0:00:04 > 0:00:06Our human senses are incredible.
0:00:09 > 0:00:10We have excellent vision...
0:00:13 > 0:00:14..precise hearing...
0:00:18 > 0:00:22..and can detect the slightest fragrance drifting on the breeze.
0:00:25 > 0:00:28But we only experience a tiny fraction of what's out there.
0:00:30 > 0:00:33Imagine a world where you could see with sound...
0:00:35 > 0:00:37These images are just phenomenal.
0:00:40 > 0:00:42ELEPHANT RUMBLES
0:00:43 > 0:00:46..hear thunderstorms from hundreds of miles away.
0:00:47 > 0:00:49That's incredible. They've all stopped.
0:00:51 > 0:00:55Imagine seeing the world in slow motion...
0:00:55 > 0:00:58or through some of the sharpest eyes in nature.
0:00:58 > 0:00:59HE GASPS
0:00:59 > 0:01:01So fast!
0:01:02 > 0:01:05Travelling to some of the wildest places on Earth...
0:01:06 > 0:01:10..we reveal the strange and wonderful world of animal senses.
0:01:10 > 0:01:13Light is emitted. Look at that!
0:01:14 > 0:01:16Another one!
0:01:16 > 0:01:18- SHE LAUGHS - This is brilliant.
0:01:18 > 0:01:20I'm Dr Helen Czerski.
0:01:20 > 0:01:22I'm a physicist,
0:01:22 > 0:01:25and I want to find out how animals tap into an amazing range
0:01:25 > 0:01:28of light, scent and sound.
0:01:31 > 0:01:33I'm Patrick Aryee,
0:01:33 > 0:01:36and as a biologist I'm fascinated by what the world appears like
0:01:36 > 0:01:39through animal senses far superior to our own.
0:01:44 > 0:01:47In this episode, we are on a journey through a world of sight.
0:01:51 > 0:01:54We'll discover the bizarre adaptations
0:01:54 > 0:01:58and ingenious ways animals use sight to survive.
0:02:01 > 0:02:03Experience the world through animal senses.
0:02:23 > 0:02:25The African grassland.
0:02:27 > 0:02:31Home to a hunter with some of the most incredible eyes on Earth...
0:02:34 > 0:02:35the cheetah.
0:02:37 > 0:02:40This female has been reared by humans,
0:02:40 > 0:02:43so I can get an unusually close-up view.
0:02:45 > 0:02:47Looking into the eyes of this cheetah,
0:02:47 > 0:02:51it's easy to imagine that they view the world like we do -
0:02:51 > 0:02:54that it sees me like I see myself in the mirror.
0:02:54 > 0:02:57But it's actually a completely different story.
0:03:00 > 0:03:02Its eyes look similar to ours...
0:03:04 > 0:03:07..but appearances can be deceptive.
0:03:17 > 0:03:19As we walk through this landscape,
0:03:19 > 0:03:22it's seeing the world very differently to me.
0:03:25 > 0:03:30Surprisingly, our eyes are sharper than the cheetah's.
0:03:31 > 0:03:33But whereas our most precise vision
0:03:33 > 0:03:36is in the centre of our field of view...
0:03:38 > 0:03:42..the cheetah sees most clearly across a long narrow band.
0:03:49 > 0:03:53This bizarre view of the world might seem odd to us,
0:03:53 > 0:03:56but it's critical for the cheetah's survival.
0:03:58 > 0:04:03It allows them to spot prey anywhere on the wide open savanna...
0:04:05 > 0:04:08..making them a formidable ambush predator.
0:04:56 > 0:05:01The way any creature sees is finely adapted to where and how it lives.
0:05:04 > 0:05:07The cheetah's view is just the tip of the iceberg.
0:05:08 > 0:05:12Some animals have a far stranger view of the world.
0:05:15 > 0:05:19Despite the fact that we have much better visual acuity than cheetahs,
0:05:19 > 0:05:22our eyes aren't better than theirs.
0:05:22 > 0:05:24And even though they are brilliant
0:05:24 > 0:05:28and can spot the slightest of movements on the horizon,
0:05:28 > 0:05:30their eyes aren't better than ours.
0:05:30 > 0:05:33We've just got different views of the world
0:05:33 > 0:05:35that are both ideally suited to our needs.
0:05:40 > 0:05:44Every tiny detail of every creature on Earth
0:05:44 > 0:05:48is adapted to give them an edge in the challenge of staying alive.
0:05:48 > 0:05:51CHEETAH PURRS
0:05:51 > 0:05:54This is the beauty of evolution -
0:05:54 > 0:05:58and through the eye, we can tell this most remarkable story.
0:06:01 > 0:06:03You may think you know what the world looks like,
0:06:03 > 0:06:05but prepare to think again.
0:06:12 > 0:06:15Animals have many weird and wonderful ways
0:06:15 > 0:06:17of seeing the world around them...
0:06:23 > 0:06:26..and it's all to do with how they detect light.
0:06:28 > 0:06:30It's easy to think we see the whole picture...
0:06:32 > 0:06:35..but there's much more to light than meets the eye.
0:06:37 > 0:06:39We get a huge amount of information about our world
0:06:39 > 0:06:41in the form of visible light -
0:06:41 > 0:06:44but that doesn't mean that's all the light there is.
0:06:44 > 0:06:47In fact, we can only see a tiny fraction of what's out there.
0:06:49 > 0:06:53I can reveal what our eyes can and, more importantly, can't see
0:06:53 > 0:06:54using a prism.
0:06:55 > 0:06:57So, the sun's up there
0:06:57 > 0:07:01and it's shining white light down onto the prism,
0:07:01 > 0:07:02and if I get the angle right...
0:07:02 > 0:07:05you can see there's a rainbow on this rock here.
0:07:09 > 0:07:11Sunlight appears white,
0:07:11 > 0:07:15but it's actually made up of a spectrum of many colours of light,
0:07:15 > 0:07:17and the prism splits them up.
0:07:18 > 0:07:21And what's happening is that as the white light comes in
0:07:21 > 0:07:23and it passes through the glass, it gets bent,
0:07:23 > 0:07:25but the different colours get bent by different amounts -
0:07:25 > 0:07:28and we can see all the colours of the rainbow lined up here.
0:07:31 > 0:07:35This is the entire spectrum of light that's visible to our eyes,
0:07:35 > 0:07:39and animals have evolved amazing ways of using every bit of it.
0:07:42 > 0:07:46But some creatures can see more,
0:07:46 > 0:07:49because there are parts of the spectrum we can't see.
