BBC.Wonders.of.Life.4of5.Size.Matters
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00:00Our world is covered in giants.
00:29The largest things that ever lived on this planet weren't the dinosaurs, they're not
00:38even blue whales, they're trees.
00:42These are mountain ash, they're the largest flowering plant in the world.
00:46They grow about a metre a year and these trees are 60, 70, even 80 metres high.
00:52To get this big, you need to face some very significant physical challenges.
01:06These giants can live to well over 300 years old, but they don't keep growing forever.
01:15There are limits to how big each tree can get.
01:19As with all living things, the structure, form and function of these trees has been
01:24shaped by the process of evolution through natural selection, but evolution doesn't have
01:31a free hand, it is constrained by the universal laws of physics.
01:42Each tree has to support its mass against the downward force of Earth's gravity.
01:50At the same time, the trees rely on the strength of the interactions between molecules to raise
01:56a column of water from the ground up to the leaves in the canopy.
02:06And it's these fundamental properties of nature that act together to limit the maximum
02:12height of a tree, which theoretically lies somewhere in the region of 130 metres.
02:29With its forests and mountains, oceans and deserts, I've come to Australia to explore
02:39the scale of life's sizes.
02:46I want to see how the laws of physics govern the lives of all living things, from the very
02:52biggest to the very smallest.
02:59The size of life on Earth spans from the tallest tree, over 100 metres tall and with a mass
03:05of over 1,000 tonnes, to the smallest bacterium cell, with a length less than a millionth
03:12of a millimetre and a mass less than a million millionths of a gram.
03:16And that spans over 22 orders of magnitude in mass.
03:24I want to see how size influences the natural world.
03:32How do the physical forces of nature dictate the lives of the big and the small?
03:40Do organisms face different challenges at different scales?
03:46And do we all experience the world differently based on our size?
03:53The size you are profoundly influences the way that you live your life.
03:57It selects for the properties of the natural world that most affect you.
04:02So I suppose that whilst we all live on the same planet, we occupy different worlds.
04:27I'm heading out to the Neptune Islands, west of Adelaide in South Australia, in search
04:42of one of nature's largest killing machines.
04:52These beasts are feared around the world, a fear not helped by Hollywood filmmakers.
05:02I'm here to swim with great white sharks.
05:05How big, how big, how wide can I open a jar?
05:16Three foot wide.
05:17That's three feet.
05:18Swallow them in.
05:19Yeah.
05:20So about three foot wide can swallow a manhole.
05:30The skipper has a special permit to use bait to lure the sharks in.
05:34The crew ready the cages.
05:46The last time I dived was in the marina in Brighton.
06:01I did see a fish.
06:02About that big.
06:03Turned out to be the largest marine predator.
06:15As the sharks start to circle, it's time to get in.
06:18There he is.
06:19Here he comes.
06:20Just look at that, he's just checking us out, he's turning straight for us.
06:41Look at those teeth.
06:47Graceful, elegant thing, shaped by natural selection, brilliant at what he does, which
06:54is to eat things.
06:55Oh, I never thought you could be that close to one of those.
07:17Great whites are highly evolved predators.
07:20Around two thirds of their brain is dedicated to their sense of smell.
07:25They can detect as little as one part per million of blood in this water.
07:34The tiniest speck of blood will attract the shark.
07:41These fish can grow to a huge size, but still move with incredible speed and agility.
07:48They've been sculpted by evolution, acting within the bounds of the physical properties
07:54of water.
07:55He's about five meters long.
07:56He weighs about a ton.
07:57And he's probably the most efficient predator on Earth.
07:58When he's attacking, he can accelerate up to over 20 miles an hour, and they can launch
08:19themselves straight out of the water.
08:20There he is.
08:21There he is.
08:23I felt the need to remove my hands.
08:51That was one of the most awe-inspiring sights I've ever seen.
08:57The great white just straight in front of me with his mouth open.
09:04With the boat moored up away from shark-infested waters, I want to explore why it's in our
09:10oceans that we find the biggest animals on Earth.
09:14From giant sharks to blue whales, the largest animals that have ever lived have lived in
09:20the sea.
09:22The reason why is down to physics.
09:26This is a container full of salt water, and I'm going to weigh it.
09:32You see that says 25 kilograms there.
09:36That's actually its mass.
09:38Its weight is the force that the Earth is exerting on it due to gravity, which is 25
09:44times about 10, which is 250 kilograms meters per second squared.
