limbs slow its descent, and its webbed feet double up as a parachute.

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The Javan flying frog goes one better.

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Its webbed feet have evolved into miniature wings.

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Instead of parachuting, it paraglides at an angle.

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But it's the Wallace frog that achieves aeronautical perfection.

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Its huge webbed feet become aerofoils that slow and control its descent.

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It glides as far forward as the distance it falls.

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As well as winged feet, its whole body is aerodynamically shaped.

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This is classic evolution.

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One feature progressively improved until perfection is achieved.

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Lizards lack webbed feet,

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so they expand other body features to get their wings.

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The flying gecko's impressive glide angle is due to wing-like fringes on its body.

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Every available edge has an aerodynamic extension.

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But it's the draco lizard whose design really flies.

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A huge aerofoil turns the draco into a living Frisbee,

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while its tail steers like a rudder.

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Foldable ribs act as support struts to create the perfect wing.

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The golden tree snake uses other aeronautical tricks for its leap of faith.

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It loops its body for the ultimate take-off,

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and projects forward to gain a head start.

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It then flattens into a ribbon and swims through the air using S-shaped waves of its body.

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The star of this jungle air show is the owner of the most extravagant wings.

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From 80m up, the Wallace frog glides 80m forward.

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The gecko soars 100m.

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But nothing beats the flying Frisbee.

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The draco reaches a full 200m.

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Even a flying snake makes 150, complete with controlled landing!

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A lack of legs has made snakes devise other weird ways of moving.

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SIREN WAILS

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In the deserts of the American South-West, off-road travelling is notoriously difficult.

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Shifting sand is one of the most challenging surfaces to negotiate.

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The horned rattlesnake's solution is to touch the sand with as little of its body as possible.

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Appropriately, its nickname is the sidewinder.

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Like tyre tread, a snake's scales usually grip the ground as it moves.

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But sand simply gives way.

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So the snake makes an S-shape with its body and lifts the loops in a rolling corkscrew.

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This continually shifts the points of contact to stop that sinking feeling.

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As waves of grip pass down the body, the snake rapidly picks up speed.

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Its hidden prey can also make a surprise move.

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Sidewinders avoid being buried by sand.

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Others welcome its smothering embrace.

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This predator spends much of its life lurking just beneath the surface.

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It can swim through shifting sand, like an eel up a river...

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..because the grains act like liquid rock.

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The sand swimmer's wedge-shaped head parts the grains like the prow of a boat.

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Its over-slung upper jaw stops it swallowing sand,

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and it can close its nostrils to avoid suffocating.

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As the snake submerges, its polished scales slip through the grains as if immersing in water.

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Nature shows many wonderful forms of travelling.

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Those that seem weird are usually the least familiar.

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But although we think we're normal, in nature, it is WE who are peculiar.

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Our two legs place us among the world's oddest animals.

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We are the only mammal to regularly walk like this.

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Sometimes, other primates hint at the origins of our strange bipedal walk.

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Proboscis monkeys use it to cross mangroves.

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A few scientists think we were once semi-aquatic.

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Walking like this kept our heads above water.

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Others believe that freeing the hands for gripping or carrying

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allowed a land ape to bring back food to a family.

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Whatever the reason, animals that walk like us seem strange.

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We find it weird to see our actions mirrored in other animals.

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In the age of the dinosaurs, we might have felt more at home. Many were also bipedal.

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Today, a few unrelated lizards continue in their footsteps.

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Oww!

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Like a miniature tyrannosaur, the collared lizard hunts on two legs.

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But it is more agile than any dinosaur,

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and, size for size, much faster.

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Being smaller gives speed and manoeuvrability.

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Most bipedal lizards live in deserts.

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This kind of running needs space.

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When hunting other lizards, two legs give it the edge.

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The tyrannosaur's modern equivalent.

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But while this lizard uses two legs for speed, we now prefer alternatives.

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The dinosaur's REAL descendants are birds.

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Because they fly, we rarely notice that they walk on two legs like we do.

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But the roadrunner is an exception.

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Its comical, human-like gait made it a cartoon character.

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To save energy for its sprint, the roadrunner exposes a solar panel.

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This dark skin patch raises its body temperature seven degrees.

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It's primed for a super-heated performance.

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To the roadrunner, our open roads are racetracks, good for chasing insects and lizards.

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Its tiny legs take it to 26mph.

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If our legs moved this fast, we would overtake any speeding bike.

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Because two-legged running limits our own speed,

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we use artificial means of travel, such as flying.

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But even though it CAN fly, the strange roadrunner chooses to run on two legs for speed.

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Our machines take us ever-faster...

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and we sometimes pay the price.

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But when we smile at the roadrunner's bipedal running,

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are we recognising our OWN weird nature?

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Our next weird journey looks at

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the strange ways animals defend themselves.