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.
12:51That's got to be... that was straight out of Jaws.
13:06That streamlined shape of the shark is something that you see echoed throughout nature.
13:11I mean, think of a whale, or a dolphin, or a tuna.
13:15All that same torpedo-like shape.
13:18And that's because they're contending with problems that arise from the same laws of
13:22physics, and convergent evolution has driven them to the same solution.
13:30For life in the sea, the evolution of giants is constrained directly by the physical properties
13:36of water.
13:37But out of the ocean, life now has to contend with the full force of Earth's gravity.
13:43And it's this force of nature that dominates the lives of giants on land.
14:08This is the hot, dry outback north of Broken Hill in New South Wales.
14:18I'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.
14:38I've come to track down one of Australia's most iconic animals.
14:44The red kangaroo.
14:48Red kangaroos are Australia's largest native land mammal.
14:52One of 50 species of macropods, so-called on account of their large feet.
14:59They're too very close there.
15:08Kangaroos are the most remarkable of mammals because they hop.
15:12And there's no record, even in the fossil record, of any other large animal that does that.
15:18But it makes them very fast.
15:20When Joseph Banks, who's one of my scientific heroes, first arrived here with Captain Cook
15:25on the Endeavour in 1770, he wrote that they moved so fast over the rocky, rough ground
15:32where they found that even my greyhound couldn't catch them.
15:36I mean, what was he doing with a greyhound?
15:40I'm going to go and find out.
15:42I'm going to go and find out.
15:44I'm going to go and find out.
15:46I'm going to go and find out.
15:48I'm going to go and find out.
15:50Kangaroos are herbivorous and scratch out a living, feeding on grasses.
15:59While foraging, they move in an ungainly fashion, using their large, muscular tail like a fifth leg.
16:10But 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.
16:41As the roos go faster, their energy consumption actually decreases.
16:49It then stays constant, even at sustained speeds of up to 40 kilometres per hour.
17:01This incredible efficiency for such a large animal comes directly from the kangaroos' anatomy.
17:10Kangaroos move so efficiently because they have an ingenious energy storage mechanism.
17:16See, when something hits the ground after falling from some height, then it has energy that it needs to dissipate.
17:22If you're a rock, that energy is dissipated as sound and a little bit of heat.
17:29But if you're a tennis ball, then some of that energy is reused.
17:33Because a tennis ball is elastic, it can deform, spring back and use some of that energy to throw itself back into the air again.
17:42Well, a kangaroo is very similar.
17:44It has very elastic tendons in its legs, particularly its Achilles tendon, and also tendons in its tail.
17:51And they store energy.
17:53And then they release it, supplementing the power of the muscles to bounce the kangaroo through the air.
18:00Now, an adult kangaroo is 85-90 kilos, which is heavier than me.
18:07And when it's going at full speed, it can jump around 9 metres.
18:13That's the distance from me to that car.
18:21The evolution of the ability to hop gives kangaroos a cheap and efficient way to get around.
18:28But not everything can move like a kangaroo.
18:33The red kangaroo is the largest animal in the world that moves in this unique way, hopping across the landscape at high speed.
18:41And there are reasons why there aren't, you know, giant hopping elephants or dinosaurs.
18:48And they're not really biological.
18:50It's not down to the details of evolution by natural selection or environmental pressures.
18:56The larger an animal gets, the more severe the restrictions on its body shape and its movement.
19:05To understand why this is the case, I want to explore what happens to the mass of a body when that body increases in size.
19:17Take a look at this block.
19:19Let's say it has width 1, length 1, and height 1.
19:23Then its volume is 1 multiplied by 1 multiplied by 1, which is 1 cubic things,
19:32whatever the measurement is.
19:34Now its mass is proportional to the volume, so we could say that the mass of this block is 1 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, double its length, and double its height.
19:53Then its volume is 2 multiplied by 2 multiplied by 2 equals 8 cubic things.
19:59Its volume is increased by a factor of 8, and so its mass is increased by a factor of 8 as well.
20:08So although I've only doubled the size of the blocks, I've increased the total mass by 8.
20:14As things get bigger, the mass of a body goes up by the cube of the increase in size.
20:22Because of this scaling relationship, the larger you get, the greater the effect.
