0:00:26 > 0:00:29Our world is covered in giants.
0:00:33 > 0:00:36The largest things that ever lived on this planet
0:00:36 > 0:00:39weren't the dinosaurs. They're not even blue whales.
0:00:39 > 0:00:41They're trees.
0:00:41 > 0:00:45These are Mountain Ash, the largest flowering plant in the world.
0:00:45 > 0:00:49They grow about a metre a year and these trees are 60, 70,
0:00:49 > 0:00:51even 80 metres high.
0:00:51 > 0:00:53But to get this big,
0:00:53 > 0:00:56you need to face some very significant physical challenges.
0:01:05 > 0:01:09These giants can live to well over 300 years old.
0:01:10 > 0:01:12But they don't keep growing forever.
0:01:14 > 0:01:18There are limits to how big each tree can get.
0:01:18 > 0:01:21As with all living things, the structure,
0:01:21 > 0:01:23form and function of these trees
0:01:23 > 0:01:28has been shaped by the process of evolution through natural selection.
0:01:28 > 0:01:31But evolution doesn't have a free hand.
0:01:31 > 0:01:35It is constrained by the universal laws of physics.
0:01:41 > 0:01:44Each tree has to support its mass
0:01:44 > 0:01:47against the downward force of Earth's gravity.
0:01:49 > 0:01:50At the same time,
0:01:50 > 0:01:53the trees rely on the strength of the interactions
0:01:53 > 0:01:58between molecules to raise a column of water from the ground
0:01:58 > 0:02:00up to the leaves in the canopy.
0:02:05 > 0:02:08And it's these fundamental properties of nature
0:02:08 > 0:02:13that act together to limit the maximum height of a tree,
0:02:13 > 0:02:18which theoretically lies somewhere in the region of 130 metres.
0:02:28 > 0:02:32With its forests and mountains...
0:02:32 > 0:02:34Oceans and deserts...
0:02:36 > 0:02:41I've come to Australia to explore the scale of life's sizes.
0:02:44 > 0:02:47I want to see how the laws of physics
0:02:47 > 0:02:51govern the lives of all living things.
0:02:51 > 0:02:54From the very biggest...
0:02:54 > 0:02:55to the very smallest.
0:02:58 > 0:03:01The size of life on Earth spans from the tallest tree,
0:03:01 > 0:03:06over 100 metres tall and with a mass of over 1,000 tonnes,
0:03:06 > 0:03:09to the smallest bacterium cell,
0:03:09 > 0:03:12with a length less than a millionth of a millimetre
0:03:12 > 0:03:16and a mass less than a million millionths of a gram.
0:03:16 > 0:03:21And that spans over 22 orders of magnitude in mass.
0:03:23 > 0:03:27I want to see how size influences the natural world.
0:03:31 > 0:03:33How do the physical forces of nature
0:03:33 > 0:03:37dictate the lives of the big and the small?
0:03:39 > 0:03:43Do organisms face different challenges at different scales?
0:03:45 > 0:03:50And do we all experience the world differently, based on our size?
0:03:52 > 0:03:53The size you are
0:03:53 > 0:03:56profoundly influences the way that you live your life.
0:03:56 > 0:03:59It selects from the properties of the natural world
0:03:59 > 0:04:01that most affect you.
0:04:01 > 0:04:05So, I suppose that whilst we all live on the same planet,
0:04:05 > 0:04:07we occupy different worlds.
0:04:32 > 0:04:34I'm heading out to the Neptune Islands,
0:04:34 > 0:04:37west of Adelaide in South Australia...
0:04:41 > 0:04:45in search of one of nature's largest killing machines.
0:04:51 > 0:04:54These beasts are feared around the world,
0:04:54 > 0:04:56a fear not helped by Hollywood filmmakers.
0:05:00 > 0:05:04I'm here to swim with great white sharks.
0:05:07 > 0:05:08ENGINE STARTS UP
0:05:13 > 0:05:16- How big... How wide can they open their jaw?- Three foot wide.
0:05:16 > 0:05:19- About three feet. - They can swallow a man whole.- Yes.
0:05:19 > 0:05:21So about three...
0:05:21 > 0:05:24Three foot wide, can swallow a man whole.
0:05:29 > 0:05:34The skipper has a special permit to use bait to lure the sharks in.
0:05:38 > 0:05:41The crew ready the cages.
0:05:57 > 0:06:01The last time I dived was in the marina in Brighton.
0:06:01 > 0:06:02I did see a fish.
0:06:02 > 0:06:04It was about that big.
0:06:06 > 0:06:09From that to the largest marine predator.
0:06:09 > 0:06:11CLEARS HIS THROAT
0:06:14 > 0:06:17As the sharks start to circle, it's time to get in.
0:06:29 > 0:06:31There he is. There he comes.
0:06:33 > 0:06:37Just look at that. He's just checking us out.
0:06:37 > 0:06:40Well, he's turning straight for us.
0:06:42 > 0:06:44Look at those teeth.
0:06:45 > 0:06:50Graceful, elegant thing. Shaped by natural selection.
0:06:50 > 0:06:54Brilliant at what it does, which is to eat things.
0:07:02 > 0:07:03HE LAUGHS
0:07:05 > 0:07:10Well, I never would've thought you could be that close to one of those.
0:07:16 > 0:07:20Great whites are highly evolved predators.
0:07:20 > 0:07:24Around two thirds of their brain is dedicated to their sense of smell.
0:07:26 > 0:07:32They can detect as little as one part per million blood.
0:07:32 > 0:07:36In this water, the tiniest speck of blood...
0:07:36 > 0:07:37will attract the shark.
0:07:40 > 0:07:44These fish can grow to a huge size.
0:07:44 > 0:07:47But still move with incredible speed and agility.
0:07:48 > 0:07:50They've been sculpted by evolution,
0:07:50 > 0:07:54acting within the bounds of the physical properties of water.
0:07:58 > 0:08:00Now, he's about five metres long.
0:08:00 > 0:08:02He weighs about a ton.
0:08:04 > 0:08:07And he's probably the most efficient predator on earth.
0:08:12 > 0:08:14When he's attacking,
0:08:14 > 0:08:17he can accelerate up to over 20 miles an hour.
0:08:17 > 0:08:21They can launch themselves straight out of the water.
0:08:21 > 0:08:22There he is! There he is.
0:08:29 > 0:08:31Whoa!
0:08:31 > 0:08:32Whoa!
0:08:34 > 0:08:37I felt the need to remove my hands.
0:08:50 > 0:08:56That was one of the most awe-inspiring sights I've ever seen.
0:08:56 > 0:09:01A great white, just straight in front of me with its mouth open.
0:09:03 > 0:09:07With the boat moored up, away from shark-infested waters,
0:09:07 > 0:09:09I want to explore why
0:09:09 > 0:09:13it's in our oceans that we find the biggest animals on Earth.