09:49That might sound pedantic, but it's going to be important in a minute.
09:53See, what happens if I lower this salt water into the ocean?
10:03Its weight has effectively disappeared.
10:06It's effectively zeroed.
10:08Now, of course, gravity is still acting on this thing, so by the strictest sense of the
10:13word, it still has the same weight as it did up here.
10:16But Mr. Archimedes told us that there's another force that's come into play.
10:21There's a force proportional to the weight of water that's been displaced by this thing.
10:27Because this thing is essentially the same density as seawater, because it's made of
10:32seawater, then that force is equal and opposite to the force of gravity, and so they cancel.
10:39So it's effectively weightless.
10:42And that is extremely important indeed for the animals that live in the ocean.
10:50The cells of all living things are predominantly made up of salty water, so in the ocean, weight
10:56is essentially unimportant.
11:13Because of Archimedes' principle, the supportive nature of water releases organisms from the
11:19constraints of Earth's gravity, allowing the evolution of marine leviathans.
11:29But this comes at a cost.
11:31Water is 800 times denser than air, and so whilst it provides support, it requires a
11:37huge amount of effort to move through it.
11:44Not only does the shark have to push the water out of the way, it also has to overcome
11:49drag forces created by the frictional contact with the water itself.
11:55The solution for the shark lies in its shape.
11:59If you look at him, that great white, it's going to be a great white shark.
12:04If you look at him, that great white, it's got a distinctive streamlined shape.
12:11His maximum width is around a third of the way down his body, and that width itself should
12:17be around about a quarter of the length.
12:21That ratio is set by the necessity for something that big to be able to swim effectively and
12:31quickly through this medium.
12:35This shape reduces drag forces to a minimum and optimises the way water flows around the
12:44shark's body.
12:46It is the result of evolution, shaped by the laws of physics.
12:52Whoa!
12:54Ha ha ha ha!
12:56That's got to be… that was straight out of Jaws!
13:09That streamlined shape of the shark is something that you see echoed throughout nature.
13:14I mean, think of a whale, or a dolphin, or a tuna.
13:18Think of a whale, or a dolphin, or a tuna, all that same torpedo-like shape.
13:24And that's because they're contending with problems that arise from the same
13:28laws of physics, and convergent evolution has driven them to the same solution.
13:37For life in the sea, the evolution of giants is constrained directly
13:41by the physical properties of water.
13:43But out of the ocean, life now has to contend with the full force of Earth's gravity.
13:53And it's this force of nature that dominates the lives of giants on land.
13:58This is the hot, dry outback north of Broken Hill in New South Wales.
14:19I'm here to explore how gravity, a force whose strength is governed by the mass of our whole
14:24planet, moulds, shapes, and ultimately limits the size of life on land.
14:41I've come to track down one of Australia's most iconic animals, the red kangaroo.
14:49Red kangaroos are Australia's largest native land mammal,
14:53one of 50 species of macropods, so-called on account of their large feet.
15:05They're too very close there.
15:07They're too very close there.
15:16The kangaroos are the most remarkable of mammals because they hop.
15:21And there's no record, even in the fossil record, of any other large animal that does that.
15:26But it makes them very fast and efficient.
15:29When Joseph Banks, who's one of my scientific heroes,
15:33first arrived here with Captain Cook on the Endeavour in 1770,
15:37he wrote that they move so fast over the rocky, rough ground where they're found
15:42that even my greyhound couldn't catch them.
15:46What was he doing with a greyhound?
15:53Kangaroos are herbivorous and scratch out a living feeding on grasses.
16:00While foraging, they move in an ungainly fashion,
16:04using their large muscular tail like a fifth leg.
16:07But when they want to, these large marsupials can cover ground at considerable speeds.
16:20To take a leap, kangaroos have to work against the downward pull of Earth's gravity.
16:26This takes a lot of energy.
16:30As animals go faster, they tend to use more energy.
16:35Not so with the kangaroos.
16:38As the roos go faster, their energy consumption actually decreases.
16:47It then stays constant, even at sustained speeds of up to 40 kilometres per hour.
16:59This incredible efficiency for such a large animal
17:02comes directly from the kangaroos' anatomy.
17:08Kangaroos move so efficiently because they have an ingenious energy storage mechanism.
17:13See, when something hits the ground after falling from some height,
17:17then it has energy that it needs to dissipate.
17:20If you're a rock, that energy is dissipated as sound and a little bit of heat.