20:28As things get bigger, the huge increase in mass has a significant impact
20:34on the way large animals support themselves against gravity and how they move about.
20:41No matter how energy efficient and advantageous it is to hop like a kangaroo,
20:46as you get bigger, it's just not physically possible.
20:53Going supersize on land comes with tremendous constraints attached.
21:01This is the left femur, the thigh bone of an extinct animal.
21:07This is the left femur, the thigh bone of an extinct animal called a diprotodon,
21:12which is the largest known marsupial ever to have existed.
21:16This would have stood as tall as me. It would have been 4 metres long,
21:20weighed between 2 and 2.5 tonnes, so the size of a rhino.
21:25It's known that it was all over Australia. It was the big herbivore.
21:31And it got progressively bigger over the 25 million years that we have fossils for it.
21:37Then, around 50,000 years ago, coincidentally, 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, the females would have carried their sheep-sized offspring in a huge pouch.
22:03To support their considerable bulk, the diprotodon's skeleton had to be very strong.
22:09This imposed significant constraints on the shape and size of its bones.
22:16This is the femur of the closest living relative of the diprotodon.
22:20It's a wombat, which is an animal around the size of a small dog.
22:24And you see that superficially, the bones are very similar.
22:29Let me take a few measurements.
22:32The length of the diprotodon femur is around 75 centimetres.
22:40The length of the wombat femur is around 15 centimetres.
22:46So this is about five times the length of the wombat femur.
22:51But now look at the cross-sectional area.
22:53Assuming the bones are roughly circular in cross-section,
22:57we can calculate their area using pi multiplied by the radius squared.
23:03It turns out that although the diprotodon femur is around five times longer,
23:09it has a cross-sectional area 40 times that of the wombat femur.
23:17A bone's strength depends directly on its cross-sectional area.
23:23The diprotodon needed thick leg bones braced in a robust skeleton
23:29just to provide enough strength to support the giant's colossal weight.
23:40As animals get more massive,
23:42the effect of gravity plays an increasingly restrictive role in their lives.
23:47The shape and form of their body is forced to change.
23:56If you look across the scale of Australian vertebrate life,
24:00you see a dramatic difference in bone thickness.
24:06This is a line of femur bones of animals of different sizes.
24:11We start with one of the smallest marsupials in Australia,
24:15the marsupial mouse, or the antechinus.
24:18Then the next one is an animal known as the potoroo.
24:21Again, it's a marsupial, around about the size of a rabbit.
24:24Then we have the Tasmanian devil, a wombat, a dingo.
24:30Then the largest marsupial in Australia today, the red kangaroo.
24:36This is the femur of the diprotodon.
24:39And then here, the femur of a rheotosaurus,
24:43which was a sauropod dinosaur, 17 metres long and weighing around 20 tonnes.
24:51And so you see, as animals get larger,
24:55from the smallest marsupial mouse all the way up to a dinosaur,
25:00the cross-sectional area of their bones increases enormously
25:04just to support that increased mass.
25:08Being big and bulky,
25:11giants are more restricted as to the shape of their body
25:15and how they get about.
25:21That's why red kangaroos are the largest animals
25:24that can move in the way that they do.
25:28At a much greater size, their bones will be very heavy,
25:32have a greater risk of fracturing,
25:34and they require far too much energy to move at high speeds.
25:41It's ultimately the strength of Earth's gravity
25:44that limits the size and manoeuvrability of land-based giants.
25:51But for the bulk of life on land,
25:53gravity is not the defining force of nature.
25:58At small scales, living things seem to bend the laws of physics,
26:03which is, of course, not possible.
26:06The world of the small is often hidden from our view.
26:11But when we look down at the world of the great,
26:15we can see it's a world of possibilities.
26:18the laws of physics which is of course not possible. The world of the small is
26:24often hidden from our view but there are ways to draw out these tiny creatures.
26:34This is the domain of the insects. These animals can clearly do things I can't do
26:43and appear to have superpowers. They can walk up walls, jump many times their own
26:52height and can lift many times their own weight. There are over 900,000 known
27:00species of insects on the planet. That's over 75% of all animal species and some
27:06biologists think that there may be an order of magnitude more yet to be
27:10discovered. There will be 10 million species and they're very small so you can fit a lot of them
27:17on planet Earth at any one time. In fact it's estimated there are over 10 billion
27:23billion individual insects alive today.