0:09:13 > 0:09:16From giant sharks to blue whales,
0:09:16 > 0:09:20the largest animals that have ever lived have lived in the sea.
0:09:21 > 0:09:24The reason why is down to physics.
0:09:25 > 0:09:28This is a container full of saltwater
0:09:28 > 0:09:30and I'm going to weigh it.
0:09:31 > 0:09:35You see, that says 25 kilograms there.
0:09:35 > 0:09:37That's actually its mass.
0:09:37 > 0:09:43Its weight is the force the Earth is exerting on it due to gravity,
0:09:43 > 0:09:45which is 25 times about ten,
0:09:45 > 0:09:48which is 250 kilogram metres per second squared.
0:09:48 > 0:09:53That might sound pedantic, but it's going to be important in a minute.
0:09:53 > 0:09:58See what happens if I lower this saltwater into the ocean.
0:10:02 > 0:10:07Its weight has effectively disappeared. It's effectively zero.
0:10:07 > 0:10:10Now, of course, gravity is still acting on this thing,
0:10:10 > 0:10:13so by the strictest sense of the word,
0:10:13 > 0:10:15it still has the same weight as it did up here,
0:10:15 > 0:10:18but Mr Archimedes told us
0:10:18 > 0:10:20that there's another force that's come into play.
0:10:20 > 0:10:22There's a force proportional
0:10:22 > 0:10:26to the weight of water that's been displaced by this thing
0:10:26 > 0:10:30and because this thing has essentially the same density as seawater,
0:10:30 > 0:10:32because it's made of seawater,
0:10:32 > 0:10:36then that force is equal and opposite to the force of gravity,
0:10:36 > 0:10:38and so they cancel,
0:10:38 > 0:10:41so it's effectively weightless
0:10:41 > 0:10:44and that is extremely important indeed
0:10:44 > 0:10:46for the animals that live in the ocean.
0:10:50 > 0:10:54The cells of all living things are predominantly made up of salty water
0:10:54 > 0:10:57so in the ocean, weight is essentially unimportant.
0:11:16 > 0:11:21Because of Archimedes' principle, the supportive nature of water
0:11:21 > 0:11:24releases organisms from the constraints of Earth's gravity,
0:11:24 > 0:11:28allowing the evolution of marine leviathans.
0:11:32 > 0:11:34But this comes at a cost.
0:11:34 > 0:11:37Water is 800 times denser than air
0:11:37 > 0:11:40and so whilst it provides support,
0:11:40 > 0:11:43it requires a huge amount of effort to move through it.
0:11:48 > 0:11:52Not only does the shark have to push the water out of the way,
0:11:52 > 0:11:54it also has to overcome drag forces
0:11:54 > 0:11:58created by the frictional contact with the water itself.
0:11:59 > 0:12:02The solution for the shark lies in its shape.
0:12:04 > 0:12:06If you look at him, that great white,
0:12:06 > 0:12:09he's got that distinctive streamlined shape.
0:12:10 > 0:12:15His maximum width is about a third of the way down his body,
0:12:15 > 0:12:19and that width itself should be around a quarter of the length.
0:12:21 > 0:12:27That ratio is set by the necessity for something that big
0:12:27 > 0:12:33to be able to swim effectively and quickly through this medium.
0:12:37 > 0:12:40This shape reduces drag forces to a minimum
0:12:40 > 0:12:45and optimises the way water flows around the shark's body.
0:12:45 > 0:12:50It is the result of evolution, shaped by the laws of physics.
0:12:54 > 0:12:56Whoa!
0:12:57 > 0:13:00HE LAUGHS
0:13:00 > 0:13:03That's cunning! That was straight out of Jaws!
0:13:11 > 0:13:14That streamlined shape of a shark
0:13:14 > 0:13:17is something that you see echoed throughout nature.
0:13:17 > 0:13:20I mean, think of a whale or a dolphin or a tuna,
0:13:20 > 0:13:24all that same torpedo-like shape,
0:13:24 > 0:13:27and that's because they're contending with problems that arise
0:13:27 > 0:13:29from the same laws of physics
0:13:29 > 0:13:34and convergent evolution has driven them to the same solution.
0:13:36 > 0:13:40For life in the sea, the evolution of giants is constrained
0:13:40 > 0:13:43directly by the physical properties of water.
0:13:46 > 0:13:48But out of the ocean,
0:13:48 > 0:13:52life now has to content with the full force of Earth's gravity.
0:13:52 > 0:13:54And it's this force of nature
0:13:54 > 0:13:58that dominates the lives of giants on land.
0:14:09 > 0:14:14This is the hot, dry outback north of Broken Hill in New South Wales.
0:14:18 > 0:14:20I'm here to explore how gravity,
0:14:20 > 0:14:25a force whose strength is governed by the mass of our whole planet,
0:14:25 > 0:14:30moulds, shapes and ultimately limits the size of life on land.
0:14:41 > 0:14:44I've come to track down one of Australia's most iconic animals...
0:14:46 > 0:14:48..the red kangaroo.
0:14:50 > 0:14:55Red kangaroos are Australia's largest native land mammal,
0:14:55 > 0:14:58one of 50 species of macropods,
0:14:58 > 0:15:00so-called on account of their large feet.
0:15:04 > 0:15:06- (WHISPERS)- There! There.
0:15:06 > 0:15:08There's two very close there.
0:15:16 > 0:15:19The kangaroos are the most remarkable of mammals
0:15:19 > 0:15:20because they hop.
0:15:20 > 0:15:23There's no record, even in the fossil record,
0:15:23 > 0:15:26of any other large animal that does that
0:15:26 > 0:15:29but it makes them very fast and efficient.
0:15:29 > 0:15:32When Joseph Banks, who's one of my scientific heroes,
0:15:32 > 0:15:37first arrived here with Captain Cook on the Endeavour in 1770,
0:15:37 > 0:15:39he wrote that "They move so fast
0:15:39 > 0:15:42"over the rocky, rough ground where they're found,
0:15:42 > 0:15:45"even my greyhound couldn't catch them."
0:15:45 > 0:15:48I mean, what was he doing with a greyhound?
0:15:52 > 0:15:54Kangaroos are herbivorous
0:15:54 > 0:15:57and scratch out a living feeding on grasses.
0:16:00 > 0:16:03While foraging, they move in an ungainly fashion,
0:16:03 > 0:16:07using their large, muscular tail like a fifth leg.
0:16:11 > 0:16:12But when they want to,
0:16:12 > 0:16:16these large marsupials can cover ground at considerable speeds.
0:16:19 > 0:16:20To take a leap,
0:16:20 > 0:16:25kangaroos have to work against the downward pull of Earth's gravity.
0:16:25 > 0:16:27This takes a lot of energy.
0:16:29 > 0:16:34As animals go faster, they tend to use more energy.
