We think we know Britain's wildlife.
But there's a secret world beneath our feet.
A hidden kingdom teems with millions of burrowing creatures...
..their lives shrouded in mystery.
Why do they live underground?
Where do they sleep and eat?
And how do they raise their young?
Their secret, subterranean world remained unseen.
Until now.
Wow! Look at this. This is our vision come true.
'We really are breaking new ground.'
'We've unearthed real animal burrows
'and created full-scale replicas of our own...
'..complete with tunnels,
'chambers
'and, of course, residents.'
To be able to crouch here this far away from a little badger
like this and observe what it's up to...
it's beyond my dreams.
'Three iconic British burrowers, each with a different home life.'
'What happens inside the riverbank burrow of the water vole?'
This is fascinating, there's an altercation going on.
'How does a rabbit warren cope with its annual baby-boom?'
I think we've uncovered something that is unknown to science.
'And how do badger cubs learn the skills they need to survive?'
Did you see the way she stuffed the nose in? Yeah.
It was really just, "What is this?"
'From their birth in winter...
'to their emergence from the burrow in summer,
'we have an unprecedented window on their lives.
'Explore this magical world with me, Chris Packham.'
'How do the burrowers create their homes?
'And how do they begin a new generation?'
This is everything that we wanted from our warren.
Our portal to this magical world
is this small farm on the edge of Dartmoor.
It's the headquarters for our remarkable year-long project
to investigate the underground world of the burrowers.
The largest man-made burrow of its kind ever built
is just down this path.
Welcome to Burrower Central,
set here in the heart of the beautiful Devon countryside.
Now, I've been studying wildlife for the last 40 years,
but there are millions of creatures that live out their lives
underground that we know very little about,
simply because we can't see inside their wild burrows without disturbing them.
But now we've set up the most extraordinary experiment -
we've recreated the underground homes of three very different
and iconic British burrowers.
It's always been impossible to watch animals
undisturbed in their burrows.
For protected species, like badgers, it's illegal.
But in our accurate replicas, we can watch rabbits, water voles
and badgers without disturbing them, throughout the seasons.
We can record their lives in a way which is new to science.
And, joining me on this great British wildlife adventure,
we've got some great British burrowing minds.
Over here, Dr Sasha Norris,
editor of the Encyclopaedia Of Mammals, and keen rabbit fan.
Hi, Chris. Well, we know a lot about what rabbits do above the ground,
they're one of the most common mammals in the British countryside,
the one people see a lot.
But what we don't know is what they do under the ground,
which is possibly the most important part of their lives,
including where these little ones come from.
And they are phenomenal digging machines,
burrowers par excellence, aren't they?
They are, and in fact our rabbits dig the most complex, labyrinthine tunnels.
So we've got a lot to learn.
We have, yeah, I'm looking forward to it, finding out all about you.
And you've got to do your work beneath the burrow.
Of course, we're not the first to capture these species on camera.
We know from what's already been filmed above ground
that wild rabbit populations expand rapidly.
A dozen or so rabbits in a winter warren
can quickly explode to 100 or more in summer.
So, we want to know exactly why it is
that the rabbit population expands...
..and how they breed so fast.
Our next burrower is a very different animal,
not common, like the rabbit, but actually endangered - the water vole.
And, Derek... Hi, Chris. ..you own the farm here,
and you've been a pioneer in water vole reintroduction in the UK.
That's right. I've been involved with captured breeding projects since the mid-1990s.
We bred tens of thousands of animals for reintroduction projects throughout the British Isles.
And, aside from being sadly endangered these days,
they are also fabulous little burrowers,
as we've just witnessed. Yes, he's completely disappeared.
They basically live in the banks of rivers and streams.
They have burrow entrances which are submerged, so they come in
underwater, go right up through the banks, right to the top of the banks.
It's adapted to that, so, yes, it's a very good burrower.
'It's also a bit prone to biting,
'which is why we've used the tank here to get a closer look.'
And, currently, what do we know
about what happens inside that riverbank?
This is still a big blank in our knowledge.
We know virtually nothing about what they do underground.
In the wild, water voles are so elusive that very little
of their behaviour has ever been filmed in their burrows.
We need to know more about these endangered creatures.
How do they use their burrow?
How do they breed and raise young?
Because this could be vital knowledge in our fight
to save the water vole from disappearing from the UK.
And, finally, perhaps my favourite burrower of all - the badger.