0:07:51 > 0:07:54Beyond blue is the hidden world of ultraviolet,
0:07:54 > 0:07:59and beyond red is the invisible world of infrared.
0:08:01 > 0:08:04And so what we can see is a tiny, tiny part
0:08:04 > 0:08:07of this enormous spectrum of light.
0:08:07 > 0:08:11And just because we can't see it doesn't mean it isn't there.
0:08:16 > 0:08:20On our journey through sight, we're going to reveal the amazing ways
0:08:20 > 0:08:22animals use every colour of the rainbow
0:08:22 > 0:08:25right across this colourful spectrum...
0:08:25 > 0:08:26and beyond.
0:08:29 > 0:08:31We start with ultraviolet
0:08:31 > 0:08:34and work our way across the spectrum to infrared
0:08:34 > 0:08:38to show nature's most bizarre and extraordinary ways
0:08:38 > 0:08:40of seeing the world.
0:08:49 > 0:08:52We begin in one of the most remote places on Earth.
0:08:56 > 0:08:59A wilderness where one unlikely animal
0:08:59 > 0:09:03has tapped into ultraviolet to get an edge over its archrival.
0:09:05 > 0:09:07Alaska.
0:09:09 > 0:09:14This is an icy world, where one creature rules supreme.
0:09:17 > 0:09:18The wolf.
0:09:25 > 0:09:29This hunter patrols much of North America's snowy landscape.
0:09:32 > 0:09:37Their camouflaged fur allows them to blend into the snowy background -
0:09:37 > 0:09:41for a hunter, disguise is essential.
0:10:09 > 0:10:12The wolf is a formidable predator,
0:10:12 > 0:10:16and in Alaska, it pursues one animal relentlessly.
0:10:19 > 0:10:23Caribou - or reindeer, as they're known in Europe.
0:10:25 > 0:10:28These gentle giants seem vulnerable to the wolf pack...
0:10:32 > 0:10:36..but the caribou have a secret that helps them to see wolves coming.
0:10:38 > 0:10:42And it's all to do with ultraviolet light that's invisible to us.
0:10:44 > 0:10:46When we think of ultraviolet light, or UV,
0:10:46 > 0:10:48we normally think about the potential it has
0:10:48 > 0:10:50to damage ourselves.
0:10:50 > 0:10:52It's what causes our skin to tan or to burn,
0:10:52 > 0:10:55and it's what sunscreen protects us against.
0:10:57 > 0:11:00We can't see UV, because our eyes have a filter
0:11:00 > 0:11:04that blocks these harmful rays, so we're blind to them.
0:11:07 > 0:11:10But caribou don't have this filter,
0:11:10 > 0:11:12so they are one of the only mammals on Earth
0:11:12 > 0:11:15that can see this ultraviolet light.
0:11:16 > 0:11:19And in this frozen landscape,
0:11:19 > 0:11:22UV vision is a particularly powerful weapon.
0:11:23 > 0:11:26Snow reflects ultraviolet light,
0:11:26 > 0:11:31so, although we can't see it, this is a world awash with UV.
0:11:32 > 0:11:35More importantly for the caribou,
0:11:35 > 0:11:38anything that doesn't reflect UV - like wolf fur -
0:11:38 > 0:11:42stands out against the bright UV background.
0:11:42 > 0:11:46Even if it looks perfectly camouflaged to our eyes.
0:11:49 > 0:11:51To show you how effective
0:11:51 > 0:11:54this hidden world of UV is for the caribou,
0:11:54 > 0:11:56I need to get close to a wolf.
0:12:04 > 0:12:06This is a habituated wolf.
0:12:06 > 0:12:09You can see that she's pretty well camouflaged against her background.
0:12:09 > 0:12:12If you were looking from a long way away against a big landscape
0:12:12 > 0:12:15you'd find this wolf pretty hard to spot.
0:12:18 > 0:12:21But that's not how the caribou see the world.
0:12:21 > 0:12:22And to get an idea of what they see,
0:12:22 > 0:12:26I'm going to take a picture using this UV camera.
0:12:26 > 0:12:29This specially modified camera can see the UV light
0:12:29 > 0:12:31that's invisible to us.
0:12:35 > 0:12:38Objects that reflect UV appear white,
0:12:38 > 0:12:41and things that absorb UV appear black.
0:12:45 > 0:12:47The pictures are really clear -
0:12:47 > 0:12:51the snow is bright, bright white because the UV is reflecting off,
0:12:51 > 0:12:54there's loads of UV around in the snowy background.
0:12:54 > 0:12:56And then when the wolf comes into shot
0:12:56 > 0:12:58it stands out, because it's so dark.
0:13:02 > 0:13:06And that's because the wolf's fur absorbs UV light.
0:13:08 > 0:13:10The image is particularly striking
0:13:10 > 0:13:12because of the huge amounts of UV
0:13:12 > 0:13:16being reflected off the snowy landscape behind.
0:13:19 > 0:13:21And so, for a caribou,
0:13:21 > 0:13:24being able to see in the ultraviolet is really useful.
0:13:24 > 0:13:27You'd see white snow and a very, very obvious wolf.
0:13:32 > 0:13:34By tapping into the ultraviolet light
0:13:34 > 0:13:37at this extreme end of the spectrum,
0:13:37 > 0:13:41caribou have stripped their archpredator of its camouflage.
0:13:46 > 0:13:50What's fascinating is that we have adaptations to protect us
0:13:50 > 0:13:54from UV light - but the caribou have evolved to use it.
0:13:54 > 0:13:57Out here in this snowy white world,
0:13:57 > 0:14:00it could be the difference between life and death.
0:14:04 > 0:14:05To survive out here,
0:14:05 > 0:14:10the caribou have pushed vision far further than ours,
0:14:10 > 0:14:14seeing light beyond the edge of the visible light spectrum.
0:14:26 > 0:14:30But as we move across the spectrum and away from ultraviolet,
0:14:30 > 0:14:34we enter the world of light our eyes can see.
0:14:34 > 0:14:36FLUTTERING
0:14:38 > 0:14:42I've come to Cuba to find one amazing little predator
0:14:42 > 0:14:47that specialises in hunting on the blue edge of the colour spectrum.
0:14:51 > 0:14:53Their bizarre, alien-like eyes
0:14:53 > 0:14:57have become highly tuned to blue light
0:14:57 > 0:15:01and can perform one of nature's most astonishing tricks...
0:15:04 > 0:15:05..warping time.
0:15:08 > 0:15:10Dragonflies -
0:15:10 > 0:15:11master predators.
0:15:13 > 0:15:15They have an incredible hunting success rate,
0:15:15 > 0:15:18capturing prey 95% of the time.