0:16:34 > 0:16:36Not so with the kangaroos.
0:16:40 > 0:16:45As the roos go faster, their energy consumption actually decreases.
0:16:48 > 0:16:50It then stays constant,
0:16:50 > 0:16:54even at sustained speeds of up to 40 kilometres per hour.
0:17:00 > 0:17:04This incredibly efficiency for such a large animal
0:17:04 > 0:17:06comes directly from the kangaroos' anatomy.
0:17:10 > 0:17:12Kangaroos move so efficiently
0:17:12 > 0:17:16because they have an ingenious energy storage mechanism.
0:17:16 > 0:17:19See, when something hits the ground after falling from some height,
0:17:19 > 0:17:22then it has energy that it needs to dissipate.
0:17:22 > 0:17:23If you're a rock...
0:17:25 > 0:17:28..that energy is dissipated as sound and a little bit of heat
0:17:28 > 0:17:30but if you're a tennis ball...
0:17:31 > 0:17:35..then some of that energy is reused because a tennis ball is elastic,
0:17:35 > 0:17:37it can deform, spring back,
0:17:37 > 0:17:40and use some of that energy to throw itself back into the air again.
0:17:42 > 0:17:44Well, a kangaroo is very similar.
0:17:44 > 0:17:47It has very elastic tendons in its legs,
0:17:47 > 0:17:51particularly its Achilles tendon and also the tendons in its tail,
0:17:51 > 0:17:55and they store energy and then they release it,
0:17:55 > 0:17:58supplementing the power of the muscles
0:17:58 > 0:18:00to bounce the kangaroo through the air.
0:18:00 > 0:18:05Now, an adult kangaroo is 85, 90 kilos,
0:18:05 > 0:18:07which is heavier than me,
0:18:07 > 0:18:13and when it's going at full speed, it can jump around nine metres.
0:18:13 > 0:18:15That's the distance from me...
0:18:16 > 0:18:17..to that car.
0:18:21 > 0:18:23The evolution of the ability to hop
0:18:23 > 0:18:28gives kangaroos a cheap and efficient way to move around.
0:18:28 > 0:18:30But not everything can move like a kangaroo.
0:18:32 > 0:18:35The red kangaroo is the largest animal in the world
0:18:35 > 0:18:37that moves in this unique way,
0:18:37 > 0:18:41hopping across the landscape at high speed,
0:18:41 > 0:18:46and there are reasons why there aren't giant hopping elephants
0:18:46 > 0:18:50or dinosaurs, and they're not really biological,
0:18:50 > 0:18:53it's not down to the details of evolution
0:18:53 > 0:18:56by natural selection or environmental pressures.
0:18:56 > 0:18:58The larger an animal gets,
0:18:58 > 0:19:03the more severe the restrictions on its body shape and its movements.
0:19:07 > 0:19:09To understand why this is the case,
0:19:09 > 0:19:13I want to explore what happens to the mass of a body
0:19:13 > 0:19:15when that body increases in size.
0:19:19 > 0:19:21Take a look at this block.
0:19:21 > 0:19:23Let's say it has width - one,
0:19:23 > 0:19:25length - one, and height - one,
0:19:25 > 0:19:29then its volume is one multiplied by one multiplied by one,
0:19:29 > 0:19:32which is one cubic...
0:19:32 > 0:19:34things, whatever the measurement is.
0:19:34 > 0:19:37Now, its mass is proportional to the volume,
0:19:37 > 0:19:41so we could say that the mass of this block is one unit as well.
0:19:41 > 0:19:44Let's say that we're going to double the size of this thing
0:19:44 > 0:19:47in the sense that we want to double its width,
0:19:47 > 0:19:50double its length,
0:19:50 > 0:19:53double its height.
0:19:53 > 0:19:56Then its volume is two multiplied by two multiplied by two,
0:19:56 > 0:19:59equals eight cubic things.
0:19:59 > 0:20:01Its volume has increased by a factor of eight,
0:20:01 > 0:20:05and so its mass has increased by a factor of eight as well.
0:20:07 > 0:20:11So although I've only doubled the size of the blocks,
0:20:11 > 0:20:13I've increased the total mass by eight.
0:20:14 > 0:20:16As things get bigger,
0:20:16 > 0:20:20the mass of a body goes up by the cube of the increase in size.
0:20:25 > 0:20:27Because of this scaling relationship,
0:20:27 > 0:20:31the larger you get, the greater the effect.
0:20:31 > 0:20:33As things get bigger,
0:20:33 > 0:20:37the huge increase in mass has a significant impact
0:20:37 > 0:20:41on the way large animals support themselves against gravity
0:20:41 > 0:20:43and how they move about.
0:20:46 > 0:20:50No matter how energy-efficient and advantageous it is
0:20:50 > 0:20:51to hop like a kangaroo,
0:20:51 > 0:20:55as you get bigger, it's just not physically possible.
0:20:58 > 0:21:03Going supersize on land comes with tremendous constraints attached.
0:21:06 > 0:21:09This is the left femur, the thigh bone
0:21:09 > 0:21:12of an extinct animal called a Diprotodon,
0:21:12 > 0:21:15which is the largest known marsupial ever to have existed.
0:21:15 > 0:21:18This would have stood as tall as me,
0:21:18 > 0:21:20it would have been four metres long,
0:21:20 > 0:21:23weighed between two and two-and-a-half tons,
0:21:23 > 0:21:24so the size of a rhino,
0:21:24 > 0:21:27and it's known that it was all over Australia,
0:21:27 > 0:21:30it was the big herbivore,
0:21:30 > 0:21:32and it got progressively bigger
0:21:32 > 0:21:36over the 25 million years that we have fossils for it,
0:21:36 > 0:21:39and then around 50,000 years ago,
0:21:39 > 0:21:42coincidentally, when humans arrived in Australia,
0:21:42 > 0:21:44the Diprotodon became extinct.
0:21:48 > 0:21:53The Diprotodon is thought to have looked like a giant wombat
0:21:53 > 0:21:55and being marsupials, the females
0:21:55 > 0:22:00would have carried their sheep-sized offspring in a huge pouch.
0:22:02 > 0:22:04To support their considerable bulk,
0:22:04 > 0:22:09the Diprotodon skeleton had to be very strong.
0:22:09 > 0:22:14This imposed significant constraints on the shape and size of its bones.
0:22:15 > 0:22:20This is the fever of the closest living relative of the Diprotodon.
0:22:20 > 0:22:23It's a wombat, which is an animal around the size of a small dog.
0:22:23 > 0:22:26And you see that superficially,
0:22:26 > 0:22:28the bones are very similar.
0:22:28 > 0:22:31But let me take a few measurements.
0:22:31 > 0:22:34The length of the Diprotodon femur
0:22:34 > 0:22:40is...what, around 75 cm.
