0:00:05 > 0:00:07'Bones.
0:00:08 > 0:00:10'They offer structure...
0:00:10 > 0:00:11'support...
0:00:11 > 0:00:13'and strength.
0:00:13 > 0:00:17'But they have a much bigger story to tell.
0:00:22 > 0:00:26'Vertebrates may look very different on the outside...
0:00:27 > 0:00:29'..but one crucial thing unites them all...
0:00:30 > 0:00:32'..the skeleton.
0:00:35 > 0:00:37'I'm Ben Garrod,
0:00:37 > 0:00:42'an evolutionary biologist with a very unusual passion.'
0:00:42 > 0:00:43This is unbelievable!
0:00:43 > 0:00:47There are too many skeletons for me to look at all at once!
0:00:47 > 0:00:50'As a child, I was fascinated by bones.
0:00:50 > 0:00:53'Now, skeletons have become my life.
0:00:58 > 0:01:03'And I put them together for museums and universities all over the world.
0:01:06 > 0:01:09'I'm going to explore the natural world...
0:01:10 > 0:01:12'..from the inside out...'
0:01:14 > 0:01:18'..to see how the skeleton has enabled animals to move...
0:01:19 > 0:01:20'..hunt...
0:01:20 > 0:01:23'and even sense the world.'
0:01:23 > 0:01:27I will take you on a very personal journey to discover how this
0:01:27 > 0:01:30one bony blueprint has shaped such massive
0:01:30 > 0:01:33diversity across the animal kingdom
0:01:33 > 0:01:36and how it has come to dominate life on planet Earth.
0:01:37 > 0:01:41'This time, we're going to uncover how bones...'
0:01:41 > 0:01:42Oh, wow!
0:01:42 > 0:01:44That's absolutely amazing.
0:01:44 > 0:01:47'..have enabled animals to do the most remarkable thing of all.
0:01:49 > 0:01:50'Take to the air.
0:01:52 > 0:01:56'I'm going to reveal the secrets of bones.'
0:02:06 > 0:02:09Pretty much every group of animals, from fish...
0:02:09 > 0:02:11to frogs...
0:02:13 > 0:02:14..and mammals...
0:02:14 > 0:02:18to snakes have had a go at getting airborne.
0:02:22 > 0:02:25But only a few have dramatically changed their skeleton
0:02:25 > 0:02:28and truly mastered powered flight.
0:02:31 > 0:02:35The ultimate flyers have to be the birds.
0:02:37 > 0:02:41Their bones have adapted not only for a life up in the sky...
0:02:43 > 0:02:45..but also down on the ground.
0:02:45 > 0:02:48And even under the water.
0:02:49 > 0:02:53How did the skeleton enable birds to become so successful?
0:02:57 > 0:03:00First up, the evolution of wings.
0:03:03 > 0:03:07Surprisingly, the blueprint for all vertebrate wings can be seen
0:03:07 > 0:03:09in the primate skeleton.
0:03:09 > 0:03:12Like this gorilla.
0:03:12 > 0:03:13And you and me.
0:03:15 > 0:03:20These five digits are known as the pentadactyl limb,
0:03:20 > 0:03:24and first appeared in land animals over 300 million years ago.
0:03:25 > 0:03:28Basically, it was from five fingers like ours
0:03:28 > 0:03:31that winged flight has evolved independently three times.
0:03:35 > 0:03:39And to see how these bones first helped animals take to the skies,
0:03:39 > 0:03:43I'm going back to the time of the dinosaurs.
0:03:45 > 0:03:49The first vertebrates to become true flyers with fully-formed wings
0:03:49 > 0:03:52and sustained flight were a type of flying reptile -
0:03:52 > 0:03:54the pterosaurs.
0:03:55 > 0:03:59This is a fossil cast of Pterodactylus antiquus,
0:03:59 > 0:04:03a young pterosaur, about as big as a starling.
0:04:03 > 0:04:06The smallest, though, were only the size of sparrows.
0:04:08 > 0:04:11But the biggest pterosaurs, they were massive.