17:26But if you're a tennis ball, then some of that energy is reused.
17:30Because a tennis ball is elastic, it can deform, spring back,
17:35and use some of that energy to throw itself back into the air again.
17:42Well, a kangaroo is very similar.
17:45It has very elastic tendons in its legs,
17:47particularly its Achilles tendon, and also the tendons in its tail.
17:52And they store energy, and then they release it,
17:56supplementing the power of the muscles to bounce the kangaroo through the air.
18:01Now, an adult kangaroo is 85, 90 kilos, which is heavier than me.
18:08And when it's going at full speed, it can jump around nine metres.
18:13That's the distance from me to that car.
18:21The evolution of the ability to hop
18:24gives kangaroos a cheap and efficient way to get around.
18:28But not everything can move like a kangaroo.
18:32The red kangaroo is the largest animal in the world
18:35that moves in this unique way,
18:37hopping across the landscape at high speed.
18:40And there are reasons why there aren't, you know,
18:43giant hopping elephants or dinosaurs.
18:47And they're not really biological.
18:49It's not down to the details of evolution
18:52by natural selection or environmental pressures.
18:55The larger an animal gets, the more severe the restrictions
19:01on its body shape and its movement.
19:07To understand why this is the case,
19:10I want to explore what happens to the mass of a body
19:13when that body increases in size.
19:20Take a look at this block.
19:21Let's say it has width one, length one, and height one.
19:25Then its volume is one multiplied by one multiplied by one,
19:30which is one cubic things, whatever the measurement is.
19:35Now, its mass is proportional to the volume,
19:37so we could say that the mass of this block is one unit as well.
19:41Let's say that we're going to double the size of this thing
19:45in the sense that we want to double its width,
19:48double its length,
19:51double its height.
19:53Then its volume is two multiplied by two multiplied by two
19:57equals eight cubic things.
19:59Its volume is increased by a factor of eight.
20:02And so its mass is increased by a factor of eight as well.
20:08So although I've only doubled the size of the blocks,
20:11I've increased the total mass by eight.
20:15As things get bigger,
20:16the mass of a body goes up by the cube of the increase in size.
20:23Because of this scaling relationship,
20:26the larger you get, the greater the effect.
20:30As things get bigger,
20:31the huge increase in mass has a significant impact
20:35on the way large animals support themselves against gravity
20:39and how they move about.
20:45No matter how energy efficient and advantageous it is
20:48to hop like a kangaroo,
20:50as you get bigger, it's just not physically possible.
20:56Going supersize on land
20:58comes with tremendous constraints attached.
21:04This is the left femur,
21:06the thigh bone of an extinct animal called a diprotodon,
21:09which is the largest known marsupial ever to have existed.
21:14This would have stood as tall as me,
21:16would have been four metres tall,
21:18as tall as me, would have been four metres long,
21:21weighed between two and two and a half tonnes,
21:23so the size of a rhino.
21:25It's known that it was all over Australia.
21:28It was the big herbivore
21:31and it got progressively bigger over the 25 million years
21:35that we have fossils for it.
21:37Then around 50,000 years ago,
21:39coincidentally when humans arrived in Australia,
21:42the diprotodon became extinct.
21:49The diprotodon is thought to have looked like a giant wombat
21:53and being marsupials,
21:55the females would have carried their sheep-sized offspring
21:58in a huge pouch.
22:03To support their considerable bulk,
22:05the diprotodon's skeleton had to be very strong.
22:09This imposed significant constraints
22:12on the shape and size of its bones.
22:15This is the femur of the closest living relative of the diprotodon.
22:19It's a wombat, which is an animal around the size of a small dog.
22:23And you see that superficially, the bones are very similar.
22:28Let me take a few measurements.
22:31The length of the diprotodon femur is...
22:36..around 75 centimetres.
22:40The length of the wombat femur is around 50 centimetres.
22:45The length of the diprotodon femur is around 15 centimetres.
22:49So this is about five times the length of the wombat femur.
22:54But now look at the cross-sectional area.
22:57Assuming the bones are roughly circular in cross-section,
23:00we can calculate their area using pi multiplied by the radius squared.
23:06It turns out that although the diprotodon femur
23:09is around five times longer,
23:12it's 40 times that of the wombat femur.
23:19A bone's strength depends directly on its cross-sectional area.