0:22:40 > 0:22:45The length of the wombat femur is around 15 cm,
0:22:45 > 0:22:50so this is about five times the length of the wombat femur.
0:22:50 > 0:22:53But now look at the cross-sectional area.
0:22:53 > 0:22:57Assuming the bones are roughly circular in cross-section,
0:22:57 > 0:23:03we can calculate their area using pi multiplied by the radius squared.
0:23:03 > 0:23:04It turns out that
0:23:04 > 0:23:08although the Diprotodon femur is around five times longer,
0:23:08 > 0:23:11it has a cross-sectional area
0:23:11 > 0:23:1440 times that of the wombat femur.
0:23:18 > 0:23:22A bone's strength depends directly on its cross-sectional area.
0:23:24 > 0:23:29The Diprotodon needed thick leg bones, braced in a robust skeleton,
0:23:29 > 0:23:34just to provide enough strength to support the giant's colossal weight.
0:23:40 > 0:23:42As animals get more massive,
0:23:42 > 0:23:44the effect of gravity
0:23:44 > 0:23:47plays an increasingly restrictive role in their lives.
0:23:49 > 0:23:53The shape and form of their body is forced to change.
0:23:57 > 0:24:01If you look across the scale of Australian vertebrate life,
0:24:01 > 0:24:04you see a dramatic difference in bone thickness.
0:24:07 > 0:24:11This is a line of femur bones of animals of different sizes.
0:24:11 > 0:24:13We start with the smallest,
0:24:13 > 0:24:15one of the smallest marsupials in Australia,
0:24:15 > 0:24:18the marsupial mouse or the Antechinus.
0:24:18 > 0:24:22Then the next one is an animal known as the Potoroo.
0:24:22 > 0:24:25Again, it's a marsupial around about the size of a rabbit.
0:24:25 > 0:24:28Then we have the Tasmanian Devil,
0:24:28 > 0:24:29a wombat,
0:24:29 > 0:24:31a dingo,
0:24:31 > 0:24:34then the largest marsupial in Austria today,
0:24:34 > 0:24:36the red kangaroo.
0:24:36 > 0:24:40And this is the femur of the Diprotodon
0:24:40 > 0:24:44and then, here, the femur of a Rhoetosaurus,
0:24:44 > 0:24:48which was a sauropod dinosaur 17 metres long
0:24:48 > 0:24:51and weighing around 20 tons.
0:24:52 > 0:24:54And so, you see,
0:24:54 > 0:24:56as animals get larger,
0:24:56 > 0:25:01from the smallest marsupial mouse, all the way up to a dinosaur,
0:25:01 > 0:25:06the cross-sectional area of their bones increases enormously,
0:25:06 > 0:25:08just to support that increased mass.
0:25:13 > 0:25:15Being big and bulky,
0:25:15 > 0:25:18giants are more restricted as to the shape of their body
0:25:18 > 0:25:20and how they get about.
0:25:24 > 0:25:26That's why red kangaroos
0:25:26 > 0:25:29are the largest animals that can move in the way that they do.
0:25:31 > 0:25:35At a much greater size, their bones would be very heavy,
0:25:35 > 0:25:37have a greater risk of fracture,
0:25:37 > 0:25:41and they'd require far too much energy to move at high speeds.
0:25:44 > 0:25:48It's ultimately the strength of Earth's gravity
0:25:48 > 0:25:49that limits the size
0:25:49 > 0:25:52and the manoeuvrability of land-based giants.
0:25:54 > 0:25:57But for the bulk of life on land,
0:25:57 > 0:26:01gravity is not the defining force of nature.
0:26:13 > 0:26:19At small scales, living things seem to bend the laws of physics,
0:26:19 > 0:26:21which is, of course, not possible.
0:26:22 > 0:26:26The world of the small is often hidden from our view,
0:26:26 > 0:26:29but there are ways to draw out these tiny creatures.
0:26:34 > 0:26:36This is the domain of the insects.
0:26:40 > 0:26:43These animals can clearly do things I can't do
0:26:43 > 0:26:46and appear to have superpowers.
0:26:47 > 0:26:49They can walk up walls,
0:26:49 > 0:26:52jump many times their own height,
0:26:52 > 0:26:55and can lift many times their own weight.
0:26:57 > 0:27:01There are over 900,000 known species of insects on the planet.
0:27:01 > 0:27:05That's over 75% of all animal species.
0:27:05 > 0:27:07Some biologists think that
0:27:07 > 0:27:11there may be an order of magnitude more yet to be discovered.
0:27:11 > 0:27:14That would be ten million species,
0:27:14 > 0:27:16and they're very small,
0:27:16 > 0:27:19so you can fit a lot of them on Planet Earth at any one time.
0:27:19 > 0:27:22In fact, it's estimated there are
0:27:22 > 0:27:27over ten billion billion individual insects alive today.
0:27:33 > 0:27:35Of all the insect groups,
0:27:35 > 0:27:38it's the beetles, or coleoptera,
0:27:38 > 0:27:40that have the greatest number of species.
0:27:45 > 0:27:47The biologist JBS Haldane said that
0:27:47 > 0:27:51if one could conclude as to the nature of the Creator
0:27:51 > 0:27:52from a study of creation,
0:27:52 > 0:27:56then it would appear that God has an inordinate fondness
0:27:56 > 0:27:58for stars and beetles.
0:28:07 > 0:28:11With so much variation in colour, form and function,
0:28:11 > 0:28:14beetles have fascinated naturalists for centuries.
0:28:16 > 0:28:21Each species is wonderfully adapted to their own unique niche.
0:28:36 > 0:28:41This is the beginnings of biology as a science that you see here,
0:28:41 > 0:28:44it's this desire to collect and classify,
0:28:44 > 0:28:48which then, over time, becomes the desire to explain and understand.
0:28:52 > 0:28:54I'm going to take a picture.
0:29:01 > 0:29:03Here in the suburbs of Brisbane,
0:29:03 > 0:29:07every February, there's an invasion of beetles.
0:29:08 > 0:29:13The rules governing their lives play out very differently to ours.
0:29:16 > 0:29:21This is the Rhinoceros Beetle, named for obvious reasons.
0:29:21 > 0:29:22But actually, it's only the males
0:29:22 > 0:29:25that have the distinctive horns on their heads.
0:29:27 > 0:29:31These beetles spend much of their lives underground as larvae,
0:29:31 > 0:29:35but then emerge en masse as adults to find a mate and breed.
0:29:36 > 0:29:40Much of this time, the males spend fighting over females.
0:29:49 > 0:29:53See that distinctive posture
0:29:53 > 0:29:54that he's adopting there?
0:29:54 > 0:29:55That's because I think
0:29:55 > 0:29:59he's seeing his reflection in the camera lens, and so he rears up.
0:29:59 > 0:30:02Look at that! He's trying to scare himself off.
0:30:04 > 0:30:05Ha-ha-ha!