0:04:11 > 0:04:15They had a wingspan of over 10m.
0:04:17 > 0:04:21They were the largest flying animals to have ever lived.
0:04:21 > 0:04:24Their wings would fill this room.
0:04:31 > 0:04:37Pterosaurs dominated the skies for 150 million years
0:04:37 > 0:04:40and had wings modified from the original five-fingered blueprint.
0:04:47 > 0:04:51This is clear when compared alongside a human hand.
0:04:53 > 0:04:56The first three digits adapted as grasping claws,
0:04:56 > 0:04:58and the fifth digit was lost.
0:04:59 > 0:05:01But the fourth digit grew really long,
0:05:01 > 0:05:03as a support for the wing membrane.
0:05:03 > 0:05:07And, in some species, could be several metres in length.
0:05:08 > 0:05:11The word 'pterodactyl' comes from Greek origins,
0:05:11 > 0:05:14and actually means 'wing finger.'
0:05:16 > 0:05:20When pterosaurs were wiped out around 65 million years ago,
0:05:20 > 0:05:24other flying animals flourished.
0:05:25 > 0:05:27Including bats.
0:05:28 > 0:05:31They developed a completely different method
0:05:31 > 0:05:33of taking to the air.
0:05:33 > 0:05:35They evolved a second way of flying...
0:05:37 > 0:05:40..once again, based on the pentadactyl limb.
0:05:42 > 0:05:45Unlike the pterosaurs, in the bats, only their first digit,
0:05:45 > 0:05:48or thumb, became hooked for grasping.
0:05:48 > 0:05:52The other four fingers grew extremely long, giving them
0:05:52 > 0:05:56superb control over the shape of their wings in flight.
0:06:04 > 0:06:06Now, on this fruit bat here,
0:06:06 > 0:06:09also known as one of the megabats - sounds cool, doesn't it? -
0:06:09 > 0:06:11you can see some amazing skeletal adaptations.
0:06:11 > 0:06:14The first, and most obvious, is up here.
0:06:14 > 0:06:16It's this wonderful forelimb,
0:06:16 > 0:06:17the wing.
0:06:17 > 0:06:19You can see the very long bones here,
0:06:19 > 0:06:21but they end in these four
0:06:21 > 0:06:23very, very elongated digits.
0:06:23 > 0:06:26And these serve to open up as much
0:06:26 > 0:06:28skin and soft tissue as possible,
0:06:28 > 0:06:32allowing for these very broad, strong wings.
0:06:32 > 0:06:35The bones are also very flexible, which helps cope with
0:06:35 > 0:06:39the extreme forces acting on the skeleton during flight.
0:06:40 > 0:06:43Basically, the whole skeleton works together to become
0:06:43 > 0:06:45as aerodynamic and as lightweight as possible.
0:06:46 > 0:06:50I'll be honest - bats amaze me because, to me,
0:06:50 > 0:06:54they're fat, little, hairy mammals, that manage to stay up in the air
0:06:54 > 0:06:56and they do it very well.
0:06:56 > 0:06:58But more impressive to me is the fact that
0:06:58 > 0:07:02there are approximately 1,000 species of bats across the globe.
0:07:02 > 0:07:05And this accounts for nearly a quarter
0:07:05 > 0:07:07of all mammal species on Earth.
0:07:15 > 0:07:20The success of bats can largely be attributed to their flying prowess.
0:07:20 > 0:07:22And that is mainly down to their skeleton.
0:07:24 > 0:07:28Flexible wings allow them to catch highly acrobatic prey.
0:07:31 > 0:07:36They can turn 180 degrees in less than half a wingspan.
0:07:44 > 0:07:46Bats may be brilliant flyers,
0:07:46 > 0:07:49but birds are the true masters of the sky...
0:07:51 > 0:07:55..with almost ten times more species inhabiting practically
0:07:55 > 0:07:57every habitat on the planet.
0:08:01 > 0:08:03So, what's special about their wings?
0:08:05 > 0:08:10They developed an entirely independent, third way of flying.
0:08:10 > 0:08:13And, again, it all began with five fingers.