0:15:18 > 0:15:22When you compare that with apex predators like Great White sharks -
0:15:22 > 0:15:24who are successful 50% of the time -
0:15:24 > 0:15:29or lions - only 40% - then dragonflies really are outstanding.
0:15:31 > 0:15:34They hunt tiny insects in midair
0:15:34 > 0:15:38by spotting their silhouettes against the bright sky.
0:15:41 > 0:15:43To help them,
0:15:43 > 0:15:47their eyes have become especially sensitive to blue light,
0:15:47 > 0:15:51which makes the sky appear dazzlingly bright.
0:15:58 > 0:16:01So much so, even the tiniest little insect...
0:16:02 > 0:16:04..casts a dark silhouette.
0:16:08 > 0:16:10Their unusual colour vision
0:16:10 > 0:16:14gives them a huge advantage spotting their prey -
0:16:14 > 0:16:16but to actually catch it,
0:16:16 > 0:16:19they need another, even more extraordinary, visual trick.
0:16:29 > 0:16:31Just by looking at them,
0:16:31 > 0:16:34you can see how vision dominates their world.
0:16:34 > 0:16:38They have the largest and possibly the best eyes of all insects,
0:16:38 > 0:16:41which takes up almost their entire head.
0:16:41 > 0:16:43And their bizarre-looking eyes
0:16:43 > 0:16:47give dragonflies almost unbelievable visual powers.
0:16:47 > 0:16:53Because these nippy little predators can see the world in slow motion.
0:16:55 > 0:16:59To show you how spectacular their time-warping vision is,
0:16:59 > 0:17:03I'm going to try to recreate one of their hunts using a high-speed lure.
0:17:13 > 0:17:16Dragonflies live in a world where every millisecond counts.
0:17:20 > 0:17:24Most of the airborne insects they eat move incredibly fast.
0:17:25 > 0:17:27If they're to stand any chance of catching one,
0:17:27 > 0:17:30they have to see it,
0:17:30 > 0:17:32anticipate what it's going to do,
0:17:32 > 0:17:34and react.
0:17:35 > 0:17:37All in a fraction of a second.
0:17:40 > 0:17:42So I'm going to test their eyes
0:17:42 > 0:17:46using a dragonfly that hunts by ambushing its prey.
0:17:48 > 0:17:51Just in front of me is a perching dragonfly.
0:17:52 > 0:17:56When they see a fly, they take off, catch it in midair,
0:17:56 > 0:17:58and return back to their perch.
0:18:00 > 0:18:04The whole hunt takes place in the blink of a human eye.
0:18:05 > 0:18:07THUNK
0:18:07 > 0:18:09This peashooter might be low-tech,
0:18:09 > 0:18:12but it's the perfect tool to recreate a high-speed target
0:18:12 > 0:18:14for our dragonfly.
0:18:14 > 0:18:16It fires a seed so quickly
0:18:16 > 0:18:18I can't possibly see it.
0:18:20 > 0:18:23But is the dragonfly's vision quick enough to spot it?
0:18:33 > 0:18:37We're going to have a look back at our slow-motion clip.
0:18:37 > 0:18:40Will the dragonfly detect the tiny pea?
0:18:40 > 0:18:42Our dragonfly is completely still.
0:18:42 > 0:18:47And the head definitely turns before we see the seed come into frame.
0:18:47 > 0:18:49And then the dragonfly almost takes off -
0:18:49 > 0:18:51but it has enough time to assess that it's not a fly,
0:18:51 > 0:18:54and it changes its mind and stays on its perch.
0:18:58 > 0:19:00That's incredible.
0:19:00 > 0:19:01The head definitely moves first.
0:19:04 > 0:19:08The dragonfly's vision is so quick it can track the flying object -
0:19:08 > 0:19:10and work out it's not prey -
0:19:10 > 0:19:12all in less than five hundredths of a second.
0:19:15 > 0:19:18It's partly due to the speed at which they process information.
0:19:25 > 0:19:29Dragonflies experience time in a completely different way to us.
0:19:29 > 0:19:33They have a reaction time of about 30 milliseconds.
0:19:33 > 0:19:35The route from their eyes to their brain
0:19:35 > 0:19:38and then back to their muscles is much shorter.
0:19:38 > 0:19:41The whole process of seeing and catching a fly
0:19:41 > 0:19:44can happen in just over 100 milliseconds.
0:19:47 > 0:19:49This is about the same time
0:19:49 > 0:19:52it takes us to simply react.
0:20:01 > 0:20:04But there's something even more astonishing
0:20:04 > 0:20:05about the dragonfly's vision.
0:20:07 > 0:20:08To demonstrate,
0:20:08 > 0:20:09I've got a flicker book.
0:20:12 > 0:20:14If I flick this book fast enough...
0:20:14 > 0:20:16the images begin to animate.
0:20:16 > 0:20:18And that's because the pages are moving so fast,
0:20:18 > 0:20:20we hardly notice them turning.
0:20:20 > 0:20:23It's essentially an optical illusion.
0:20:24 > 0:20:26Although our vision appears seamless,
0:20:26 > 0:20:27our eyes actually work
0:20:27 > 0:20:30by capturing up to 60 images a second,
0:20:30 > 0:20:32which our brain then combines
0:20:32 > 0:20:36to create the illusion of a continuous moving image.
0:20:36 > 0:20:38And because the pages are turning faster than that,
0:20:38 > 0:20:41it brings the animation to life.
0:20:42 > 0:20:46To a dragonfly, however, this would look completely different.
0:20:47 > 0:20:50The images would appear slowed down,
0:20:50 > 0:20:53and it would see each individual page turning.
0:20:53 > 0:20:57And that's because dragonflies see faster than we do.
0:20:58 > 0:21:01Whereas we see 60 images per second,
0:21:01 > 0:21:03they see around 200.
0:21:04 > 0:21:06And so they can observe things
0:21:06 > 0:21:08that are just too fast for us to even process.
0:21:10 > 0:21:12In real time, it's impossible for us
0:21:12 > 0:21:15to see exactly what this dragonfly's doing.
0:21:19 > 0:21:24But using our high-speed camera that slows down the action 80 times,
0:21:24 > 0:21:27we're able to reveal the astonishing accuracy
0:21:27 > 0:21:29of a dragonfly's vision...
0:21:30 > 0:21:33..as it catches a tiny midge in midair.
0:21:37 > 0:21:41Dragonflies have been around for 300 million years -
0:21:41 > 0:21:43since before the dinosaurs.