0:30:07 > 0:30:08INSECT BRISTLES
0:30:09 > 0:30:11You also heard that hissing sound.
0:30:11 > 0:30:17That's him contract in his abdomen which again is a defensive
0:30:17 > 0:30:21posture that he adopts to scare other males.
0:30:21 > 0:30:22INSECT HISSES
0:30:24 > 0:30:29Gramme for gramme, these insects are among the strongest animals alive.
0:30:32 > 0:30:36I can demonstrate that I just getting hold of the top of his head.
0:30:39 > 0:30:43It doesn't hurt him at all, but watch what he is able to do.
0:30:48 > 0:30:50Look at that.
0:30:50 > 0:30:52So he is hanging on to this branch,
0:30:52 > 0:30:54which is many times his own bodyweight.
0:30:56 > 0:30:58Absolutely no distress at all.
0:31:01 > 0:31:03As things get smaller, it is
0:31:03 > 0:31:07a rule of nature that they inevitably get stronger.
0:31:08 > 0:31:10The reason is quite simple.
0:31:10 > 0:31:13Small things have relatively large muscles compared
0:31:13 > 0:31:17to their tiny body mass and this makes them very powerful.
0:31:25 > 0:31:29The beetles also appear to have a cavalier attitude to
0:31:29 > 0:31:30the effects of gravity.
0:31:34 > 0:31:36They fight almost like sumo wrestlers,
0:31:36 > 0:31:40their aim is to throw each other off the branch.
0:31:42 > 0:31:45If they should fall...
0:31:45 > 0:31:49they just bounce and walk off.
0:31:52 > 0:31:56If I fail a similar distance relative to my size, I'd break.
0:31:59 > 0:32:02So why does size make such a difference?
0:32:09 > 0:32:12Time for a bit of fundamental physics.
0:32:12 > 0:32:16All things fall at the same rate under gravity.
0:32:16 > 0:32:18That's because they they're following geodesics
0:32:18 > 0:32:21through curved space-time, but that's not important.
0:32:21 > 0:32:24The important thing for biology is that although everything falls at
0:32:24 > 0:32:30the same rate, it doesn't meet the same fate when it hits the ground.
0:32:35 > 0:32:37A grape bounces.
0:32:42 > 0:32:46A melon...
0:32:49 > 0:32:51Doesn't bounce.
0:32:54 > 0:32:58The reasons for that are quite complex actually.
0:32:58 > 0:33:04First of all, the grape has a larger surface area in relation
0:33:04 > 0:33:08to its volume and therefore its mass than the melon.
0:33:08 > 0:33:11Although, in a vacuum, if you took away the air,
0:33:11 > 0:33:14they would both fall at the same rate. Actually, in reality,
0:33:14 > 0:33:17the grape falls slower than the melon.
0:33:17 > 0:33:21Also, the melon is more massive so it has more kinetic energy
0:33:21 > 0:33:24when it hits the ground. Remember physics class.
0:33:24 > 0:33:28Kinetic energy is ½ MV squared,
0:33:28 > 0:33:30so you reduce M, you reduce the energy.
0:33:30 > 0:33:33The upshot of that is that the melon has a lot more energy
0:33:33 > 0:33:35when it hits the ground.
0:33:35 > 0:33:38It has to dissipate it in some way and it dissipates it by exploding.
0:33:44 > 0:33:48The influence of Earth's gravity in your life becomes progressively
0:33:48 > 0:33:50diminished the smaller you get.
0:33:59 > 0:34:01For life at the small scale,
0:34:01 > 0:34:06a second fundamental force of nature starts to dominate.
0:34:06 > 0:34:11And it's this that explains many of those apparent superpowers.
0:34:13 > 0:34:19For me, the force of gravity is a thing that defines my existence.
0:34:19 > 0:34:22It's the force that I really feel the effects of.
0:34:23 > 0:34:25But there are other forces at work.
0:34:25 > 0:34:29For example if I lick my finger and wet it, I can pick up a piece
0:34:29 > 0:34:34of paper and can hold up against the downward pull of gravity.
0:34:34 > 0:34:38That's because the force of electromagnetism is important.
0:34:38 > 0:34:42In fact, it is the cohesive forces between water molecules
0:34:42 > 0:34:44and the molecules that make up my finger
0:34:44 > 0:34:47and the molecules that make up the paper,
0:34:47 > 0:34:51that are dominating this particular situation.
0:34:51 > 0:34:54That's why this piece of paper doesn't fall to the floor.
0:34:54 > 0:34:57Many insects can use a similar effect.
0:34:58 > 0:35:00Take a common fly for example.
0:35:06 > 0:35:09Their feet have especially enlarged pads onto which
0:35:09 > 0:35:11they secrete a sticky fluid.
0:35:13 > 0:35:17And that allows them to adhere to rather slippery
0:35:17 > 0:35:20surfaces like the glass of this jam jar.
0:35:20 > 0:35:24It allows them to do things that for me would be absolutely impossible.
0:35:24 > 0:35:28It's all down to the relative influence of the different
0:35:28 > 0:35:30forces of nature on the animal.
0:35:34 > 0:35:39So the capacity to walk up walls and fall from a great height without
0:35:39 > 0:35:44breaking, plus supers trength, are not super powers at all.
0:35:46 > 0:35:49They're just abilities gained naturally by animals
0:35:49 > 0:35:52that are small and lightweight.
0:35:55 > 0:35:59But this is just the beginning of my journey into the world of the small.
0:36:02 > 0:36:05Down at the very small scale, it becomes possible to live
0:36:05 > 0:36:09within the lives of other individuals, worlds within worlds.
0:36:13 > 0:36:15But just how small can animals get?
0:36:28 > 0:36:32This macadamia nut plantation, an hour outside of Brisbane,
0:36:32 > 0:36:36is home to one of the very smallest members of the animal kingdom.
0:36:45 > 0:36:47These are a species of micro-hymenoptera
0:36:47 > 0:36:48known as Trichogramma.
0:36:50 > 0:36:56They're basically very small wasps and when I say small,
0:36:56 > 0:36:58I mean small.
0:36:58 > 0:37:03Can you see that? They're like specks of dust.
0:37:03 > 0:37:06They're less than half a millimetre long,
0:37:06 > 0:37:08but each one of those is a wasp.
0:37:09 > 0:37:13It's got compound eyes, six legs and wings.
0:37:13 > 0:37:18They've even got a little stripe on their abdomen.
0:37:19 > 0:37:23And they're very precisely adapted to a specific evolutionary niche.
0:37:25 > 0:37:29The Trichogramma wasps may be small, but they're very useful.
0:37:30 > 0:37:34Theyr're natural parasites of an insect pest species
0:37:34 > 0:37:37called the nut borer moth which attacks the macadamia nuts.