And to help me get to grips with these enigmatic creatures,
one of my badger heroes, Dr Chris Cheeseman. Chris, welcome.
Nice to meet you. Welcome.
This is going to be a very different challenge
than with our other burrowers, because these animals are nocturnal,
shy, difficult to get to know.
It's a fantastic opportunity.
I've spent the majority of my career watching badgers above ground,
at night, foraging, doing what comes naturally to badgers,
and it's always fascinated me
to know what happens when they go down their setts,
the behaviours that badgers develop, like digging,
and forming bonds and associations with the other badgers
in their social group - this is a wonderful opportunity.
Of all of our burrowers, I know badgers best of all.
I spent my youth studying them.
But they spend three-quarters of their lives
out of sight underground, and their cubs don't surface at all
for the first three months.
So, we have to ask, what goes on during this vital period of development?
How do they bond, grow, and learn to be adult badgers?
'To create perfect replicas of the underground homes
'of our three secretive species,
'we had to investigate what happens in nature, first of all.
'Our research began nearly a year ago.'
We started in autumn with a rabbit warren
at Devon's Bicton Park Botanic Gardens.
This is the perfect place for us to begin to understand
the structure of a rabbit warren, because this is indeed a warren.
It was covered with brambles, they've been cleared,
but all the other signs are here.
Look at all the nettles growing in the field here
where there've been fertilised by years of rabbit droppings.
But, just like many places in the UK,
the rabbit population here got too large and it's needed to be managed,
so there are no longer any rabbits living in this warren.
This empty warren gives us a unique chance to find out
what we need to create our replica warren.
And our first task is simply to find out how many tunnels there are.
That is definitely going in there.
So we definitely know that this is joined through to here.
'Park manager Cliff Clogger fills some tunnel entrances with smoke.'
So that's one, two, three, four, five
six, seven, eight, nine, ten,
11, 12, 13.
Yeah, that's a lot of exits and entrances.
'So, smoke reveals the number of entrances,
'but we now need to know how deep these tunnels go,
'so for this we need another experienced warren expert -
'Tanya the ferret.'
Moment of truth, then, Cliff.
Well, she's never let us down up until now.
Tanya's collar works like an avalanche rescue beacon.
It gives us a precise reading of her position and depth.
Pop Tanya down this one. It looks a well-used hole.
It's brilliant, isn't it?
So she's actually down to about four and a half feet here.
So that's flag number one.
She's now on the move again. I've got six feet here.
Six feet deep there. OK.
She's out again here.
Oh, no, she's straight back down again.
What a ferret professional, honestly! She knows what to do.
I'm very surprised at these depths... There she is!
'So, the warren's up to two and a half metres deep,
'but how wide are the tunnels?'
We've taken one very high-quality drain camera here,
and amended it by adding these wires so that Sasha can steer it
when I stick it down... Supposedly. ..the burrow.
If you look at the size of the leaf,
I think the tunnel's about that diameter.
There we are.
It's about 20 centimetres, just about the right height
to give a rabbit, a large male rabbit, room to move through.
'But there aren't just tunnels.'
Ah, there we are. There we go. Is that a tunnel either side?
That's a little chamber. It is, look, definitely.
'There are chambers,
'and each one of them is about twice the size of a football.'
I am interested to know, do rabbits have friends?
Do rabbits hang out with particular other rabbits?
Yes, are there four or five in one chamber, or are they all in separate chambers?
I don't know, one of the things we don't know about,
what goes on under the ground, so it'll be absolutely
fascinating to find out through the building of the artificial warren.
Now we have all the data we need to build our own warren -
numerous exits and entrances,
a network of tunnels and chambers,
some of them up to two and a half metres in depth.
So, now I'm really confident that we can sculpt a burrow,
a system of chambers and tunnels that will keep our rabbits
really happy so that we can watch them living naturally.
And then we can learn all the things we want to know.
But there's one thing that's become very clear, and that is
that for this species of burrower, we'll need to think big.
In fact, very big.
At Burrower Central, our base in Devon,
huge mechanical burrowers begin work.
They shift tonnes of soil...
..and an area bigger than a tennis court is cleared.
We're going to replicate the incredible insulation
of a real burrow with four walls and a roof...
..and then install a giant cross-section
through our replica warren.
This is our cutaway through the hillside.
This is our slice through the rabbit warren.