0:08:15 > 0:08:18Rather than the elongated digits found in bats and pterosaurs,
0:08:18 > 0:08:21some bird bones fused together.
0:08:21 > 0:08:24Others disappeared completely.
0:08:25 > 0:08:27This gave rigidity to the wing
0:08:27 > 0:08:31and provided a platform for feathers to generate lift.
0:08:31 > 0:08:33But it's not all about wings.
0:08:35 > 0:08:37Birds had to make other significant changes
0:08:37 > 0:08:41within their skeletons to become such successful flyers.
0:08:42 > 0:08:44In order to overcome gravity,
0:08:44 > 0:08:48it's important to become both lightweight and strong.
0:08:48 > 0:08:50This is exactly what birds have done.
0:08:51 > 0:08:54You can see on this pheasant how many of their bones
0:08:54 > 0:08:57have fused together for strength.
0:08:58 > 0:09:02The first of which is this area here, where they've fused
0:09:02 > 0:09:06a load of their vertebrae and their pelvis into one big superstructure.
0:09:06 > 0:09:09And you can very clearly see the edge of the wing
0:09:09 > 0:09:11where you've got, not only a loss of some of the digits,
0:09:11 > 0:09:15but also a fusion of several bones into one, yet again.
0:09:15 > 0:09:19There's another skeletal adaptation which I love.
0:09:19 > 0:09:21It's these little processes you get between the ribs,
0:09:21 > 0:09:24linking one rib to the next, to the next.
0:09:24 > 0:09:27This all serves to stiffen the whole ribcage,
0:09:27 > 0:09:29again, making it really, really strong.
0:09:30 > 0:09:33Now, you're a bird, you can fly, you've got these big wings,
0:09:33 > 0:09:35you've got these massive muscles,
0:09:35 > 0:09:37you need somewhere to attach these things to.
0:09:37 > 0:09:38And what birds have evolved
0:09:38 > 0:09:41and developed is this wonderful structure here, as well.
0:09:41 > 0:09:43This is the breastbone, or the keel.
0:09:43 > 0:09:47This big, flattened projection you can see here serves to anchor
0:09:47 > 0:09:49all of these big muscle attachments
0:09:49 > 0:09:51which, ultimately, allows the bird to fly.
0:09:53 > 0:09:58One other crucial adaptation has helped birds take to the air.
0:10:00 > 0:10:03And, this time, the secret is INSIDE their skeleton.
0:10:05 > 0:10:09Their bones have evolved to be as light as possible.
0:10:12 > 0:10:16Here, we've got a wonderful image from a scanning electron microscope
0:10:16 > 0:10:19from within a bird's bone.
0:10:19 > 0:10:22And you can see this whole network of rigid, internal strut-like
0:10:22 > 0:10:27supports which actually prevent the bones from buckling during flight.
0:10:27 > 0:10:30Compare this to a human bone,
0:10:30 > 0:10:32and you can see the difference instantly.
0:10:32 > 0:10:35It's much thicker, it's very dense, there's lots of marrow.
0:10:35 > 0:10:37Ultimately, it's incredibly heavy.
0:10:39 > 0:10:41It's the last thing you want when trying to fly.
0:10:43 > 0:10:47To see how all these adaptations to the skeleton have come together,
0:10:47 > 0:10:51I'm going to look at an extraordinary bird
0:10:51 > 0:10:52that we see everyday -
0:10:52 > 0:10:55and is often taken for granted.
0:10:57 > 0:10:58The humble pigeon.
0:11:00 > 0:11:02It's hugely successful.
0:11:02 > 0:11:05There are over ten million pigeons in the UK alone.
0:11:07 > 0:11:11And there are thought to be around 260 million in the world.
0:11:13 > 0:11:16There's one particular survival technique that has allowed
0:11:16 > 0:11:18the pigeon to thrive.
0:11:20 > 0:11:24Specialist bird handler Lloyd Buck is going to reveal the secret
0:11:24 > 0:11:27with a very short and simple flight.
0:11:27 > 0:11:29- Ready?- Yeah.