27:33Of all the insect groups, it's the beetles or Coleoptera that have the greatest number of
27:40species. The biologist JBS Haldane said that if one could conclude as to the
27:50nature of the creator from a study of creation then it would appear that God
27:55has an inordinate fondness for stars and beetles.
28:01With so much variation in colour, form and function, beetles have fascinated
28:13naturalists for centuries. Each species is wonderfully adapted to their own
28:20unique niche.
28:31And this is the beginnings of biology as a science that you see here. It's this
28:41desire to collect and classify which then over time becomes the desire to
28:47explain and understand.
29:01Here in the suburbs of Brisbane, every February there's an invasion of beetles.
29:08The rules governing their lives play out very differently to ours.
29:16This is the rhinoceros beetle, named for obvious reasons but actually it's only
29:22the males that have the distinctive horns on their heads. The beetles spend
29:28much of their lives underground as larvae but then emerge en masse as
29:33adults to find a mate and breed. Much of this time the males spend fighting over
29:40females.
29:48See that distinctive posture that he's adopting there? That's because I think
29:55he's seeing his reflection in the camera lens and so he rears up. Look at that,
30:01he's trying to scare himself off.
30:09If you also hear that hissing sound, that's him contracting his abdomen, which
30:15again is a defensive posture that he adopts to scare other males.
30:22Gram for gram, these insects are among the strongest animals alive. I can
30:33demonstrate that by just getting hold the top of his head, doesn't hurt him at
30:37all, but watch what he is able to do.
30:46Look at that, so he's hanging on to this branch which is many times his own body
30:55weight. Absolutely no distress at all.
31:01As things get smaller it's a rule of nature that they inevitably get
31:07stronger. The reason is quite simple, small things have relatively large
31:13muscles compared to their tiny body mass and this makes them very powerful.
31:25The beetles also appear to have a cavalier attitude to the effects of
31:30gravity. They fight almost like sumo wrestlers, their aim is to throw each
31:39other off the branch. If they should fall, they just bounce and walk off. If I fell
31:53a similar distance relative to my size, I'd break. So why does size make such a
32:01difference?
32:09Time for a bit of fundamental physics. All things fall at the same rate under
32:15gravity, that's because they're following geodesics through curved space-time but
32:19that's not important. The important thing for biology is that although everything
32:25falls at the same rate, it doesn't meet the same fate when it hits the ground.
32:31A grape bounces.
32:43A melon doesn't bounce.
32:54Now the reasons for that are quite complex actually. First of all the grape
33:01has a larger surface area in relation to its volume and therefore its mass than
33:07the melon and so although in a vacuum if you took away the air they would both
33:12fall at the same rate, actually in reality the grape falls a bit slower than the
33:17melon. Also the melon is more massive and so it has more kinetic energy when
33:23it hits the ground. Remember from physics class, kinetic energy is a half
33:26MV squared, so if you reduce M you reduce the energy. The upshot of that is that
33:32the melon has a lot more energy when it hits the ground. It has to dissipate it
33:36in some way and it dissipates it by exploding.
33:44The influence of Earth's gravity on your life becomes progressively diminished
33:49the smaller you get.
33:59For life at the small scale, a second fundamental force of nature starts to
34:05dominate and it's this that explains many of those apparent superpowers.
34:13For me, the force of gravity is the thing that defines my existence. It's the
34:19force that I really feel the effects of. But there are other forces at work, for
34:25example if I lick my finger and wet it, I can pick up a piece of paper. I can hold
34:31it up against the downward pull of gravity. That's because the force of
34:36electromagnetism is important. In fact it's the cohesive forces between water
34:42molecules and the molecules that make up my finger and the molecules that make up
34:46the paper that are dominating this particular situation and that's why this
34:52piece of paper doesn't fall to the floor. Now many insects can use a similar
34:57effect. Take a common fly for example.
35:06Their feet have specially enlarged pads onto which they secrete a sticky fluid
35:13and that allows them to adhere to rather slippery surfaces like the glass
35:19of this jam jar and allows them to do things that for me would be absolutely
35:24impossible and it's all down to the relative influence of the different
35:28forces of nature on the animal.