0:21:47 > 0:21:50In this time, they've finely tuned their eyes
0:21:50 > 0:21:52to see their world in slow motion.
0:21:54 > 0:21:58Dragonflies clearly have the edge when it comes to high-speed vision,
0:21:58 > 0:22:02which explains why they're so difficult for us to catch.
0:22:02 > 0:22:05With their time-warping eyesight, they can definitely see us coming.
0:22:05 > 0:22:07And it's this visual adaptation
0:22:07 > 0:22:10that has enabled these spellbinding insects
0:22:10 > 0:22:11to become masters of their world.
0:22:20 > 0:22:23In the battle to spot their prey,
0:22:23 > 0:22:27these little predators have evolved astonishing eyes.
0:22:27 > 0:22:32But the dragonfly's super slow-mo vision comes at a cost.
0:22:33 > 0:22:37For peak performance, they need bright, blue skies.
0:22:43 > 0:22:46On our journey across the light spectrum,
0:22:46 > 0:22:49we show the many unique ways that animals see.
0:22:51 > 0:22:55Some see colours beyond our vision -
0:22:55 > 0:22:58others see in slow motion.
0:23:01 > 0:23:04But they all rely on one thing -
0:23:04 > 0:23:06light from the sun.
0:23:08 > 0:23:13Darkness poses the single biggest challenge for sight,
0:23:13 > 0:23:16so nocturnal animals have evolved special adaptations
0:23:16 > 0:23:18to allow them to see.
0:23:22 > 0:23:26Many have huge eyes - because, often,
0:23:26 > 0:23:30the bigger the eye, the better it can detect tiny traces of light.
0:23:35 > 0:23:39But some animals have come up with an even more unusual way
0:23:39 > 0:23:42of dealing with darkness -
0:23:42 > 0:23:44by illuminating the world around them.
0:23:47 > 0:23:49I'm in Puerto Rico,
0:23:49 > 0:23:51because it's the perfect place
0:23:51 > 0:23:54to show you one of nature's most dazzling displays.
0:23:56 > 0:23:59All around me now there's something incredible.
0:23:59 > 0:24:01I can see it,
0:24:01 > 0:24:04but for you to see it too, we'll have to switch to a special camera.
0:24:06 > 0:24:07Rather like big eyes,
0:24:07 > 0:24:11our camera is very sensitive in low light conditions.
0:24:14 > 0:24:17The water around me might appear pitch black -
0:24:17 > 0:24:20but looks can be deceptive.
0:24:24 > 0:24:25THUNDER ROLLS
0:24:34 > 0:24:37This is bioluminescence.
0:24:40 > 0:24:43It's found out in the deep ocean and also in coastal waters,
0:24:43 > 0:24:47and this is one of the best places in the world to see it.
0:24:48 > 0:24:50And it's magical! Look at this!
0:24:50 > 0:24:53I feel like I'm a witch casting spells.
0:24:54 > 0:24:57This is all my childhood dreams come true.
0:24:57 > 0:24:59And what's happening is,
0:24:59 > 0:25:02down here there are thousands of little organisms
0:25:02 > 0:25:05that each give out a little bit of light when they're disturbed.
0:25:05 > 0:25:09And all together, they make these amazing plumes.
0:25:15 > 0:25:17This is living light.
0:25:22 > 0:25:25These tiny organisms are one of nature's wonders -
0:25:25 > 0:25:29but what is the secret to making light?
0:25:33 > 0:25:38It comes down to some simple chemistry with beautiful results.
0:25:41 > 0:25:43What they need is a way of storing energy
0:25:43 > 0:25:47that they can convert into light just at the moment they need it.
0:25:47 > 0:25:49So they produce a chemical called luciferin,
0:25:49 > 0:25:51and also an enzyme called luciferase,
0:25:51 > 0:25:54And when you mix those two things together...
0:25:59 > 0:26:02..light is emitted. Look at that!
0:26:03 > 0:26:04Wow.
0:26:07 > 0:26:09This is a fascinating adaptation,
0:26:09 > 0:26:14because it allows animals to generate light in complete darkness.
0:26:16 > 0:26:21What's even cleverer is that most ocean bioluminescence is blue,
0:26:21 > 0:26:24because blue light travels further through water
0:26:24 > 0:26:27than any other colour in the visible light spectrum.
0:26:29 > 0:26:32So if you want to light up your underwater world -
0:26:32 > 0:26:34blue is the colour to go for.
0:26:38 > 0:26:40But what's in it for these little organisms?
0:26:40 > 0:26:45Why go to all the effort of creating this strange light?
0:26:48 > 0:26:51I'm hoping to demonstrate using these cardinal fish
0:26:51 > 0:26:54and some tiny bioluminescent creatures called ostracods.
0:26:56 > 0:26:58These fish feed on plankton,
0:26:58 > 0:27:01so ostracods are just the sort of thing that they might normally eat.
0:27:01 > 0:27:04I'm going to put some ostracods in the tank,
0:27:04 > 0:27:06and that might sound a little bit mean,
0:27:06 > 0:27:08but have a look at what happens.
0:27:21 > 0:27:22SHE LAUGHS
0:27:23 > 0:27:25It's like a little fish firework!
0:27:30 > 0:27:31Oh, look at that!
0:27:41 > 0:27:42Oh there we go, look!
0:27:43 > 0:27:45So what's happening here
0:27:45 > 0:27:49is that the cardinal fish ate an ostracod,
0:27:49 > 0:27:54and the ostracod immediately put out an intense burst of light.
0:27:54 > 0:27:57And that made the cardinal fish spit it out.
0:27:59 > 0:28:04And the reason for that is that cardinal fish also has predators,
0:28:04 > 0:28:06and if they can see it, they'll come and eat it.
0:28:06 > 0:28:09So the cardinal fish don't want to be seen.
0:28:09 > 0:28:12So the ostracod lets out a burst of light, the fish spits it out,
0:28:12 > 0:28:14and then both go their separate ways.
0:28:18 > 0:28:19Oh, there we go!
0:28:21 > 0:28:22SHE LAUGHS
0:28:22 > 0:28:25Oh, that time I saw the ostracod swim away!
0:28:31 > 0:28:32Oh, another one!
0:28:32 > 0:28:34- SHE LAUGHS - This is brilliant.
0:28:36 > 0:28:39I never thought that watching fish vomit could be this much fun!
0:28:39 > 0:28:40SHE LAUGHS
0:28:40 > 0:28:43You can see it coming out through its gills!
0:28:43 > 0:28:44SHE LAUGHS
0:28:50 > 0:28:53These bioluminescent creatures illuminate when threatened
0:28:53 > 0:28:57because their dazzling glare acts like a security light
0:28:57 > 0:28:58deterring predators.