0:11:29 > 0:11:31There's a little pea there.
0:11:31 > 0:11:33- I'll try and get out of the way.- OK.
0:11:38 > 0:11:40By filming Smudge in slow motion,
0:11:40 > 0:11:44we can see how pigeons have an explosive takeoff,
0:11:44 > 0:11:48able to fly vertically, upwards, for more than 20m.
0:11:50 > 0:11:54This exceptional flying ability is down to their complex physiology.
0:11:56 > 0:11:58So, when they do their vertical takeoff, what's going on?
0:11:58 > 0:12:01Well, it's amazing to watch, isn't it?
0:12:01 > 0:12:03And when you see it slowed down,
0:12:03 > 0:12:05you get more of an idea what's happening.
0:12:05 > 0:12:09And you see, she's putting all her energy into one purpose,
0:12:09 > 0:12:12- to clear the ground and go up as fast as she possibly can.- Yeah.
0:12:14 > 0:12:19This slow, you can see how Smudge first bends her knees.
0:12:21 > 0:12:25Then pushes off the ground,
0:12:25 > 0:12:27whilst flinging her wings above her head.
0:12:29 > 0:12:32This powerful jump allows her to clear the ground enough
0:12:32 > 0:12:35to make a complete downward stroke
0:12:35 > 0:12:37without her wings touching the floor.
0:12:41 > 0:12:46She can accelerate from 0 to 60mph in less than two seconds.
0:12:48 > 0:12:50To see how pigeons are such skilled flyers,
0:12:50 > 0:12:53we need to take a closer look at their bones.
0:12:53 > 0:12:55I thought it'd be nice to have a real good
0:12:55 > 0:13:00comparison between the live bird and my sort of bird.
0:13:00 > 0:13:03The first thing that sticks out is this massive keel.
0:13:03 > 0:13:05Quite big for a bird that size.
0:13:05 > 0:13:07I've never seen a pigeon's skeleton before today.
0:13:07 > 0:13:11I'm only used to seeing live ones, like Smudge here.
0:13:11 > 0:13:14When I first looked at it, I thought, "That's not quite right, is it?"
0:13:14 > 0:13:16But, actually, when you look at Smudge, look...
0:13:16 > 0:13:19Right down there, you can really see it.
0:13:19 > 0:13:21And there are so many adaptations that just points to this
0:13:21 > 0:13:24being an absolute powerhouse of muscle in flight.
0:13:24 > 0:13:26If you look at the bones in the upper arm, the humerus,
0:13:26 > 0:13:30they're really short. Having that short, stubby, stocky little bone
0:13:30 > 0:13:32really allows that power, again.
0:13:32 > 0:13:34Even that one bone says this is a very strong bird.
0:13:34 > 0:13:37- Incredible.- Brilliant.
0:13:37 > 0:13:40You can see these massive legs. They just keep going up.
0:13:40 > 0:13:42- And they're really strong, aren't they?- Yeah.
0:13:42 > 0:13:45- You can tell that this bird is one hell of a flyer.- Yeah.
0:13:49 > 0:13:53Their stocky and flexible legs, big, muscular keel
0:13:53 > 0:13:56and short, manoeuvrable wing bones allow them to perform
0:13:56 > 0:13:58powerful vertical take-offs.
0:14:00 > 0:14:02But why do they need this skill?
0:14:03 > 0:14:05It's as much being a feral pigeon.
0:14:05 > 0:14:08They spend a lot of time feeding on the ground.
0:14:08 > 0:14:11So, if a ground predator or an aerial predator comes in to try
0:14:11 > 0:14:15and kill them, they need to be able to get away as quickly as possible.
0:14:16 > 0:14:20- And the vertical take-off is a brilliant method.- Yeah.
0:14:20 > 0:14:23They can out-climb a peregrine, if they need to.
0:14:23 > 0:14:27A peregrine has no chance of matching them for climbing speed.
0:14:29 > 0:14:31And also, they can just keep going over distance,
0:14:31 > 0:14:34getting close to 100km an hour.