23:24The diprotodon needed thick leg bones braced in a robust skeleton
23:30just to provide enough strength to support the giant's colossal weight.
23:35As animals get more massive,
23:37the effect of gravity plays an increasingly restrictive role in their lives.
23:44The shape and form of their body is forced to change.
23:52If you look across the scale of Australian vertebrate life,
23:55you see a dramatic difference in bone size.
23:58If you look across the scale of Australian vertebrate life,
24:02you see a dramatic difference in bone thickness.
24:08This is a line of femur bones of animals of different sizes.
24:12We start with the smallest, one of the smallest marsupials in Australia,
24:17the marsupial mouse, or the antechinus.
24:20Then the next one is an animal known as the potoroo.
24:23Again, it's the marsupial, around about the size of a rabbit.
24:26Then we have the Tasmanian devil, a wombat, a dingo.
24:31Then the largest marsupial in Australia today, the red kangaroo.
24:38This is the femur of the diprotodon.
24:41And then here, the femur of a rheotosaurus,
24:45which was a sauropod dinosaur, 17 metres long and weighing around 20 tonnes.
24:53And so, you see, as animals get larger,
24:57from the smallest marsupial mouse all the way up to a dinosaur,
25:02the cross-sectional area of their bones increases enormously
25:06just to support that increased mass.
25:13Being big and bulky, giants are more restricted
25:17as to the shape of their body and how they get about.
25:23That's why red kangaroos are the largest animals
25:26that can move in the way that they do.
25:30At a much greater size, their bones will be very heavy,
25:34have a greater risk of fracture,
25:36and they require far too much energy to move at high speeds.
25:43It's ultimately the strength of Earth's gravity
25:47that limits the size and the manoeuvrability of land animals.
25:52They're called land-based giants.
25:56But for the bulk of life on land,
25:58gravity is not the defining force of nature.
26:15At small scales, living things seem to bend the laws of physics,
26:21but that's, of course, not possible.
26:24The world of the small is often hidden from our view,
26:28but there are ways to draw out these tiny creatures.
26:36This is the domain of the insects.
26:41These animals can clearly do things I can't do
26:45and appear to have superpowers.
26:48They can walk up walls, jump many times their own height,
26:53and can lift many times their own weight.
26:57There are over 900,000 known species of insects on the planet.
27:02That's over 75% of all animal species,
27:06and some biologists think that there may be
27:08an order of magnitude more yet to be discovered.
27:11That would be 10 million species.
27:14And they're very small,
27:15so you can fit a lot of them on planet Earth at any one time.
27:19In fact, it's estimated there are over 10 billion billion
27:23individual insects alive today.
27:33Of all the insect groups, it's the beetles, or Coleoptera,
27:37that have the greatest number of species.
27:45The biologist JBS Haldane said that if one could conclude
27:49as to the nature of the creator from a study of creation,
27:52then it would appear that God has an inordinate fondness
27:56for stars and beetles.
28:06With so much variation in colour, form and function,
28:10beetles have fascinated naturalists for centuries.
28:16Each species is wonderfully adapted to their own unique niche.
28:36This is the beginnings of biology as a science.
28:39Biology as a science that you see here is this desire to collect and classify,
28:44which then over time becomes the desire to explain and understand.
28:53Let me take a picture.
29:02Here in the suburbs of Brisbane,
29:05every February there's an invasion of beetles.
29:09The rules governing their lives play out very differently to ours.
29:17This is the rhinoceros beetle, named for obvious reasons,
29:22but actually it's only the males that have the distinctive horns on their heads.
29:28The beetles spend much of their lives underground as larvae,
29:32but then emerge en masse as adults to find a mate and breed.
29:37Much of this time, the males spend fighting over females.
29:49See that distinctive posture that he's adopting there?
29:54That's because I think he's seeing his reflection in the camera lens,
29:58and so he rears up.
30:00Look at that. He's trying to scare himself off.
30:06If you also hear that hissing sound, that's him contracting his abdomen,
30:12which again is a defensive posture that he adopts to scare other males.
30:22Gram for gram, these insects are among the strongest animals alive.
30:30I can demonstrate that by just getting hold of the top of his head.
30:34It doesn't hurt him at all, but watch what he is able to do.
30:46Look at that.
30:48So he's hanging on to this branch, which is many times his own body weight.
30:54Absolutely no distress at all.
30:56As things get smaller, it's a rule of nature that they inevitably get stronger.
31:02The reason is quite simple.
