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- Hello, I'm Lindsay Turnbull

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and I teach biology at
the University of Oxford.

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In this video, I want to explain

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how and why some animals
large it over others.

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And when it comes to producing giants,

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there's really only one
group we have to look at.

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They're the vertebrates

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and they're the subject of
chapter eight of my book,

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Biology: The Whole Story.

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(birds chirping)

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(frogs croaking)

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So, who are the vertebrates?

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Well, vertebrates include a whole bunch

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of very familiar animals.

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In the oceans we have fish and sharks,

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and on land we have frogs,
salamanders, lizards,

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snakes, birds, mammals,
including us of course.

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The key defining feature of
vertebrates is the backbone,

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which serves two important functions.

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It's a crucial part of the skeleton,

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but it also protects a
bundle of nerve fibres

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that connect the brain to the body.

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Now, vertebrates are
all large, active animals

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packed with muscle.

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The ancestral vertebrate
strangely may have looked

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just a little bit like
this modern basking shark.

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The shark swims around
with its mouth open,

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water enters through the mouth

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and then out through those
slits in the side of its neck.

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It's a type of feeding
called filter feeding

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in which it traps any
particles in the water.

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Of course, there's one
really big difference

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between the ancestral vertebrate

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that lived around 500 million years ago

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and this modern basking shark.

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The ancestral vertebrate was tiny,

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just a few centimetres long,

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whereas this shark can
grow up to 12 metres.

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So that's one of the big questions

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that we need to answer in today's video,

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how is it that animals
become really large?

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What are the challenges
they face in doing so?

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And what are the advantages
to becoming a giant?

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The second thing we need to
look at is to consider, okay,

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so there's a load of
vertebrates in the ocean,

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but there's also a load
of four-limbed vertebrates

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called tetrapods on land.

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Just how did they get there?

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So we know that the tetrapods

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evolved from a fishy
ancestor, but which one?

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Well, let's go back 400 million years ago

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to a time when the oceans
were teeming with fish

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of all shapes and sizes.

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There are sharks, ancestors
of the modern sharks,

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but they're not the
meanest fish on the block

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because incredibly there's
another group of fish,

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the armoured fish,

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and some of those are truly massive,

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including this top predator, Dunkleosteus.

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It's the size of a modern great white.

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Now actually, those armoured
fish left no modern descendants,

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but swimming around
lower in the food chain

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were a load of bony fish,
and they come in two kinds.

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There are the ray-finned fish,

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which have small fine bones in the fins,

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and they've become very, very successful.

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So nearly every fish in today's
ocean that you can think of

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is actually a ray-finned fish.

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But there was another kind of bony fish

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that was rather common back then,

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and they're called lobe-finned fish.

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And in today's ocean,

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you'd have to look
really hard to find one.

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In fact, this is pretty
much the only species

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that you can find in the
oceans of a lobe-finned fish.

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It's called a coelacanth,

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and it was only discovered
around a hundred years ago.

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People call it a living fossil

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because it looks similar
to many fossil fish,

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but nothing like most of the
ray-finned fish around it.

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Now, this fish or something rather like it

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gave rise to the tetrapods, but
how exactly did that happen?

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So around 380 million years ago,

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a lot of fish are living in coastal waters

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and there are forests on land now,

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and some of that vegetation

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is coming into the water and rotting away.

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And when vegetation rots in
water, it uses up oxygen.

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That means that for
the fish in that water,

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they're struggling for oxygen.

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What they need to do is get
their noses into the air,

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like many modern fish do today.

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They can actually breathe air

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to supplement the oxygen in the water.

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But if you're gonna get
your snout into the air,

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it would be really useful to be able

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to push up from the bottom with your fins.

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And the fish that are best suited to that

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are the lobe-finned fish,

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because in those lobes

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there is quite a significant,
substantial arm bone.

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And so that's what happened.

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One group of lobe-finned
fish started to push up

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and develop legs on the
front of their bodies

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while they still had a fishy tail.

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And then eventually,

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a four-legged tetrapod
appeared with four limbs

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paddling around in those
swampy coastal waters.

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Now, the early tetrapod was
actually a very large animal,

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more than a metre long.

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So is that coelacanth,

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that lobe-finned fish. It's two metres long!

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So these are large animals

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and they need some
adaptations to their bodies.

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Those bodies are packed with muscles,

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and muscle cells are actively respiring.

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That means they need a lot of
glucose and a lot of oxygen,

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and they're also producing carbon dioxide

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which they need to get rid of.

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But the muscle cells are
buried deep in the body,

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and that's a long way
from the source of oxygen,

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which is either the water or the air,

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and a long way from the gut of the animal

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where the glucose is being absorbed.