0:14:34 > 0:14:36- They could do up to 800km in a day.- In one day?
0:14:36 > 0:14:39- At that speed.- That's phenomenal, isn't it? It really is.
0:14:39 > 0:14:40PIGEON WARBLES
0:14:40 > 0:14:41HE WHISTLES
0:14:43 > 0:14:45The pigeon is the ultimate all-rounder.
0:14:48 > 0:14:51But the basic bird skeleton has adapted in other species
0:14:51 > 0:14:55for extremely specialised forms of flight.
0:14:55 > 0:14:56WINGS SCUD RAPIDLY
0:14:59 > 0:15:00For speed.
0:15:02 > 0:15:04Manoeuvrability.
0:15:08 > 0:15:09And long-haul travel.
0:15:13 > 0:15:16At more than 3.5m,
0:15:16 > 0:15:19the albatross has the longest wingspan of any bird.
0:15:20 > 0:15:22They require a good run-up
0:15:22 > 0:15:26to allow enough air to move over their wings to generate lift.
0:15:28 > 0:15:32Once airborne, they rarely need to flap their wings,
0:15:32 > 0:15:34using a soaring technique
0:15:34 > 0:15:38to glide on wind currents for thousands of kilometres.
0:15:41 > 0:15:44How can they undertake such epic journeys?
0:15:46 > 0:15:49The wing bones are very, very long and very, very straight.
0:15:49 > 0:15:52This wing allows the animal to soar and glide,
0:15:52 > 0:15:55in much the same way that an aeroplane's wing would.
0:15:57 > 0:16:00On top of that, there is another very specific adaptation,
0:16:00 > 0:16:04and that's a very large tendon that sits in the shoulder area
0:16:04 > 0:16:09and travels all the way down the humerus, up and over the elbow.
0:16:10 > 0:16:13This allows the wing to be locked into place.
0:16:15 > 0:16:19This ability to effectively lock their wings during gliding
0:16:19 > 0:16:22allows them to fly effortlessly,
0:16:22 > 0:16:25conserving valuable energy.
0:16:25 > 0:16:27We can see from this X-ray image
0:16:27 > 0:16:31that part of their tendon has also become hard and bony.
0:16:31 > 0:16:36Known as a spreader bone, it offers stability and support to their wings
0:16:36 > 0:16:40during long periods of flight, and reduces muscle fatigue.
0:16:43 > 0:16:46This bird can glide like almost no other.
0:16:46 > 0:16:49And it can travel for 15,000km,
0:16:49 > 0:16:54from the moment they take off to the moment they return to the ground.
0:16:55 > 0:16:57As one of the heaviest flying birds,
0:16:57 > 0:17:00the albatross needs a colossal wingspan to cope.
0:17:03 > 0:17:07But some big birds have tiny wings.
0:17:09 > 0:17:12I'm on my way to the Royal Veterinary College near London
0:17:12 > 0:17:17to understand why their skeletons have specialised in this way.
0:17:20 > 0:17:24- Hello! Be careful here. It's really muddy as you go in.- OK.
0:17:24 > 0:17:29Professor John Hutchinson is an expert in animal locomotion.
0:17:29 > 0:17:30It's safe.
0:17:30 > 0:17:31Hello!
0:17:32 > 0:17:33They're pretty mellow.
0:17:35 > 0:17:37Cheeky animals, too!
0:17:37 > 0:17:40The peck won't hurt you, it's the kick you've got to worry about.
0:17:40 > 0:17:42But they won't use it unless they're threatened.
0:17:42 > 0:17:44Why do you have a field full of emus?
0:17:44 > 0:17:47Yeah, yeah, so emus are just really cool birds.
0:17:47 > 0:17:51Although they look kind of dinosaur-like, and they don't fly,
0:17:51 > 0:17:53so that seems primitive, actually, they're specialised,
0:17:53 > 0:17:56they're advanced, for a bird. Because most birds fly.
0:17:56 > 0:18:00They've lost their flight and become an extreme runner,
0:18:00 > 0:18:02a real athlete on land.
0:18:03 > 0:18:05Oh-ho, looky!