31:04Small things have relatively large muscles compared to their tiny body mass,
31:10and this makes them very powerful.
31:12The beetles also appear to have a cavalier attitude to the effects of gravity.
31:20They fight almost like sumo wrestlers.
31:22Their aim is to throw each other off the branch.
31:28And they're not afraid of getting hurt.
31:30They're not afraid of getting hurt.
31:32They're not afraid of getting hurt.
31:34They're not afraid of getting hurt.
31:36They're not afraid of getting hurt.
31:38They're trying to throw each other off the branch.
31:42If they should fall,
31:46they just bounce and walk off.
31:52If I fell a similar distance relative to my size, I'd break.
31:59So why does size make such a difference?
32:08Time for a bit of fundamental physics.
32:12All things fall at the same rate under gravity,
32:16and that's because they're following geodesics through curved spacetime,
32:19but that's not important.
32:21The important thing for biology
32:23is that although everything falls at the same rate,
32:26it doesn't meet the same fate when it hits the ground.
32:34A grape bounces.
32:38A melon...
32:45...doesn't bounce.
32:52Now, the reasons for that are quite complex, actually.
32:56First of all, the grape has a larger surface area
33:00in relation to its volume, and therefore its mass, than the melon.
33:05And so, although in a vacuum, if you took away the air,
33:08they would both fall at the same rate,
33:11actually, in reality, the grape falls a bit slower than the melon.
33:14Also, the melon is more massive,
33:17and so it has more kinetic energy when it hits the ground.
33:20Remember from physics class,
33:23kinetic energy is a half mv squared.
33:26So if you reduce m, you reduce the energy.
33:29The upshot of that is that the melon has a lot more energy
33:33when it hits the ground.
33:35It has to dissipate it in some way,
33:38and it dissipates it by exploding.
33:44The influence of Earth's gravity on your life
33:47becomes progressively diminished the smaller you get.
33:52For life at the small scale,
33:55a second fundamental force of nature starts to dominate,
33:59and it's this that explains many of those apparent superpowers.
34:06For me, the force of gravity is the thing that defines my existence.
34:11It is the force that I really feel,
34:14and it's the force that I really feel.
34:17The force of gravity defines my existence.
34:20It is the force that I really feel the effects of.
34:23But there are other forces at work.
34:26For example, if I lick my finger and wet it,
34:29I can pick up a piece of paper.
34:32I can hold it up against the downward pull of gravity.
34:35That's because the force of electromagnetism is important.
34:39In fact, it's the cohesive forces
34:42between water molecules and the molecules that make up my finger
34:45and the molecules that make up the paper
34:48that are dominating this particular situation,
34:51and that's why this piece of paper doesn't fall to the floor.
34:55Many insects can use a similar effect.
34:59Take a common fly, for example.
35:07Their feet have specially enlarged pads
35:10onto which they secrete a sticky fluid.
35:14And that allows them to adhere to rather slippery surfaces,
35:18like the glass of this jam jar,
35:21and allows them to do things that, for me, would be absolutely impossible,
35:25and it's all down to the relative influence
35:28of the different forces of nature on the animal.
35:35So the capacity to walk up walls
35:38and fall from a great height without breaking,
35:41this super strength, are not superpowers at all.
35:46They're just abilities gained naturally
35:49by animals that are small and lightweight.
35:55But this is just the beginning of my journey into the world of the small.
36:02Down at the very small scale,
36:05it becomes possible to live within the lives of other individuals,
36:09worlds within worlds.
36:13But just how small can animals get?
36:28This macadamia nut plantation, an hour outside of Brisbane,
36:32is home to one of the very smallest members of the animal kingdom.
36:40These are a species of microhymenoptera known as trichogramma.
36:45They're basically very small wasps.
36:48And when I say small, I mean small.
36:52Can you see that?
36:55They're like specks of dust.
36:58They're less than half a millimetre long.
37:02But each one of those is a wasp.
37:05It's got compound eyes, it's got six legs, it's got wings.
37:09They've even got a little stripe on their abdomen.
37:14And they're very precisely adapted to a specific evolutionary niche.
37:20The trichogramma wasps may be small, but they're very useful.
37:25They're natural parasites of an insect pest species
37:29called the nut borer.
37:33The micro wasps lay their eggs inside the eggs of the moths,
37:37killing the developing moth larvae.
37:42So what you're seeing here is the surface of a macadamia nut,
37:46and here's a small cluster of moth eggs.