0:37:43 > 0:37:48The micro-wasps lay their eggs inside the eggs of the moths,
0:37:48 > 0:37:50killing the developing moth larvae.
0:37:53 > 0:37:56What you're seeing here is the surface of the macadamia nut
0:37:56 > 0:38:01and here's a small cluster of moth eggs and there,
0:38:01 > 0:38:04you see the wasp is walking over the eggs.
0:38:04 > 0:38:07They're almost pacing out the size to see
0:38:07 > 0:38:12whether the eggs are suitable for their eggs to be laid inside.
0:38:12 > 0:38:17And if we're lucky, there you go, you see that...
0:38:17 > 0:38:19That...
0:38:19 > 0:38:21There we go.
0:38:23 > 0:38:28The wasps emerge just nine days later as full-grown adults.
0:38:29 > 0:38:33At this scale, they live a very sticky world,
0:38:33 > 0:38:37dominated by strong intermolecular forces.
0:38:38 > 0:38:42To them, even the air is a thick fluid through which
0:38:42 > 0:38:45they essentially swim, using paddle-like wings.
0:38:48 > 0:38:53Incredibly, these tiny animals can move about across several trees,
0:38:53 > 0:38:54seeking out the moth eggs.
0:38:57 > 0:38:59But what I find more remarkable
0:38:59 > 0:39:04is that they do all this operating with very restricted brain power.
0:39:05 > 0:39:09One of the limiting factors that determines the minimum
0:39:09 > 0:39:13size of insects is the volume of their central nervous system.
0:39:13 > 0:39:17In other words, the processing power you can fit inside their bodies
0:39:17 > 0:39:20and these little wasps are pretty much at their limit.
0:39:20 > 0:39:25They've less than 10,000 neurons in their whole nervous system.
0:39:25 > 0:39:26To put it into perspective,
0:39:26 > 0:39:30most tiny insects have 100 times that many, but that's still
0:39:30 > 0:39:34enough to allow them to exhibit quite complex behaviour.
0:39:36 > 0:39:39These micro-wasps exist at almost the minimum possible size
0:39:39 > 0:39:42for multicellular animals.
0:39:42 > 0:39:48But the scale of life on our planet gets much, much smaller.
0:39:48 > 0:39:50The wasps are giants
0:39:50 > 0:39:55compared to life at the very limit of size on earth.
0:40:07 > 0:40:11The smallest organisms on our planet are also our oldest
0:40:11 > 0:40:13and most abundant type of lifeforms.
0:40:18 > 0:40:21These weird, rocky blobs in the shallows of Lake Clifton,
0:40:21 > 0:40:25just south of Perth, are made by bacteria.
0:40:30 > 0:40:33These mounds are called thrombolites,
0:40:33 > 0:40:35on account of their clotted structure,
0:40:35 > 0:40:37and they're built up over centuries
0:40:37 > 0:40:41by colonies of microscopic bacterial cells.
0:40:43 > 0:40:46Although these colonies are rare, by most definitions,
0:40:46 > 0:40:50bacteria are THE dominant form of life on our planet.
0:40:50 > 0:40:55On every surface across every landscape, you find bacteria.
0:40:55 > 0:40:58In fact, numerically speaking, then there are more bacteria
0:40:58 > 0:41:03living on and inside my body than there are human cells.
0:41:04 > 0:41:07Bacteria come in many shapes and forms
0:41:07 > 0:41:11and are not actually animals or plants,
0:41:11 > 0:41:14instead sitting in their own unique taxonomic kingdom.
0:41:16 > 0:41:18Compared to the cells we're made of,
0:41:18 > 0:41:24bacteria are structurally much simpler and far, far smaller.
0:41:24 > 0:41:28Bacteria are typically around two microns in size.
0:41:28 > 0:41:33That's two millionths of a metre, which is very hard to picture
0:41:33 > 0:41:36but it means that you could fit around half a million of them
0:41:36 > 0:41:39on the head of a pin or, to look at it another way,
0:41:39 > 0:41:42if I took a single bacterium and scaled it up to
0:41:42 > 0:41:48the size of this coin, then I would be 25 kilometres high.
0:41:48 > 0:41:49SPLASH
0:41:51 > 0:41:54Bacterial-type organisms were the first life on Earth
0:41:54 > 0:41:57and they've dominated our planet ever since.
0:41:58 > 0:42:02Excluding viruses, which by most definitions are not alive,
0:42:02 > 0:42:06bacteria are the smallest free-living lifeforms we know of.
0:42:07 > 0:42:12But what ultimately puts the limit on the smallest size of life?
0:42:13 > 0:42:17Single-cell life needs to be big enough to accommodate all
0:42:17 > 0:42:20the molecular machinery of life
0:42:20 > 0:42:24and that size ultimately depends on the basic laws of physics.
0:42:24 > 0:42:27It depends on the size of molecules which
0:42:27 > 0:42:29depends on the size of atoms
0:42:29 > 0:42:32which depends on fundamental properties of the universe
0:42:32 > 0:42:35like the strength of the force of electromagnetism
0:42:35 > 0:42:38and the mass of an electron.
0:42:38 > 0:42:43And when you do those calculations, you find out that the minimum size
0:42:43 > 0:42:46of a free-living organism should be around 200 nanometres
0:42:46 > 0:42:51which is around 200 billionths of a metre.
0:42:51 > 0:42:52And that should be universal,
0:42:52 > 0:42:55it shouldn't only apply to life on Earth
0:42:55 > 0:42:58but it should apply to any carbon-based life
0:42:58 > 0:43:00anywhere in the universe
0:43:00 > 0:43:05because it depends on fundamental properties of the universe.
0:43:14 > 0:43:19From the smallest bacterium to the largest tree,
0:43:19 > 0:43:22it's your size that determines how the laws of physics
0:43:22 > 0:43:27govern your life. Gravity imposes itself on the large,
0:43:27 > 0:43:32and the electromagnetic force rules the world of the small.
0:43:36 > 0:43:39But the consequences of scale for life on Earth
0:43:39 > 0:43:42extend beyond dictating the relationship
0:43:42 > 0:43:44you have with the world around you.
0:43:46 > 0:43:52Your size also influences how energy itself flows through your body.
0:43:59 > 0:44:03BATS SQUEAK FAINTLY
0:44:08 > 0:44:11These are southern bent-wing bats...
0:44:12 > 0:44:15..one of the rarest bat species in Australia.
0:44:18 > 0:44:21Every evening, they emerge in their thousands
0:44:21 > 0:44:24from this cave, in order to feed.
0:44:26 > 0:44:30When fully grown, these bats are just 5.5cm long,
0:44:30 > 0:44:33and weigh around 18 grams.
0:44:33 > 0:44:39Because of their size, they face a constant struggle to stay alive.
0:44:42 > 0:44:45BATS SQUEAK, CRICKETS CHIRP
0:44:47 > 0:44:50We're using a thermal camera here to look at the bats,
0:44:50 > 0:44:53and you can see that they appear as streaks across the sky.