0:18:07 > 0:18:11John and his team have been studying how the emu's anatomy
0:18:11 > 0:18:13is adapted for running and not flight.
0:18:15 > 0:18:18- Being this close, you can really see, there's almost no wing.- Yep.
0:18:18 > 0:18:21- Where is it? - Well, it's just a little nubbin,
0:18:21 > 0:18:24dangling down in front of the knee here.
0:18:24 > 0:18:26Really small, really fragile.
0:18:26 > 0:18:28I happen to have a wing of an emu.
0:18:30 > 0:18:34- Check that out, isn't that cool? - So tiny, yeah.
0:18:35 > 0:18:38There's the humerus, forearm bones, radius and ulna,
0:18:38 > 0:18:40and there's the wrist and the hand.
0:18:40 > 0:18:42Very, very short hand. And the claw.
0:18:42 > 0:18:44They don't get much bigger than that.
0:18:44 > 0:18:46It really shows you close-up just how small,
0:18:46 > 0:18:49and just that massive reduction they've had.
0:18:49 > 0:18:51It makes sense to lose flight
0:18:51 > 0:18:54when it's no longer favoured by natural selection.
0:18:54 > 0:18:57Because flight is energetically expensive.
0:18:57 > 0:18:59Yeah, I guess so. It's almost like a trade-off.
0:18:59 > 0:19:01They've lost the ability to fly,
0:19:01 > 0:19:03but they're compensated by having massive legs.
0:19:03 > 0:19:08Exactly. A bird can't be both a super-fast runner and a great flyer.
0:19:08 > 0:19:12It's one or the other. And emus really are at the one extreme
0:19:12 > 0:19:14- of being a great runner, not a flyer at all.- OK.
0:19:17 > 0:19:19Show us the wings, come on...
0:19:19 > 0:19:23Emus aren't alone when it comes to being superb runners.
0:19:23 > 0:19:26Ostriches are the fastest birds in the world,
0:19:26 > 0:19:28when it comes to sprinting.
0:19:28 > 0:19:33They can reach speeds of over 40mph.
0:19:33 > 0:19:36And once again, the secret is in their bones.
0:19:40 > 0:19:43These are the leg bones of five different land animals.
0:19:43 > 0:19:47Now, they're the femur, which is the bone in the upper thigh.
0:19:47 > 0:19:52We've got a camel, a horse, dog,
0:19:52 > 0:19:54ostrich and emu.
0:19:54 > 0:19:57Now, they all look incredibly similar to each other
0:19:57 > 0:19:58and that's because, technically,
0:19:58 > 0:20:02they all have the same functional role, which is support
0:20:02 > 0:20:04and a lot of weight-bearing at the top of the leg.
0:20:04 > 0:20:08But what's weird and quite interesting is that these two,
0:20:08 > 0:20:10from the flightless birds here, the ostrich and the emu,
0:20:10 > 0:20:13they look really heavy, robust, thick-set.
0:20:13 > 0:20:17But they're actually really light.
0:20:17 > 0:20:19HOLLOW CLACK
0:20:24 > 0:20:29To understand why, I need to saw one open and take a close-up look.
0:20:44 > 0:20:45Ha-ha! Oh, wow!
0:20:47 > 0:20:49Look at that.
0:20:49 > 0:20:51That's absolutely amazing.
0:20:51 > 0:20:56Now, this ostrich bone perfectly demonstrates why it's so light.
0:20:56 > 0:20:58These big, flightless birds have retained so many
0:20:58 > 0:21:02of the characteristics that you'd see in the original flying birds.
0:21:02 > 0:21:06These bones genuinely are more air than they are bone.
0:21:06 > 0:21:10That same honeycomb structure found in flying birds
0:21:10 > 0:21:13is still here in one that lives on land.
0:21:23 > 0:21:25But what happens when birds take to the oceans?
0:21:32 > 0:21:35Penguins lost flight around 65 million years ago.
0:21:36 > 0:21:40About the same time as when the dinosaurs died out.