0:29:08 > 0:29:11Most ostracods live near the surface of the ocean,
0:29:11 > 0:29:15but the vast majority of bioluminescent animals
0:29:15 > 0:29:17live far deeper down.
0:29:23 > 0:29:27As you descend further into the dark depths of the abyss,
0:29:27 > 0:29:31the world of bioluminescence gets far weirder.
0:29:35 > 0:29:40Down here, this is the only light there is.
0:29:44 > 0:29:46And in the deep ocean abyss,
0:29:46 > 0:29:50one of the planet's most mysterious giants
0:29:50 > 0:29:53uses this alien light to survive.
0:29:57 > 0:30:02To do this it has supersized its eye to a grotesque dimension,
0:30:02 > 0:30:05far beyond any other animal on Earth.
0:30:08 > 0:30:11Just think about all the creatures on planet Earth
0:30:11 > 0:30:12that need to be able to see.
0:30:12 > 0:30:15They go right the way from tiny insects,
0:30:15 > 0:30:18all the way up to gigantic blue whales.
0:30:18 > 0:30:20And if you look at the eyes themselves,
0:30:20 > 0:30:23they start with tiny, microscopic, light-sensitive patches,
0:30:23 > 0:30:26and get bigger and bigger and bigger,
0:30:26 > 0:30:28until you get to about nine or ten centimetres -
0:30:28 > 0:30:31and that's about the size of an orange.
0:30:31 > 0:30:33And there they stop.
0:30:33 > 0:30:37So all the eyes on planet Earth are this size or smaller.
0:30:40 > 0:30:43But there is one massive exception -
0:30:43 > 0:30:47an animal whose eye has grown to the size of a human head.
0:30:53 > 0:30:56The mysterious giant squid.
0:30:58 > 0:31:02These images, filmed over 600 metres below the ocean surface,
0:31:02 > 0:31:07show it in its natural habitat for the first time ever.
0:31:08 > 0:31:10They are true giants,
0:31:10 > 0:31:13measuring over 14 metres in length -
0:31:13 > 0:31:16and even relative to their colossal body size,
0:31:16 > 0:31:20their eyes are still enormous.
0:31:26 > 0:31:29But what's in it for the squid?
0:31:29 > 0:31:34Why is seeing this bioluminescent light so vital to its survival?
0:31:37 > 0:31:41As I row across this bay, the water offers a clue.
0:31:42 > 0:31:44As my oars move through the water,
0:31:44 > 0:31:46they're disturbing these organisms,
0:31:46 > 0:31:49and they feel threatened so they light up,
0:31:49 > 0:31:51producing this wonderful glow.
0:31:52 > 0:31:55And that brings us back to the giant squid eye.
0:31:55 > 0:31:57The biggest eye in the world.
0:31:57 > 0:32:00Because what those eyes are really good at
0:32:00 > 0:32:05is detecting the faint glow of bioluminescence in the deep ocean.
0:32:14 > 0:32:17The squid needs gigantic eyes
0:32:17 > 0:32:21because a fearsome predator is searching for it in the darkness.
0:32:25 > 0:32:29A giant that descends into the pitch-black abyss
0:32:29 > 0:32:31to hunt it down -
0:32:31 > 0:32:33the sperm whale.
0:32:40 > 0:32:43Now, sperm whales also don't produce bioluminescence,
0:32:43 > 0:32:46but as such a big animal moves through the water,
0:32:46 > 0:32:49it'll disturb little organisms like this,
0:32:49 > 0:32:52and so it'll be surrounded by a faint ghostly glow -
0:32:52 > 0:32:55and that is what the giant squid eyes are looking out for.
0:33:04 > 0:33:06The giant squid's huge eyes
0:33:06 > 0:33:09allows it to peer further through the gloom
0:33:09 > 0:33:13and detect these traces of blue bioluminescent light
0:33:13 > 0:33:15as the sperm whale approaches.
0:33:16 > 0:33:19It's this spectacular eye
0:33:19 > 0:33:23that has allowed them to survive in this inky black world.
0:33:28 > 0:33:31And so that is why the largest eye on the planet
0:33:31 > 0:33:34has evolved in the darkest place on Earth.
0:33:43 > 0:33:46But as we leave the dark blue depths
0:33:46 > 0:33:48and return into bright, white sunlight...
0:33:52 > 0:33:55..we enter a world where most eyes perform at their peak.
0:33:57 > 0:34:01It's in these bright conditions that our eyes work at their best.
0:34:03 > 0:34:05And I've come to the Welsh highlands
0:34:05 > 0:34:10to pit mine against an animal with some of the sharpest eyes on Earth.
0:34:11 > 0:34:13Our eyes are pretty impressive.
0:34:13 > 0:34:16In fact, sight is our dominant sense.
0:34:16 > 0:34:19We can see things in incredible detail.
0:34:19 > 0:34:20But there's a group of predators
0:34:20 > 0:34:23with an even sharper view of the world.
0:34:33 > 0:34:34Meet Moses!
0:34:34 > 0:34:38He's a Peale's peregrine falcon, and he's pretty spectacular.
0:34:55 > 0:34:57Hey-up, hup!
0:34:57 > 0:34:59His owner, Lloyd, is using a lure
0:34:59 > 0:35:02to help demonstrate exactly what he's capable of.
0:35:09 > 0:35:11He's flying so close,
0:35:11 > 0:35:12and so fast...
0:35:13 > 0:35:14HE GASPS
0:35:17 > 0:35:19But he's using those amazing eyes
0:35:19 > 0:35:20to help avoid a collision.
0:35:22 > 0:35:24Good lad.
0:35:24 > 0:35:27We're going to put a peregrine to the test,
0:35:27 > 0:35:29and try and find out what it's like
0:35:29 > 0:35:32to see through some of the sharpest eyes on Earth!
0:35:33 > 0:35:38But first I want to show you why they need such precise vision.
0:35:41 > 0:35:44These birds are the fastest animals on Earth.
0:35:48 > 0:35:52In a dive, they can hit 180mph.
0:35:56 > 0:36:00Flying at these speeds demands exceptional visual coordination.
0:36:00 > 0:36:03Any misjudgement could be fatal.
0:36:05 > 0:36:09To show me how exceptional, Lloyd has an unusual test.
0:36:12 > 0:36:14And it requires nerves of steel.
0:36:38 > 0:36:39That was fantastic!
0:36:44 > 0:36:46'Even travelling at 50mph,
0:36:46 > 0:36:48'his incredible vision allows him
0:36:48 > 0:36:51'to fly through my legs effortlessly.'