37:50And there, you can see the eggs.
37:54And you can see the larvae.
37:58And here's another cluster of moth eggs.
38:02And there, you see the wasp is walking over the eggs.
38:06They're almost pacing out the size to see whether the eggs are suitable
38:10for their eggs to be laid inside.
38:13And if we're lucky...
38:16There you go, you see that?
38:20There we go.
38:24The wasps emerge just nine days later as full-grown adults.
38:30At this scale, they live in a very sticky world,
38:34dominated by strong intermolecular forces.
38:38To them, even the air is a thick fluid
38:42through which they essentially swim using paddle-like wings.
38:48Incredibly, these tiny animals can move about across several trees,
38:53seeking out the moth eggs.
38:57But what I find more remarkable is that they do all this
39:01operating with very restricted brain power.
39:06One of the limiting factors that determines the minimum size of insects
39:10is the volume of their central nervous system.
39:13In other words, the processing power you can fit inside their bodies.
39:17And these little wasps are pretty much at the limit.
39:21There are more than 10,000 neurons in our whole nervous system.
39:25To put that into perspective, most tiny insects have 100 times that many.
39:30But that's still enough to allow them to exhibit quite complex behaviour.
39:36These micro wasps exist at almost the minimum possible size
39:40for multicellular animals.
39:44But the scale of life on our planet gets much, much smaller.
39:48The wasps are giants compared to life at the very limit of size on Earth.
40:07The smallest organisms on our planet are also our oldest
40:11and most abundant type of life forms.
40:18These weird rocky blobs in the shallows of Lake Clifton,
40:22just south of Perth, are made by bacteria.
40:31These mounds are called thrombolytes on account of their clotted structure.
40:36And they're built up over centuries by colonies of microscopic bacterial cells.
40:43Now, although these colonies are rare, by most definitions,
40:47bacteria are the dominant form of life on our planet.
40:51On every surface, across every landscape, you find bacteria.
40:55And, in fact, numerically speaking, then there are more bacteria
40:59living on and inside my body than there are human cells.
41:05Bacteria come in many shapes and forms
41:08and are not actually animals or plants,
41:11instead sitting in their own unique taxonomic kingdom.
41:17But compared to the cells we're made of,
41:20bacteria are structurally much simpler and far, far smaller.
41:26Bacteria are typically around two microns in size.
41:30That's two millionths of a metre, which is very hard to picture,
41:34but it means that you could fit around half a million of them on the head of a pin.
41:38Or, to look at it another way, if I took a single bacterium
41:42and scaled it up to the size of this coin,
41:45it would be 25 kilometres high.
41:51Bacterial-type organisms were the first life on Earth
41:55and they've dominated our planet ever since.
41:58Excluding viruses, which by most definitions are not alive,
42:02bacteria are the smallest free-living life forms we know of.
42:08But what ultimately puts the limit on the smallest size of life?
42:13Single-cell life needs to be big enough
42:16to accommodate all the molecular machinery of life.
42:19And that size ultimately depends on the basic laws of physics.
42:24It depends on the size of molecules, which depends on the size of atoms,
42:28which depends on fundamental properties of the universe,
42:32like the strength of the force of electromagnetism
42:35and the mass of an electron.
42:37And when you do those calculations,
42:39you find out that the minimum size of a free-living organism
42:43should be around 200 nanometres,
42:46which is about 200 billionths of a metre.
42:50And that should be universal.
42:52It shouldn't only apply to life on Earth,
42:55but it should apply to any carbon-based life anywhere in the universe
43:00because it depends on fundamental properties of the universe.
43:10From the smallest bacterium to the largest tree,
43:15it's your size that determines how the laws of physics govern your life.
43:22Gravity imposes itself on the large
43:25and the electromagnetic force rules the world of the small.
43:33But the consequences of scale for life on Earth
43:36extend beyond dictating the relationship you have with the world around you.
43:43Your size also influences how energy itself flows through your body.
43:49These are southern bent-wing bats,
43:54one of the rarest bat species in Australia.
44:00They're known for their ability to survive in the cold.
44:07They're also known for their ability to survive in the cold,
44:11and they're also known for their ability to survive in the cold.
44:15They're very rare in Australia.
44:19Every evening, they emerge in their thousands from this cave in order to feed.
44:27When fully grown, these bats are just 5.5 centimetres long
44:32and weigh around 18 grams.
44:35Because of their size, they face a constant struggle to stay alive.