0:21:41 > 0:21:43It's thought they lost this ability
0:21:43 > 0:21:46because they no longer had any sort of land predators.
0:21:48 > 0:21:50You can see from this little guy,
0:21:50 > 0:21:52he's really not too fussed that I'm next to him.
0:21:54 > 0:21:59PENGUIN HONKS
0:21:59 > 0:22:02HONKING FADES
0:22:02 > 0:22:03Exactly!
0:22:06 > 0:22:08But it wasn't just the lack of land predators
0:22:08 > 0:22:11that led to penguins becoming flightless.
0:22:13 > 0:22:16It was also their need to swim.
0:22:24 > 0:22:28Penguins can travel at over 20mph.
0:22:28 > 0:22:30They need to be fast
0:22:30 > 0:22:32to dodge predators like leopard seals
0:22:32 > 0:22:34and hunt down their prey.
0:22:41 > 0:22:44But, to be this manoeuvrable underwater,
0:22:44 > 0:22:48something happened to their bones that then made it impossible to fly.
0:22:51 > 0:22:53It's only when you look at their skeletons
0:22:53 > 0:22:55and their bones, specifically,
0:22:55 > 0:22:59that you can really see the actual story behind what's going on here.
0:23:01 > 0:23:04Every single bone is heavier than you'd expect in a bird.
0:23:04 > 0:23:07When you've got flying birds, they've got very dynamic
0:23:07 > 0:23:10and lightweight bones. And that's perfect for them.
0:23:10 > 0:23:13If you live underwater a lot of the time and you hunt underwater,
0:23:13 > 0:23:15you need heavy bones.
0:23:15 > 0:23:18This allows the skeleton to act as ballast.
0:23:18 > 0:23:22If you take a close-up look at a penguin bone under a microscope,
0:23:22 > 0:23:25you can see just how dense it is,
0:23:25 > 0:23:27compared to that of a flying bird.
0:23:30 > 0:23:33If you look at specific areas of the skeleton as well, you can see there
0:23:33 > 0:23:37are some perfect adaptations for this hunting, underwater lifestyle.
0:23:37 > 0:23:40First of all, if you look at the wings.
0:23:40 > 0:23:42Now, they're not very long, but they're very broad
0:23:42 > 0:23:45and the leading edge and trailing edge are actually quite sharp.
0:23:45 > 0:23:47This allows the penguin
0:23:47 > 0:23:50to have a very rigid wing, that you can see here.
0:23:50 > 0:23:54And again, this is perfect for slicing through the water.
0:23:55 > 0:23:58One of my favourite adaptations in the penguin, though,
0:23:58 > 0:24:01are these things. These are massive scapula.
0:24:01 > 0:24:03These are the shoulder blades.
0:24:03 > 0:24:06They're huge! They're absolutely monstrous!
0:24:06 > 0:24:08If you watch a penguin power through the water,
0:24:08 > 0:24:11they're constantly paddling and paddling,
0:24:11 > 0:24:13and because water offers much more resistance than air,
0:24:13 > 0:24:17they really need a lot of power up in the shoulder area,
0:24:17 > 0:24:19to really pull themselves through the water.
0:24:22 > 0:24:24With their large scapulae, paddle-like wings
0:24:24 > 0:24:28and heavy bones, penguins have traded the ability
0:24:28 > 0:24:33to fly in the air, to effectively fly underwater.
0:24:39 > 0:24:40When you see one shoot past,
0:24:40 > 0:24:43they're like a little fat, feathered torpedo!
0:24:45 > 0:24:47It's only when you finally see the bubbles
0:24:47 > 0:24:50that you kind of remember they're underwater.
0:24:52 > 0:24:55We see penguins in an almost comedy light.
0:24:55 > 0:24:57And it's wrong, because they're not.
0:24:57 > 0:25:00They're predators, and they're good predators.
0:25:00 > 0:25:03They live in the Southern Ocean, round Antarctica,
0:25:03 > 0:25:05and it's not easy to live down there.
0:25:05 > 0:25:06They're tough, tough animals.