0:36:55 > 0:36:57To fly at such speed,
0:36:57 > 0:37:00peregrines have fantastic depth perception.
0:37:02 > 0:37:04But where their eyes really come into their own
0:37:04 > 0:37:08is in spotting prey over incredibly long distances.
0:37:29 > 0:37:32There's no doubt that excellent vision is vital
0:37:32 > 0:37:34to the peregrines' existence.
0:37:34 > 0:37:36But exactly how precise is their eyesight?
0:37:42 > 0:37:46To find out, we've set up the ultimate long-distance sight test.
0:37:47 > 0:37:49The peregrine falcon is on a ridge,
0:37:49 > 0:37:52just under a mile away, over there.
0:37:55 > 0:37:57We're going to see if he can spot his lure
0:37:57 > 0:37:59from the other side of the valley.
0:38:01 > 0:38:05To make it even tougher, the visibility is poor...
0:38:07 > 0:38:09..and he has no idea where we are.
0:38:11 > 0:38:14But we've fitted him with a tracking device,
0:38:14 > 0:38:16so we can monitor where he is.
0:38:19 > 0:38:20Using this radio tracker,
0:38:20 > 0:38:23we'll be able to tell when Moses is getting closer.
0:38:23 > 0:38:25Fingers crossed, when his hood comes off,
0:38:25 > 0:38:28he'll spot the lure and come swooping in.
0:38:31 > 0:38:35It's time to put some of the sharpest eyes on Earth to the test.
0:38:37 > 0:38:40Will he spot our tiny lure from so far away?
0:38:43 > 0:38:44SHE SHOUTS COMMAND
0:39:03 > 0:39:05I can't see anything at all.
0:39:08 > 0:39:10It's quite nerve-racking, actually!
0:39:10 > 0:39:12I don't know where he's coming from.
0:39:16 > 0:39:17TRACKER BEEPS
0:39:26 > 0:39:27I'm getting him from over here.
0:39:27 > 0:39:29Here he is, here he is!
0:39:30 > 0:39:32Hup, hup, hup!
0:39:37 > 0:39:38- Oh!- Hey-up!
0:39:38 > 0:39:40Good boy.
0:39:40 > 0:39:42That was phenomenal.
0:39:42 > 0:39:43There.
0:39:43 > 0:39:45He spotted us from nearly a mile away,
0:39:45 > 0:39:47and flew straight to us.
0:39:49 > 0:39:51- Wow!- There's a good boy, you clever boy!
0:39:53 > 0:39:56- Look how he picked us up almost instantly.- Yeah.
0:39:56 > 0:39:58It's worth remembering, he's never flown here before,
0:39:58 > 0:40:00he's never even been here before,
0:40:00 > 0:40:02he has no idea, when his hood comes off,
0:40:02 > 0:40:04where I'm going to be in this landscape.
0:40:04 > 0:40:06So you can't get any more definitive proof
0:40:06 > 0:40:09about how good their eyesight is than that, I don't think.
0:40:09 > 0:40:11That was really quite astonishing.
0:40:11 > 0:40:15I could barely make out that there's anyone over there on that ridge,
0:40:15 > 0:40:18but Moses was able to spot us and this small lure.
0:40:20 > 0:40:24So what's the secret to their amazing vision?
0:40:24 > 0:40:27It's all down to a brilliant adaptation they share with us
0:40:27 > 0:40:31and other sharp-sighted animals called a fovea.
0:40:33 > 0:40:37This is an area in the retina where light-sensitive cells -
0:40:37 > 0:40:41called photoreceptors - are particularly tightly concentrated.
0:40:44 > 0:40:48Like pixels on a screen, the more receptors you have,
0:40:48 > 0:40:51the higher the resolution or visual acuity.
0:40:53 > 0:40:55Because the fovea's so packed with receptors,
0:40:55 > 0:40:58it creates a super-sharp image.
0:41:01 > 0:41:05Our fovea contains 200,000 receptors per square millimetre.
0:41:08 > 0:41:11But birds of prey can have nearly twice as many...
0:41:20 > 0:41:23..so their vision is much sharper than ours.
0:41:25 > 0:41:30This is why these birds can spot prey over such incredible distances.
0:41:35 > 0:41:38Our vision is pretty impressive,
0:41:38 > 0:41:39but when it comes to acuity,
0:41:39 > 0:41:42the peregrine is in a class of its own.
0:41:45 > 0:41:46In daylight conditions,
0:41:46 > 0:41:49birds of prey have an unrivalled view of the world.
0:41:50 > 0:41:55They have evolved the sharpest eyes of any animal on Earth.
0:42:00 > 0:42:04We've seen how colour vision is critical for many creatures.
0:42:06 > 0:42:08As we continue our journey through sight,
0:42:08 > 0:42:11we enter the world of red light.
0:42:12 > 0:42:14And it's here our human eyes
0:42:14 > 0:42:17have evolved a surprising power of their own.
0:42:20 > 0:42:23It's easy to take our vision for granted -
0:42:23 > 0:42:26but compared to almost every other mammal on Earth,
0:42:26 > 0:42:29our eyes can see far more colour.
0:42:31 > 0:42:34Even within our close primate family,
0:42:34 > 0:42:36we see things very differently.
0:42:38 > 0:42:41You might think the more colours you see, the better.
0:42:42 > 0:42:45But this pygmy marmoset is about to demonstrate
0:42:45 > 0:42:49that sometimes seeing less is more.
0:42:51 > 0:42:56He's the smallest monkey on Earth - just the size of my hand -
0:42:56 > 0:42:59and he's a supreme visual predator.
0:43:01 > 0:43:04And that's partly because he sees fewer colours than us.
0:43:08 > 0:43:12So how does this marmoset's limited colour vision
0:43:12 > 0:43:15make him such a keen little predator?
0:43:15 > 0:43:18I can demonstrate this with the help of these pictures.
0:43:18 > 0:43:19On this first one,
0:43:19 > 0:43:22there's a cross hidden in this pattern,
0:43:22 > 0:43:24but it's hard to make out at first glance
0:43:24 > 0:43:26because of the two colours.
0:43:26 > 0:43:29But when you take away one of those colours...
0:43:29 > 0:43:30all of a sudden,
0:43:30 > 0:43:33the cross becomes more prominent.
0:43:33 > 0:43:37And that's because colour is a very powerful visual cue,
0:43:37 > 0:43:39so when it comes to looking for patterns,
0:43:39 > 0:43:41it can be quite distracting.