0:44:53 > 0:44:55They appear as brightly as me -
0:44:55 > 0:44:58that's because they're roughly the same temperature as me.
0:44:58 > 0:45:00They're known as endotherms -
0:45:00 > 0:45:04animals that maintain their body temperature.
0:45:04 > 0:45:06And that takes a lot of effort.
0:45:06 > 0:45:08These bats have to eat something like
0:45:08 > 0:45:11three-quarters of their own body weight every night,
0:45:11 > 0:45:15and a lot of that energy goes into maintaining their temperature.
0:45:17 > 0:45:19As with all living things,
0:45:19 > 0:45:24the bats eat to provide energy to power their metabolism.
0:45:24 > 0:45:25Although, like us,
0:45:25 > 0:45:28they have a high body temperature when they're active,
0:45:28 > 0:45:33keeping warm is a considerable challenge, on account of their size.
0:45:36 > 0:45:41The bats lose heat mostly through the surface of their bodies.
0:45:42 > 0:45:45But because of simple laws governing the relationship
0:45:45 > 0:45:48between the surface area of a body and its volume,
0:45:48 > 0:45:51being small creates a problem.
0:45:51 > 0:45:53BATS SQUEAK
0:45:53 > 0:45:56So, let's look at our blocks again,
0:45:56 > 0:45:58but this time for surface area to volume.
0:45:58 > 0:45:59Here's a big thing -
0:45:59 > 0:46:02it's made of eight blocks so its volume is eight units,
0:46:02 > 0:46:07and its surface area is two by two on each side, so that's four,
0:46:07 > 0:46:10multiplied by the six faces is 24.
0:46:10 > 0:46:14so, the surface area to volume ratio is 24 to eight,
0:46:14 > 0:46:17which is 3:1.
0:46:17 > 0:46:20Now, look at a smaller thing. This is one block,
0:46:20 > 0:46:22so its volume is one unit.
0:46:22 > 0:46:26Its surface area is one by one by one, six times, so it's six.
0:46:26 > 0:46:32So, this has a surface area to volume ratio of 6:1.
0:46:32 > 0:46:35So, as you go from big to small,
0:46:35 > 0:46:39your surface area to volume ratio increases.
0:46:40 > 0:46:43Small animals, like bats,
0:46:43 > 0:46:46have a huge surface area compared to their volume.
0:46:46 > 0:46:50As a result, they naturally lose heat at a very high rate.
0:46:52 > 0:46:56To help offset the cost of losing so much energy in the form of heat,
0:46:56 > 0:47:00the bats are forced to maintain a high rate of metabolism.
0:47:00 > 0:47:04They breathe rapidly, their little heart races,
0:47:04 > 0:47:07and they have to eat a huge amount.
0:47:07 > 0:47:10So, a bat's size clearly affects
0:47:10 > 0:47:13the speed at which it lives its life.
0:47:21 > 0:47:23Right across the natural world,
0:47:23 > 0:47:27the size you are has a profound effect on your metabolic rate -
0:47:27 > 0:47:30or your "speed of life".
0:47:32 > 0:47:36- EXTREMELY FAST HEARTBEAT - For Australia's small marsupial mouse,
0:47:36 > 0:47:39even at rest, his heart is racing away.
0:47:41 > 0:47:44- SLOWER HEARTBEAT - For the fox-sized Tasmanian devil,
0:47:44 > 0:47:46he ticks along at a much slower rate.
0:47:47 > 0:47:51And then there's me, living life at a languid 60 beats a minute.
0:47:54 > 0:47:56Looking beyond heart rate,
0:47:56 > 0:48:01your size influences the amount of energy you need to consume,
0:48:01 > 0:48:04and the rate at which you need to consume it.
0:48:06 > 0:48:09Bigger bodies have more cells to feed.
0:48:09 > 0:48:12So, you might expect that the total amount of energy needed
0:48:12 > 0:48:16goes up at the same rate as any increase in size.
0:48:18 > 0:48:20But that's not what happens.
0:48:24 > 0:48:28If you plot the amount of energy an animal uses against its mass,
0:48:28 > 0:48:34for a huge range of sizes, from animals as small as flies,
0:48:34 > 0:48:37and even smaller, all the way up to whales,
0:48:37 > 0:48:40then you DO get a straight line, but the slope
0:48:40 > 0:48:45is less than one. So, that implies that gramme for gramme,
0:48:45 > 0:48:49large animals use less energy than small animals.
0:48:52 > 0:48:55This relationship between metabolism and size
0:48:55 > 0:48:58significantly affects the amount of food
0:48:58 > 0:49:02larger animals have to consume to stay alive.
0:49:05 > 0:49:10Now, if my metabolic rate scaled one-to-one with that of a mouse,
0:49:10 > 0:49:14then I would need to eat about four kilograms of food a day.
0:49:14 > 0:49:19In my language, that's around 67,000 kilojoules of energy,
0:49:19 > 0:49:22which more colloquially is 16,000 calories.
0:49:22 > 0:49:25That is eight times the amount that I take in
0:49:25 > 0:49:28on average on a daily basis.
0:49:30 > 0:49:33Each of the cells in my body requires less energy
0:49:33 > 0:49:37than the equivalent cells in a smaller-sized mammal.
0:49:40 > 0:49:44The reason why this should be so is not fully understood.
0:49:44 > 0:49:48It's also not clear whether this rule of nature
0:49:48 > 0:49:50gives an advantage to big things,
0:49:50 > 0:49:54or is actually a constraint placed on larger animals.
0:49:56 > 0:49:58Take the relationship between
0:49:58 > 0:50:01an animal's surface area and its volume.
0:50:02 > 0:50:06Big animals have a much smaller surface area to volume ratio
0:50:06 > 0:50:10than small animals, and that means that their rate of heat loss
0:50:10 > 0:50:11is much smaller.
0:50:11 > 0:50:15And that means that there's an opportunity there for large animals.
0:50:15 > 0:50:18They don't have to eat as much food to stay warm,
0:50:18 > 0:50:22and therefore they can afford a lower metabolic rate.
0:50:25 > 0:50:27Now this helps explain the lives of large,
0:50:27 > 0:50:32warm-blooded endotherms, like birds and mammals,
0:50:32 > 0:50:35but doesn't hold so well for large ectotherms,
0:50:35 > 0:50:38life's cold-blooded giants.
0:50:41 > 0:50:44Now, there's another theory that says that it wasn't really
0:50:44 > 0:50:46an evolutionary opportunity
0:50:46 > 0:50:49that large animals took to lower their metabolic rate.
0:50:49 > 0:50:52It was forced on them. It was a constraint, if you like.