35:34So the capacity to walk up walls and fall from a great height without breaking
35:40plus super strength are not superpowers at all. They're just abilities gained
35:48naturally by animals that are small and lightweight.
35:55But this is just the beginning of my journey into the world of the small.
36:01Down at the very small scale it becomes possible to live within the lives of
36:07other individuals, worlds within worlds.
36:13But just how small can animals get?
36:28This macadamia nut plantation, an hour outside of Brisbane, is home to one of
36:34the very smallest members of the animal kingdom.
36:45These are a species of micro hymenoptera known as trichogramma.
36:50They're basically very small wasps and when I say small, I mean small. Can you
36:59see that? They're like specks of dust. They're less than half a millimetre long
37:06but each one of those is a wasp. It's got compound eyes, it's got six legs, it's got
37:13wings, they've even got a little stripe on their abdomen and they're very
37:20precisely adapted to a specific evolutionary niche.
37:25The trichogramma wasps may be small but they're very useful. They're natural
37:31parasites of an insect pest species called the nut borer moth which attacks
37:37the macadamia nuts.
37:43The micro wasps lay their eggs inside the eggs of the moths, killing the
37:49developing moth larvae. So what you're seeing here is the surface of a
37:56macadamia nut and here's a small cluster of moth eggs and there you see the wasp
38:03is walking over the eggs. They're almost pacing out the size to see whether the
38:08eggs are suitable for their eggs to be laid inside and if we're lucky, there you
38:15can see that. There we go.
38:23The wasps emerge just nine days later as full-grown adults. At this scale they
38:31live in a very sticky world dominated by strong intermolecular forces. To them
38:39even the air is a thick fluid through which they essentially swim using
38:44paddle-like wings. Incredibly these tiny animals can move about across several
38:52trees seeking out the moth eggs. But what I find more remarkable is that they do
39:00all this operating with very restricted brain power. One of the limiting factors
39:08that determines a minimum size of insects is the volume of their central
39:13nervous system. In other words the processing power you can fit inside
39:16their bodies and these little wasps are pretty much at the limit. They have less
39:21than 10,000 neurons in their whole nervous system. To put that into perspective most
39:27tiny insects have a hundred times that many but that's still enough to allow
39:32them to exhibit quite complex behaviour. These micro wasps exist at almost the
39:39minimum possible size for multicellular animals. But the scale of life on our
39:45planet gets much, much smaller. The wasps are giants compared to life at the very
39:53limit of size on Earth.
39:57The smallest organisms on our planet are also our oldest and most abundant type
40:13of life forms. These weird rocky blobs in the shallows of Lake Clifton just south
40:23of Perth are made by bacteria.
40:30These mounds are called thrombolytes on account of their clotted structure and
40:36they're built up over centuries by colonies of microscopic bacterial cells.
40:43Now although these colonies are rare by most definitions bacteria are the
40:48dominant form of life on our planet. On every surface across every landscape you
40:54find bacteria and in fact numerically speaking then there are more bacteria
40:59living on and inside my body than there are human cells. Bacteria come in many
41:06shapes and forms and are not actually animals or plants. Instead sitting in
41:12their own unique taxonomic kingdom. Compared to the cells we're made of
41:19bacteria are structurally much simpler and far, far smaller. Bacteria are typically
41:27around two microns in size that's two millionths of a meter which is very hard
41:33to picture but it means that you could fit around half a million of them on the
41:36head of a pin. Or to look at it another way if I took a single bacterium and
41:42scaled it up to the size of this coin then I would be 25 kilometres high.
41:51Bacterial type organisms were the first life on earth and they've dominated our
41:56planets ever since. Excluding viruses which by most definitions are not alive
42:02bacteria are the smallest free-living life forms we know of. But what ultimately
42:09puts the limit on the smallest size of life? Single-cell life needs to be big
42:16enough to accommodate all the molecular machinery of life and that size
42:22ultimately depends on the basic laws of physics. It depends on the size of
42:26molecules, which depends on the size of atoms, which depends on fundamental
42:31properties of the universe like the strength of the force of electromagnetism
42:36and the mass of an electron. And when you do those calculations you find out that
42:41the minimum size of a free-living organism should be around 200 nanometres
42:47which is about 200 billionths of a meter and that should be universal. It shouldn't
42:53only apply to life on earth but it should apply to any carbon-based life
42:58anywhere in the universe because it depends on fundamental properties of the
43:05universe.