0:25:17 > 0:25:20'One bird has adapted for life underwater,
0:25:20 > 0:25:22'on land,
0:25:22 > 0:25:23'and in the sky.
0:25:26 > 0:25:28'It really is my ultimate bird skeleton.'
0:25:32 > 0:25:36This little bird is such a paradox.
0:25:36 > 0:25:40It doesn't look as though it's very good at flying,
0:25:40 > 0:25:43and it doesn't look as though it's very good at swimming.
0:25:43 > 0:25:46But actually, this wonderful
0:25:46 > 0:25:50little interesting bird is both.
0:25:50 > 0:25:51'It's the guillemot.'
0:25:59 > 0:26:03'Guillemots live in large colonies on coastal cliffs.
0:26:06 > 0:26:09'Although they appear quite clumsy when taking off,
0:26:09 > 0:26:12'they're surprisingly good flyers,
0:26:12 > 0:26:16'capable of speeds of over 40mph.
0:26:21 > 0:26:26'And when they hit the water, their versatility really becomes apparent.
0:26:29 > 0:26:33'You'd think that their wings would be too cumbersome for diving,
0:26:33 > 0:26:37'but they swim with them half closed to reduce turbulence.
0:26:38 > 0:26:43'Guillemots can reach depths of over 150m.
0:26:43 > 0:26:47'This puts them amongst the deepest divers of all birds.
0:26:51 > 0:26:55'To really understand how they can be both skilful flyers
0:26:55 > 0:26:59'and impressive divers, you have to look at their bones.'
0:27:02 > 0:27:05Now, we've got a specialist flyer, the pigeon here,
0:27:05 > 0:27:07and we've got a specialist diver, the penguin.
0:27:07 > 0:27:10We've just got a few bones of the wing,
0:27:10 > 0:27:11that's enough.
0:27:11 > 0:27:15I think probably my favourite way to look at the differences here,
0:27:15 > 0:27:17and I love this technique,
0:27:17 > 0:27:20is to get a torch and shine it through the bones.
0:27:20 > 0:27:23You can see, if I shine it through the pigeon,
0:27:23 > 0:27:25this light shines through them perfectly,
0:27:25 > 0:27:29and you can really see, they're almost translucent.
0:27:29 > 0:27:32And this is what you'd associate with an animal that has
0:27:32 > 0:27:35lightweight bones which is essential for flight.
0:27:35 > 0:27:38If we go the opposite end of the scale and look at the wing bones
0:27:38 > 0:27:41from a penguin, you can barely see that light coming through.
0:27:41 > 0:27:45This is because they're incredibly dense bones to counteract buoyancy.
0:27:45 > 0:27:47Where will the guillemot fit?
0:27:47 > 0:27:49You can see, if you have a good look with a torch,
0:27:49 > 0:27:52it's somewhere between the two, it's a happy medium.
0:27:52 > 0:27:55You can see through slightly, but it's much more dense.
0:27:55 > 0:27:56It's not as dense as the penguin,
0:27:56 > 0:27:59but it's definitely more dense than the pigeon.
0:28:00 > 0:28:02'This amazing adaptation,
0:28:02 > 0:28:04'bones light enough to fly
0:28:04 > 0:28:07'and yet heavy enough to dive,
0:28:07 > 0:28:11'makes this one of the most impressive birds on the planet.'
0:28:12 > 0:28:15My little guillemot here really is the ultimate flyer,
0:28:15 > 0:28:19and for that reason, I'm in love with this bird, it's brilliant.
0:28:24 > 0:28:27'The skeleton has enabled birds to conquer the sky.
0:28:30 > 0:28:31'And also the land.
0:28:32 > 0:28:34'And even the sea.
0:28:36 > 0:28:41'Next time, we'll discover how bones have evolved to detect prey...'
0:28:41 > 0:28:43What you've got, in effect,
0:28:43 > 0:28:46is a 40 or 50-tonne rigid swimming radar gun.
0:28:46 > 0:28:49'..and to sense the world around us.'
0:28:53 > 0:28:58These eyes are so large that each one is larger than
0:28:58 > 0:28:59the animal's own brain.