0:50:52 > 0:50:56The capillaries, the supply network to cells,
0:50:56 > 0:51:00branches in such a way that it gets more and more difficult
0:51:00 > 0:51:03to get oxygen and nutrients to cells in a big animal
0:51:03 > 0:51:05than in a small animal.
0:51:05 > 0:51:10Therefore, those cells must run at a lower rate.
0:51:10 > 0:51:13They must have a lower metabolic rate.
0:51:17 > 0:51:19Or it could just be that as you get bigger,
0:51:19 > 0:51:23then more of your mass is taken up by the stuff that supports you,
0:51:23 > 0:51:27and support structures, like bones, are relatively inert.
0:51:27 > 0:51:29They don't use much energy.
0:51:32 > 0:51:35But whatever the reason, it's certainly true to say
0:51:35 > 0:51:39that the only way that large animals can exist on planet Earth
0:51:39 > 0:51:43is to operate at a reduced metabolic rate.
0:51:45 > 0:51:47If this wasn't the case,
0:51:47 > 0:51:51the maximum size of a warm-blooded endotherm like me or you
0:51:51 > 0:51:54would be around that of a goat.
0:51:55 > 0:51:59And cold-blooded animals, or ectotherms like dinosaurs,
0:51:59 > 0:52:01could only get as big as a pony.
0:52:02 > 0:52:05Any bigger, and giants would simply overheat.
0:52:08 > 0:52:12Now, there's one last consequence of all these scaling laws
0:52:12 > 0:52:16that I suspect you'll care about more than anything else,
0:52:16 > 0:52:19and it's this - there's a strong correlation
0:52:19 > 0:52:23between the effective cellular metabolic rate of an animal
0:52:23 > 0:52:26and its lifespan. In other words,
0:52:26 > 0:52:30as things get bigger, they tend to live longer.
0:52:45 > 0:52:49To explore this connection between size and longevity,
0:52:49 > 0:52:52I've left the mainland behind.
0:52:52 > 0:52:54For my final destination,
0:52:54 > 0:52:57I've come to one of Australia's remotest outposts.
0:53:02 > 0:53:07Named Christmas Island when it was spotted on Christmas Day in 1643,
0:53:07 > 0:53:13this isolated lump of rock in the Indian Ocean is a land of crabs.
0:53:26 > 0:53:31And in their midst lurks a giant wonder of the natural world.
0:53:35 > 0:53:37This is a Christmas Island robber crab,
0:53:37 > 0:53:40the largest land crab anywhere on the planet.
0:53:40 > 0:53:44These things can grow to around 50 centimetres in length,
0:53:44 > 0:53:47they can weigh over four kilograms,
0:53:47 > 0:53:52and they are supremely adapted as an adult to life on land.
0:53:53 > 0:53:55They can even climb trees.
0:53:58 > 0:54:00Over the years, the crabs have become
0:54:00 > 0:54:03well adapted to human co-habitation.
0:54:05 > 0:54:07These things are called robber crabs
0:54:07 > 0:54:12because they have a reputation for curiosity and for stealing things,
0:54:12 > 0:54:14anything that isn't bolted down.
0:54:14 > 0:54:20They'll steal food and cameras if they can get half a chance.
0:54:30 > 0:54:34These giants live on a diet of seeds and fruit,
0:54:34 > 0:54:37and occasionally other small crabs.
0:54:38 > 0:54:40Their large, powerful claws mean
0:54:40 > 0:54:43they can also rip open fallen coconuts.
0:54:45 > 0:54:49They're really quite a menacing animal, actually, for a crab!
0:54:52 > 0:54:54What's wonderful about these crabs
0:54:54 > 0:54:57is that they live through a range of scales.
0:54:57 > 0:54:59At different times of their lives,
0:54:59 > 0:55:02they have a completely different relationship
0:55:02 > 0:55:06with the world around them, simply down to their size.
0:55:07 > 0:55:10Throughout their lives, robber crabs take on many different forms.
0:55:10 > 0:55:13They begin their lives as small larvae,
0:55:13 > 0:55:17swept around by the ocean currents, and as they grow,
0:55:17 > 0:55:20some of them get swept up onto the beaches of Christmas Island,
0:55:20 > 0:55:25where they find a shell, because they are, in fact, hermit crabs.
0:55:25 > 0:55:27They live inside their shell for a while,
0:55:27 > 0:55:30they continue to grow, and eventually, as adults,
0:55:30 > 0:55:33they roam the forests like this chap here.
0:55:33 > 0:55:39So these crabs, over that lifespan, inhabit many different worlds.
0:55:42 > 0:55:45On land, the adults continue to grow
0:55:45 > 0:55:48and now have to support their weight against gravity.
0:55:50 > 0:55:53Compared to the smaller crabs whizzing around,
0:55:53 > 0:55:56these giants move about much more slowly,
0:55:56 > 0:55:59but they also live far longer.
0:56:02 > 0:56:05Of all the species of land crab here on Christmas Island,
0:56:05 > 0:56:07robber crabs are not only the biggest,
0:56:07 > 0:56:09they're also the longest-living.
0:56:09 > 0:56:13So this chap here is probably about as old as me,
0:56:13 > 0:56:18and he might live to 60, 70, even 80 years old.
0:56:20 > 0:56:23Because of the robber crab's overall body size,
0:56:23 > 0:56:27its individual cells use less energy
0:56:27 > 0:56:29and they run at a slower rate
0:56:29 > 0:56:34than the cells of their much smaller, shorter-lived cousins.
0:56:37 > 0:56:40The pace of life is slower for robber crabs,
0:56:40 > 0:56:43and it's this that's thought to allow them
0:56:43 > 0:56:46to live to a ripe old age.
0:56:53 > 0:56:57Your size influences every aspect of your life...
0:57:00 > 0:57:02..from the way you were built...
0:57:04 > 0:57:07..to the way you move...
0:57:08 > 0:57:11..and even how long you live.
0:57:12 > 0:57:17Your size dictates how you interact with the universal laws of nature.
0:57:20 > 0:57:22So there's a minimum size,
0:57:22 > 0:57:26which is set ultimately by the size of atoms and molecules,
0:57:26 > 0:57:29the fundamental building blocks of the universe.
0:57:31 > 0:57:34And there's a maximum size which, certainly on land,
0:57:34 > 0:57:37is set by the size and the mass of our planet,
0:57:37 > 0:57:42because it's gravity that restricts the emergence of giants.
0:57:44 > 0:57:47But within those constraints, evolution has conspired to produce
0:57:47 > 0:57:51a huge range in size of animals and plants,
0:57:51 > 0:57:56each beautifully adapted to exploit the niches available to them.
0:57:59 > 0:58:03Your size influences your form and constriction.
0:58:03 > 0:58:06It determines how you experience the world,
0:58:06 > 0:58:10and ultimately, how long you have to enjoy it.
0:58:34 > 0:58:37Subtitles by Red Bee Media Ltd