0:43:45 > 0:43:47If you're a small monkey that eats insects
0:43:47 > 0:43:50camouflaged amongst leaves and branches,
0:43:50 > 0:43:51seeing fewer colours
0:43:51 > 0:43:56dramatically helps you to recognise the shape of your insect prey.
0:43:59 > 0:44:02Surprisingly, this little marmoset sees colour
0:44:02 > 0:44:06the same way virtually every other mammal on Earth does.
0:44:07 > 0:44:10Every cat, dog, bear -
0:44:10 > 0:44:14they all share the same limited colour vision.
0:44:17 > 0:44:19But there's one exception.
0:44:19 > 0:44:23A small group of primates - including us -
0:44:23 > 0:44:25have pushed colour vision even further
0:44:25 > 0:44:30and evolved the ability to see a colour that no other mammal can.
0:44:34 > 0:44:36BABOONS GIBBER
0:44:41 > 0:44:45These chacma baboons should help me demonstrate...
0:44:47 > 0:44:49..if they cooperate, that is.
0:44:53 > 0:44:55I've got a little test.
0:44:55 > 0:44:57Baboons love tomatoes,
0:44:57 > 0:45:00and I've got two of them here - one red, and one green.
0:45:00 > 0:45:03So let's see which one they decide to go for.
0:45:07 > 0:45:08It seems odd,
0:45:08 > 0:45:11but most mammals wouldn't be able to see the difference
0:45:11 > 0:45:13between these two tomatoes.
0:45:26 > 0:45:29So he's taken the red one straightaway.
0:45:29 > 0:45:30Chomping away!
0:45:30 > 0:45:34He knows that red fruit is the sweetest and most ripe.
0:45:36 > 0:45:38Let's try again.
0:45:42 > 0:45:45Every time, they're taking the red one first.
0:45:53 > 0:45:55It's clear that they see the difference,
0:45:55 > 0:45:57and they definitely prefer the ripe ones.
0:46:03 > 0:46:05That's because, like us,
0:46:05 > 0:46:07baboons can see the colour red.
0:46:07 > 0:46:10We take it for granted,
0:46:10 > 0:46:13but this ability we share is remarkable -
0:46:13 > 0:46:15and something we can only do
0:46:15 > 0:46:17thanks to a bizarre twist
0:46:17 > 0:46:19in our distant past.
0:46:22 > 0:46:25Deep in our eyes, we can unravel what happened.
0:46:27 > 0:46:29We see the colours we do
0:46:29 > 0:46:31because of light receptors in the back of the eye
0:46:31 > 0:46:33called cones.
0:46:33 > 0:46:37We have three main types of cones
0:46:37 > 0:46:40that detect red, green and blue light.
0:46:43 > 0:46:46So we can see any combination of those colours.
0:46:49 > 0:46:52But our eyes haven't always been this advanced.
0:46:57 > 0:46:59Going back in evolutionary time,
0:46:59 > 0:47:02primates only had two types of colour receptor -
0:47:02 > 0:47:05one for blue light and one for green.
0:47:05 > 0:47:08But it's thought that about 40 million years ago,
0:47:08 > 0:47:11a tiny genetic mutation caused a shift
0:47:11 > 0:47:13from the green colour receptor to the red.
0:47:20 > 0:47:25As a result, some primates - including us and the baboons -
0:47:25 > 0:47:28developed the ability to see a new colour...
0:47:28 > 0:47:29red...
0:47:31 > 0:47:35..and this turned out to be very important.
0:47:37 > 0:47:42This small change gave our primate ancestors a huge advantage,
0:47:42 > 0:47:46because it allowed them to pick out ripe red fruits
0:47:46 > 0:47:48from the dense, green foliage.
0:47:53 > 0:47:55The way each and every animal sees colour
0:47:55 > 0:47:57is slightly different,
0:47:57 > 0:48:00and as a result of millions of years of evolution.
0:48:02 > 0:48:06It's all about what works best in that creature's world.
0:48:21 > 0:48:23We have seen the diverse ways
0:48:23 > 0:48:26animals tap into every parts of the light spectrum
0:48:26 > 0:48:28to survive.
0:48:32 > 0:48:36But as we reach the very edge of the visible spectrum,
0:48:36 > 0:48:40one predator has evolved an almost supernatural ability
0:48:40 > 0:48:43to see light that's invisible to us.
0:48:51 > 0:48:55And it can do so without using its eyes at all.
0:48:59 > 0:49:00I'm in Cuba,
0:49:00 > 0:49:03in a beautiful landscape that's teeming with wildlife,
0:49:03 > 0:49:08and tonight I'm hoping to witness a special hunting spectacle.
0:49:08 > 0:49:12This predator's view of the world is different to mine,
0:49:12 > 0:49:16and it's evolved a way of hunting in complete darkness.
0:49:20 > 0:49:23To find this remarkable animal,
0:49:23 > 0:49:26I'm on my way to a cave deep in the jungle.
0:49:30 > 0:49:34And to show you how well it can spot its prey in this pitch-black world,
0:49:34 > 0:49:37I'm going to use a camera that works in the dark.
0:49:46 > 0:49:49Down here, my eyes can't see a thing...
0:49:52 > 0:49:56..yet these are the perfect hunting conditions for our predator.
0:50:01 > 0:50:04It's not long before they make an appearance.
0:50:15 > 0:50:17I'm in the mouth of a cave,
0:50:17 > 0:50:20and over there is a Cuban boa.
0:50:23 > 0:50:26It's been curled up in the crevice in the rock all day,
0:50:26 > 0:50:29and now it's night-time and it's come out to hunt.
0:50:33 > 0:50:37It's after an animal that seems virtually impossible to catch.
0:50:37 > 0:50:40The snake's prey are just starting to come out of the cave,
0:50:40 > 0:50:45and I can hear them zooming past my ears and out into the night.
0:50:47 > 0:50:49They're bats,
0:50:49 > 0:50:51and the snake is hoping to catch one.
0:50:57 > 0:51:01To survive, these snakes have to feed.
0:51:06 > 0:51:08They must somehow pinpoint...
0:51:09 > 0:51:10..strike...
0:51:12 > 0:51:16..and catch a bat as it speeds past -
0:51:16 > 0:51:18all in complete darkness.
0:51:24 > 0:51:27But these Cuban boas are specialists.
0:51:41 > 0:51:43This one's got one...
0:51:44 > 0:51:47..and he's coiling around it to suffocate it.
0:51:50 > 0:51:54And the snake's just tucked up underneath the rock there.
0:52:05 > 0:52:08To survive in this underground world,
0:52:08 > 0:52:11these snakes have evolved a bizarre and brilliant visual trick.