43:14From the smallest bacterium to the largest tree it's your size that
43:21determines how the laws of physics govern your life. Gravity imposes itself
43:27on the large and the electromagnetic force rules the world of the small.
43:36But the consequences of scale for life on earth extend beyond dictating the
43:42relationship you have with the world around you. Your size also influences how
43:49energy itself flows through your body.
44:05These are southern bent-wing bats, one of the rarest bat species in Australia.
44:18Every evening they emerge in their thousands from this cave in order to
44:24feed. When fully grown these bats are just five and a half centimetres long
44:31and weigh around 18 grams. Because of their size they face a constant struggle
44:38to stay alive.
44:47Now we're using a thermal camera here to look at the bats and you can see that
44:52they appear as streaks across the sky. They appear as brightly as me. That's
44:56because they're roughly the same temperature as me. They're known as
45:00endotherms. They're animals that maintain their body temperature and that
45:05takes a lot of effort. I mean these bats have to eat something like three
45:09quarters of their own body weight every night and a lot of that energy goes
45:13into maintaining their temperature. As with all living things the bats eat to
45:21provide energy to power their metabolism. Although like us they have a high body
45:27temperature when they're active, keeping warm is a considerable challenge on
45:32account of their size. The bats lose heat mostly through the surface of their
45:41bodies but because of simple laws governing the relationship between the
45:46surface area of a body and its volume, being small creates a problem. So let's
45:55look at our blocks again but this time for surface area to volume. Here's a big
46:00thing. It's made of eight blocks so its volume is eight units and its surface
46:04area is two by two on each side so that's four multiplied by the six faces
46:09is 24. So the surface area to volume ratio is 24 to 8 which is 3 to 1. Now
46:18look at a smaller thing. This is one block so its volume is one unit. Its
46:23surface area is one by one by one six times so it's six. So this has a surface
46:29area to volume ratio of six to one. So as you go from big to small your surface
46:37area to volume ratio increases. Small animals like bats have a huge surface
46:44area compared to their volume. As a result they naturally lose heat at a
46:50very high rate. To help offset the cost of losing so much energy in the form of
46:56heat, the bats are forced to maintain a high rate of metabolism. They breathe
47:02rapidly, their little heart races and they have to eat a huge amount. So a bat's
47:09size clearly affects the speed at which it lives its life.
47:21Right across the natural world, the size you are has a profound effect on your
47:27metabolic 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. And
47:48then 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
48:00to consume and the rate at which you need to consume it. Bigger bodies have
48:08more cells to feed, so you might expect that the total amount of energy needed
48:13goes up at the same rate as any increase in size. But that's not what happens.
48:25If you plot the amount of energy an animal uses against its mass for a huge
48:30range of sizes, from animals as small as flies and even smaller, all the way up to
48:37whales, then you do get a straight line, but the slope is less than one. So that
48:43implies that gram for gram, large animals use less energy than small animals.
48:53This relationship between metabolism and size significantly affects the amount of
48:58food larger animals have to consume to stay alive.
49:02Now if my metabolic rate scaled one to one, with that of a mouse, then I would need to eat about four kilograms of food a day.
49:15In my language that's around 67,000 kilojoules of energy, which more
49:20colloquially is 16,000 calories. That is eight times the amount that I take in, on
49:27average, on a daily basis. Each of the cells in my body requires less energy
49:34than the equivalent cells in a smaller-sized mammal. The reason why this
49:42should be so is not fully understood. It's also not clear whether this rule of
49:48nature gives an advantage to big things, or is actually a constraint placed on
49:54larger animals. Take the relationship between an animal's surface area and its
50:01volume. Big animals have a much smaller surface area to volume ratio than small
50:07animals, and that means that their rate of heat loss is much smaller, and that
50:12means that there's an opportunity there for large animals. They don't have to eat
50:17as much food to stay warm, and therefore they can afford a lower metabolic rate.
50:25Now this helps explain the lives of large warm-blooded endotherms, like
50:30birds and mammals, but doesn't hold so well for large ectotherms, life's cold-
50:37blooded giants.