44:46Now, we're using a thermal camera here to look at the bats,
44:50and you can see that they appear as streaks across the sky.
44:53They appear as brightly as me.
44:55That's because they're roughly the same temperature as me.
44:58They're known as endotherms.
45:00They're animals that maintain their body temperature,
45:03and that takes a lot of effort.
45:05I mean, these bats have to eat something like three-quarters of their own body weight every night,
45:11and a lot of that energy goes into maintaining their temperature.
45:18As with all living things, the bats eat to provide energy to power their metabolism.
45:24Although, like us, they have a high body temperature when they're active,
45:28keeping warm is a considerable challenge on account of their size.
45:34The bats lose heat mostly through the surface of their bodies,
45:39but because of simple laws governing the relationship
45:42between the surface area of a body and its volume,
45:46being small creates a problem.
45:51So let's look at our blocks again, but this time for surface area to volume.
45:56Here's a big thing. It's made of eight blocks, so its volume is eight units,
46:00and its surface area is 2 by 2 on each side, so that's 4,
46:05multiplied by the six faces is 24.
46:08So the surface area to volume ratio is 24 to 8, which is 3 to 1.
46:15Now look at a smaller thing.
46:17This is one block, so its volume is one unit.
46:20Its surface area is 1 by 1 by 1, six times, so it's 6.
46:25Six times, so it's 6.
46:27So this has a surface area to volume ratio of 6 to 1.
46:32So as you go from big to small, your surface area to volume ratio increases.
46:41Small animals like bats have a huge surface area compared to their volume.
46:46As a result, they naturally lose heat at a very high rate.
46:52To help offset the cost of losing so much energy in the form of heat,
46:57the bats are forced to maintain a high rate of metabolism.
47:01They breathe rapidly, their little heart races, and they have to eat a huge amount.
47:07So a bat's size clearly affects the speed at which it lives its life.
47:21Right across the natural world, the size you are has a profound effect
47:26on your metabolic rate, or your speed of life.
47:33For Australia's small marsupial mouse, even at rest his heart is racing away.
47:40For the fox-sized Tasmanian devil, he tips along at a much slower rate.
47:47And then there's me, living life at a languid 60 beats a minute.
47:55Looking beyond heart rate, your size influences the amount of energy you need to consume
48:01and the rate at which you need to consume it.
48:06Bigger bodies have more cells to feed,
48:09so you might expect that the total amount of energy needed
48:13goes up at the same rate as any increase in size.
48:19But that's not what happens.
48:25If you plot the amount of energy an animal uses against its mass
48:29for a huge range of sizes, from animals as small as flies,
48:34and even smaller, all the way up to whales,
48:37then you do get a straight line, but the slope is less than one.
48:42So that implies that gram for gram, large animals use less energy than small animals.
48:52This relationship between metabolism and size
48:56significantly affects the amount of food larger animals have to consume to stay alive.
49:06Now if my metabolic rate scaled one to one, with that of a mouse,
49:10then I would need to eat about four kilograms of food a day.
49:14In my language, that's around 67,000 kilojoules of energy,
49:19which more colloquially is 16,000 calories.
49:22That is eight times the amount that I take in, on average, on a daily basis.
49:30Each of the cells in my body requires less energy
49:34than the equivalent cells in a smaller-sized mammal.
49:40The reason why this should be so is not fully understood.
49:44It's also not clear whether this rule of nature gives an advantage to big things,
49:50or is actually a constraint placed on larger animals.
49:57Take the relationship between an animal's surface area and its volume.
50:02Big animals have a much smaller surface area to volume ratio than small animals,
50:07and that means that their rate of heat loss is much smaller.
50:11And that means that there's an opportunity there for large animals.
50:15They don't have to eat as much food to stay warm,
50:18and therefore they can afford a lower metabolic rate.
50:25Now this helps explain the lives of large, warm-blooded endotherms,
50:29like birds and mammals, but doesn't hold so well for large ectotherms,
50:35life's cold-blooded giants.
50:41Now there's another theory that says that it wasn't really an evolutionary opportunity
50:46that large animals took to lower their metabolic rate.
50:49It was forced on them. It was a constraint, if you like.
50:52The capillaries, the supply network to cells, branches in such a way
50:58that it gets more and more difficult to get oxygen and nutrients to cells in a big animal
51:03than in a small animal.
51:05Therefore, those cells must run at a lower rate.