0:52:17 > 0:52:20Because even though it's pitch black,
0:52:20 > 0:52:24the snakes have found an ingenious way to use light to find their prey.
0:52:26 > 0:52:28But how are they doing it?
0:52:31 > 0:52:34Their secret is that they can see heat.
0:52:36 > 0:52:40Off the edge of the visible spectrum beyond red is infrared.
0:52:42 > 0:52:44It's invisible to our eyes...
0:52:47 > 0:52:50..but this is a bizarre part of the spectrum
0:52:50 > 0:52:51where heat becomes light.
0:52:56 > 0:52:58I've brought a special camera to the jungle
0:52:58 > 0:53:00which can see this heat.
0:53:06 > 0:53:08This is a thermal imaging camera,
0:53:08 > 0:53:11and, just like the snakes, it can detect infrared radiation.
0:53:11 > 0:53:14So the world looks almost the same, but a little bit different -
0:53:14 > 0:53:17because this is an image in heat.
0:53:17 > 0:53:18So, you can see that, for example,
0:53:18 > 0:53:22my forehead and my neck are giving off lots of heat,
0:53:22 > 0:53:24so they're bright white.
0:53:24 > 0:53:27But my clothes have a thin layer of insulating air,
0:53:27 > 0:53:30so the clothes themselves are cooler, so they look darker,
0:53:30 > 0:53:32and my nose is also quite dark.
0:53:32 > 0:53:34The secret to the snake's ability to hunt
0:53:34 > 0:53:38is that it can find warm-blooded mammals like bats
0:53:38 > 0:53:39by sensing their heat.
0:53:41 > 0:53:46And to show you how expert they are at detecting this infrared,
0:53:46 > 0:53:48I've got a test.
0:53:48 > 0:53:50I've put a Cuban boa in a box that's blacked out.
0:53:53 > 0:53:55I've got a balloon here, a black balloon,
0:53:55 > 0:53:56which is filled with warm water,
0:53:56 > 0:53:59so it's a reasonable mock-up of a small warm-blooded mammal
0:53:59 > 0:54:00like a bat.
0:54:00 > 0:54:04And what we're going to do is turn off all the lights...
0:54:04 > 0:54:06put this in the box with the snake
0:54:06 > 0:54:10and watch with the thermal camera to see how the snake reacts.
0:54:12 > 0:54:14Despite the darkness,
0:54:14 > 0:54:17he immediately zeroes in on our fake bat.
0:54:21 > 0:54:23So, here comes the snake.
0:54:25 > 0:54:27And it's completely dark in the box.
0:54:33 > 0:54:36And he's just turned towards the balloon.
0:54:37 > 0:54:39Gone right up to have a good look.
0:54:46 > 0:54:48- Oh! - SHE LAUGHS
0:54:48 > 0:54:51So, the snake struck, burst the balloon,
0:54:51 > 0:54:54and now there's hot water all over the bottom.
0:54:54 > 0:54:57This snake clearly found that balloon
0:54:57 > 0:55:01and struck at it accurately in complete darkness.
0:55:10 > 0:55:13The snake's ability to see infrared light is incredible.
0:55:16 > 0:55:21But even more remarkable is that it doesn't use its eyes to see it.
0:55:24 > 0:55:25The snake's eyes are on top here.
0:55:25 > 0:55:27They're really obvious.
0:55:27 > 0:55:29But that's not what the snake is using
0:55:29 > 0:55:31to detect this infrared radiation.
0:55:31 > 0:55:34If you look along the top and bottom of its jaw,
0:55:34 > 0:55:36in between the scales,
0:55:36 > 0:55:38there are pits like little dimples.
0:55:38 > 0:55:41And the back of each pit is sensitive to heat,
0:55:41 > 0:55:44and that's what the snake's using.
0:55:44 > 0:55:49Each pit generates a very crude infrared image.
0:55:49 > 0:55:51Because there are so many pits,
0:55:51 > 0:55:54by combining the information from all of them,
0:55:54 > 0:55:57the snake can build a deadly accurate picture
0:55:57 > 0:55:59of where its prey is.
0:55:59 > 0:56:02So, actually, the infrared-sensing organ
0:56:02 > 0:56:04is all along the front of the jaw.
0:56:04 > 0:56:06It's actually much bigger than the eye.
0:56:06 > 0:56:09It's a really ingenious physical solution.
0:56:13 > 0:56:15But what do they actually see?
0:56:15 > 0:56:17What does their world look like to them?
0:56:20 > 0:56:24To show you, I'm taking my thermal camera into the cave
0:56:24 > 0:56:26to try and see these bats.
0:56:28 > 0:56:31I can hear the bats whooshing past me, but I can't see them.
0:56:32 > 0:56:35But we can get an idea of how this scene looks to the snake.
0:56:41 > 0:56:44The bats are like flying beacons in the blackness.
0:56:47 > 0:56:52Like all mammals, their warm bodies radiate infrared light...
0:56:52 > 0:56:54so there's no way for them to hide.
0:57:03 > 0:57:05The snakes here have a huge advantage
0:57:05 > 0:57:08by tapping into light that's all around us,
0:57:08 > 0:57:10but that most animals just can't see.
0:57:10 > 0:57:13And it lets them hunt here, even in complete darkness.
0:57:16 > 0:57:18These Cuban boas have mastered
0:57:18 > 0:57:20one of the most challenging places on Earth...
0:57:24 > 0:57:27..because they've pushed sight to the edge of possibility.
0:57:38 > 0:57:43On our journey we've shown that there's far more to sight
0:57:43 > 0:57:44than meets the eye.
0:57:45 > 0:57:48We live in a world that's bathed in light.
0:57:51 > 0:57:54Every animal has evolved to use a part of that light,
0:57:54 > 0:57:57that gives them the best chance to survive.
0:58:00 > 0:58:03They can each see a part of the puzzle,
0:58:03 > 0:58:06but no one animal can see it all.
0:58:08 > 0:58:09Next time...
0:58:10 > 0:58:13..we travel through the sound spectrum.
0:58:15 > 0:58:16From the deepest bellows...
0:58:16 > 0:58:18ELEPHANT RUMBLES
0:58:18 > 0:58:19..to the highest pitches.
0:58:19 > 0:58:20FROG CHIRPS
0:58:20 > 0:58:22When I look out there, I see blackness,
0:58:22 > 0:58:23but when I look here,
0:58:23 > 0:58:26there's these really bright splotches of light.
0:58:27 > 0:58:32We reveal the most extraordinary ways animals use sound to survive.