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So what do they do?

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Well, they have to invent organs,

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specialist groups of cells

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that are going to provide resources

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to other cells in the body.

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So one of the things they
need is a circulatory system,

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and that's a set of pipes
that's going to deliver oxygen

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from either the lungs or the
gills to the muscle cells,

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bringing oxygen and taking
carbon dioxide away.

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And those pipes can also pass
by the gut and pick up glucose

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and transport that to the muscle cells.

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Now, the liquid flowing in
those pipes is of course blood,

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and the flow is kept moving

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by another organ called the heart.

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The heart is just a muscular pump

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that keeps blood flowing around the pipes.

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Now, the tetrapod circulatory
system has two parts.

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There's the part that pumps
the blood around the body,

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and there's the part that takes
the blood just to the lungs.

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And the heart is in two halves.

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The left hand side of the heart

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is much bigger and more muscular,

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and that pumps blood around the body,

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while the right hand side of the heart

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is smaller and not as muscular,

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and that just pumps blood to the lungs.

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So we've seen that as animals get larger

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they have to make fundamental
changes to their bodies,

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developing new organ systems for example.

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But there are even more
fundamental changes that occur

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as animals get bigger.

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So let's start by looking at a
really tiny animal, this ant.

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Now, ants are famous for being
able to carry heavy weights.

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These are leafcutter ants,

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and they can carry a piece of leaf

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that's up to nine times
their own body mass.

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But you or I can't carry half a tonne

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on our heads for a mile or two.

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Why not?

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Well, as animals get larger,

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their volume increases faster
than their surface area,

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and there are lots of biological things

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that scale with volume
or with surface area.

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So weight increases with volume,

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whereas the strength that an animal has

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is proportional to the
cross-sectional area of the muscle.

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That means as animals get bigger,

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they're becoming more
and more underpowered.

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Their strength is not increasing
as fast as their weight.

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And you can see this

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when you look at large
animals moving around.

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Look at this elephant.

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Look how slow and ponderous it is

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just moving one heavy leg after another,

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whereas this hamster in a wheel

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looks almost bionic in comparison.

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Now, these kinds of facts can
be used to help us reconstruct

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the way animals that are
now extinct once moved.

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So for example, this huge, heavy sauropod,

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we know that it must have moved
like this

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and not like this.

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So if the rewards of getting larger

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are just to get slow and ponderous,

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then why would any
animal want to get large?

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There must be some advantages
too, and of course there are.

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So we said as an animal gets larger

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the surface does not increase
as fast as the volume.

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But that also means that
something like an elephant

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has quite a small external
surface for its weight.

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And that means, because an
elephant is a warm-blooded animal,

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that it doesn't lose heat very
rapidly to the environment.

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And that's an advantage that
can make you very efficient.

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And that means, for example,

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that an elephant only has to
eat its own body weight in food

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every 30 days.

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Okay, it's still got to eat a lot of food,

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but it can focus on
eating low-quality food,

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of which there's plenty on the planet.

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It can just browse on branches and leaves.

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The mouse, in contrast, has a
large surface for its volume,

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and that means it's got to
eat its own body mass in food

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every six days.

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And it can't just eat low-quality food.

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It's got to focus on finding
really energy-dense food

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to keep that fast metabolism going.

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Now that's one advantage
then of being large,

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but there's a second one as well,

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especially if you are a herbivore.

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If you become very, very large,

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then you're going to become very difficult

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for a predator to kill.

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And you can see that even lions

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can be kept off by elephants.

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They are large and intimidating,

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and we believe that even in the past,

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those huge sauropods
were pretty much immune

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from the most fearsome
predator of all time,

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Tyrannosaurus rex.

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So we've seen that there
are many advantages

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to large body size,

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and we see in the fossil record that often

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groups of vertebrates do
evolve large body size.

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But in those first steps
that the vertebrates took

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when they came out of the water,

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what tempted them to come onto land?

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Well, there obviously must
have been opportunities there

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and food there,

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and those opportunities were only there

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because before the vertebrates
came out onto the land,

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the plants did the same.

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And that's what we're going to
look at in the next episode.

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Well, I really hope
you enjoyed that video,

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and if you did, please do share the link

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with colleagues and friends.

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If you'd like to get your own
copy of the book of course,

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there's also a link below.

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There's a lot more in chapter
eight about vertebrates,

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including more about those
scaling relationships

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and exactly how they work,

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and also a section on animal development.

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How do animals go from
just being a single cell

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to being a complex body
full of organs and tissues?

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Otherwise, look out for the next episode,

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which is all about plants.

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(birds chirping)