51:10They must have a lower metabolic rate.
51:17Or it could just be that as you get bigger,
51:19then more of your mass is taken up by the stuff that supports you.
51:23And support structures like bones are relatively inert.
51:27They don't use much energy.
51:30But whatever the reason, it's certainly true to say
51:33that the only way that large animals can exist on planet Earth
51:37is to operate at a reduced metabolic rate.
51:43If this wasn't the case, the maximum size of a warm-blooded endotherm,
51:48like me or you, would be around that of a goat.
51:52And cold-blooded animals, or ectotherms like dinosaurs,
51:56could only get as big as a pony.
51:59Any bigger, and giants would simply overheat.
52:06Now, there's one last consequence of all these scaling laws
52:09that I suspect you'll care about more than anything else, and it's this.
52:15There's a strong correlation between the effective cellular metabolic rate
52:20of an animal and its lifespan.
52:23In other words, as things get bigger, they tend to live longer.
52:43To explore this connection between size and longevity,
52:47I've left the mainland behind.
52:50For my final destination,
52:52I've come to one of Australia's remotest outposts.
53:00Named Christmas Island when it was spotted on Christmas Day in 1643,
53:05this isolated lump of rock in the Indian Ocean is a land of crabs.
53:21And in their midst lurks a giant wonder of the natural world.
53:30This is a Christmas Island robber crab,
53:33the largest land crab anywhere on the planet.
53:36These things can grow to around 50cm in length,
53:40they can weigh over 4kg,
53:43and they are supremely adaptive,
53:46as an adult, to life on land.
53:48They can even climb trees.
53:53Over the years, the crabs have become well adapted to human cohabitation.
54:00These things are called robber crabs because they have a reputation
54:04for curiosity and for stealing things.
54:08Anything that isn't bolted down will steal food.
54:11And cameras, if they can get half a chance.
54:26These giants live on a diet of seeds and fruit,
54:30and occasionally other small crabs.
54:33Their large, powerful claws mean they can also be used as bait.
54:38Their large, powerful claws mean they can also rip open fallen coconuts.
54:46They're quite a menacing animal, actually, for a crab.
54:52What's wonderful about these crabs is that they live through a range of scales.
54:58At different times of their lives,
55:00they have a completely different relationship with the world around them,
55:04simply down to their size.
55:07Throughout their lives, robber crabs take on many different forms.
55:11They begin their lives as small larvae swept around by the ocean currents.
55:16And as they grow, some of them get swept up onto the beaches of Christmas Island,
55:21where they find a shell, because they are, in fact, hermit crabs.
55:25They live inside their shell for a while, they continue to grow,
55:29and eventually, as adults, they roam the forest like this chap here.
55:34So these crabs, over that lifespan, inhabit many different worlds.
55:43On land, the adults continue to grow
55:46and now have to support their weight against gravity.
55:51Compared to the smaller crabs whizzing around,
55:54these giants move about much more slowly,
55:57but they also live far longer.
56:01Of all the species of land crab here on Christmas Island,
56:05the robber crabs are not only the biggest, they're also the longest living.
56:09So this chap here is probably about as old as me,
56:13and he might live to 60, 70, even 80 years old.
56:20Because of the robber crab's overall body size,
56:24its individual cells use less energy
56:27and they run at a slower rate
56:30than the cells of their much smaller, shorter-lived cousins.
56:37The pace of life is slower for robber crabs,
56:40and it's this that's thought to allow them to live to a ripe old age.
56:47Your size influences every aspect of your life.
56:53From the way you are built...
56:58..to the way you move...
57:02..and even how long you live.
57:05Your size dictates how you interact with the universe around you.
57:12Your size dictates how you interact with the universal laws of nature.
57:21So there's a minimum size,
57:23which is set ultimately by the size of atoms and molecules,
57:27the fundamental building blocks of the universe.
57:31And there's a maximum size, which certainly on land
57:35is set by the size and the mass of our planet,
57:38because it's gravity that restricts the emergence of giants.
57:45But within those constraints, evolution has conspired
57:48to produce a huge range in size of animals and plants,
57:52each beautifully adapted to exploit the niches available to them.
58:00Size influences your form and construction.
58:04It determines how you experience the world
58:07and ultimately how long you have to enjoy it.
58:15High drama on the menu tonight.
58:17Jess is in hospital and it's not looking good.
58:20Dancing on the Edge continues on BBC HD next.
58:37.