50:41Now there's another theory that says that it wasn't really an evolutionary
50:46opportunity that large animals took to lower their metabolic rate. It was forced
50:51on them. It was a constraint, if you like. The capillaries, the supply network to
50:56cells, branches in such a way that it gets more and more difficult to get
51:01oxygen and nutrients to cells in a big animal than in a small animal. Therefore
51:06those cells must run at a lower rate. They must have a lower metabolic rate.
51:17Or it could just be that as you get bigger, then more of your mass is taken
51:21up by the stuff that supports you, and support structures like bones are
51:26relatively inert. They don't use much energy. But whatever the reason, it's
51:34certainly true to say that the only way that large animals can exist on planet
51:39Earth is to operate at a reduced metabolic rate. If this wasn't the case,
51:47the maximum size of a warm-blooded endotherm, like me or you, would be
51:53around that of a goat. And cold-blooded animals, or ectotherms like dinosaurs,
51:59could only get as big as a pony. Any bigger and giants would simply overheat.
52:09Now there's one last consequence of all these scaling laws that I suspect you'll
52:14care about more than anything else, and it's this. There's a strong correlation
52:20between the effective cellular metabolic rate of an animal and its lifespan.
52:26In other words, as things get bigger, they tend to live longer.
52:39To explore this connection between size and longevity, I've left the mainland
52:51behind. For my final destination, I've come to one of Australia's remotest
52:57outposts. Named Christmas Island when it was spotted on Christmas Day in 1643,
53:08this isolated lump of rock in the Indian Ocean is a land of crabs.
53:26And in their midst lurks a giant wonder of the natural world.
53:34This is a Christmas Island robber crab, the largest land crab anywhere on the
53:41planet. These things can grow to around 50 centimetres in length, they can weigh
53:45over four kilograms, and they are supremely adapted as an adult to life on
53:52land. They can even climb trees. Over the years, the crabs have become well adapted
54:02to human cohabitation. These things are called robber crabs because they have a
54:09reputation for curiosity and for stealing things, anything that isn't
54:14bolted down to fill food, and cameras if they can get half a chance.
54:23These giants live on a diet of seeds and fruit, and occasionally other small
54:37crabs. Their large, powerful claws mean they can also rip open fallen coconuts.
54:46Quite a menacing animal actually for a crab.
54:53What's wonderful about these crabs is that they live through a range of scales.
54:57At different times of their lives, they have a completely different relationship
55:03with the world around them, simply down to their size. Throughout their lives,
55:08robber crabs take on many different forms. They begin their lives as small
55:13larvae swept around by the ocean currents, and as they grow, some of them
55:18get swept up onto the beaches of Christmas Island where they find a shell
55:22because they are, in fact, hermit crabs. They live inside their shell for a while,
55:28they continue to grow, and eventually, as adults, they roam the forest like this
55:33chap here. So, these crabs, over that lifespan, inhabit many different worlds.
55:42On land, the adults continue to grow and now have to support their weight against
55:48gravity. Compared to the smaller crabs whizzing around, these giants move about
55:55much more slowly, but they also live far longer. Of all the species of land crab
56:04here on Christmas Island, the robber crabs are not only the biggest, they're
56:08also the longest living. So, this chap here is probably about as old as me, and
56:14he might live to 60, 70, even 80 years old.
56:21Because of the robber crab's overall body size, its individual cells use less
56:27energy, and they run at a slower rate than the cells of their much smaller,
56:32shorter-lived cousins.
56:37The pace of life is slower for robber crabs, and it's this that's thought to
56:43allow them to live to a ripe old age.
56:54Your size influences every aspect of your life,
57:00from the way you are built,
57:05to the way you move, and even how long you live.
57:12Your size dictates how you interact with the universal laws of nature.
57:21So, there's a minimum size, which is set ultimately by the size of atoms and
57:26molecules, the fundamental building blocks of the universe, and there's a
57:32maximum size, which certainly on land is set by the size and the mass of our
57:37planet, because it's gravity that restricts the emergence of giants.
57:45But within those constraints, evolution has conspired to produce a huge range in
57:50size of animals and plants, each beautifully adapted to exploit the
57:55niches available to them.
58:00Size influences your form and construction, it determines how you
58:05experience the world, and ultimately how long you have to enjoy it.
58:15High drama on the menu tonight. Jess is in hospital and it's not looking good.
58:20Dancing on the Edge continues on BBC HD next.