It is big - it's at least two and a half metres down to here,
because we know they were going that deep.
It's got the tunnels at about the right diameter.
It's essential the rabbits feel they're in a natural warren.
Then they'll behave naturally,
and we will hopefully learn all the things that we want to.
Now all we need are some rabbits.
But that's not so easy.
Hey, you're supposed to be in your box. Are you hungry?
You see, wild rabbits can be very aggressive,
specifically with other rabbits.
But Sasha has a solution closer to home.
Rabbits were only domesticated very recently,
and so inside each household rabbit is a wild rabbit waiting to come out,
and in fact a whole suite of genetic behaviours are in there,
and once put into a natural-type situation
they will express themselves.
There's one more challenge.
You see, underground it's dark,
so how can we create enough light to observe them,
but not disturb them?
I think it looks OK.
The key to this project's success lies with this.
It's an LED light.
Now, it is a cool light,
so we can leave these on in the burrows
and they'll never get hot enough to burn any of our animals -
got to put their safety first, of course.
The other thing is, we can control the brightness,
so we can start with it low, and when they're happy
we can increase that brightness until we can see
exactly what's going on in that burrow.
It's February.
At Burrower Central, work begins on the second of our burrows.
It's a bit more complicated than the first one.
Our second burrower is the smallest of the three,
but it's such a powerful digger that we need to line everything with concrete and steel.
And, for it to be happy and to behave as it would in the wild,
we also need to build it a river.
It's the water vole.
How do they create their homes?
And how do male and female water voles breed and raise their young?
There is one problem, though, and that is that no-one
seems to know anything about the structure of water vole burrows.
But Derek has come up with a big idea -
a big idea that's never been tested before.
'He wants to encourage the water voles to dig out a new burrow,
'one that we can lift the lid on to see
'how they build a home inside the riverbank.'
Look at that, they have been busy.
'We put a pair of voles into this box of sand...'
Let's lift that glass up. Right.
'..and they soon made themselves a home.'
Oh, look at that, Derek!
'After a month, we see exactly how the water voles built their burrow.'
It's fantastic, absolutely fantastic.
And when you think this is perhaps the first time anyone has had
the opportunity to look at this sort of structure for this species, as well.
It really is quite remarkable.
One thing that surprises me is the size of the chambers
they've excavated. These are... Yeah, these are big.
..the size of a rugby ball.
What about the diameter of the tunnels, though, Derek?
Because to me they look about water vole size, don't they?
Yeah, well, I mean they're quite Plasticine animals, quite a slinky shape.
So even when you look at them sitting
it's this kind of dumpy little furry, round vole.
When they actually decide to slink through something,
yeah, they'd pass their bodies through that.
'Remarkably, we discovered that these meticulous creatures have
'separate rooms, each with a separate function.
'There's a bedroom.
'There's a larder.
'And that's not all.'
A latrine! look!
That's really interesting, because these are the territorial markers
water voles leave to tell other water voles the territory is occupied.
The partitioning nature of everything they've done, they're so OCD!
They can move in, as far as I'm concerned.
I could live with these voles, they have everything in the right place.
It's pretty neat.
The Feng Shui when it comes to this is right. I'm up for that.
But the thing is, Derek, this is just two animals.
If we introduce a pair and they breed throughout the course of the season,
by the end of that season we could end up with more than 20 animals.
Absolutely, yes.
So this would not be big enough for a family group of that size. No.
We're going to have to build something on an altogether larger scale.
Yeah, this is basically a plan of a bungalow.
What we now have to do is sort of build a housing estate.
Using our sandbox experiment to guide us,
we can now build our burrow.
Outside, we're going to create the all-important river.
And we'll connect it to a network of tunnels
and rugby ball-sized chambers.
So, we're beginning to get a sense of how our burrowers plan their lives underground.
The large, abandoned rabbit warren at Bicton
revealed that rabbits create these complex underground structures.
The sandbox experiment with our water voles
showed that they make tunnels which are very clean and tidy -
something I quite admire myself.
But, of course, there is one more burrow that we need to make.
The sett for our badgers.
We've chosen to build our badger sett 100 miles away in Somerset
at a wildlife rescue centre.
Hello, little badger.
Here we have a remarkable opportunity
to give a temporary home to orphaned badgers
rescued from all over the country.
Hello.
Each winter, vet nurse Sarah Cowan nurses abandoned cubs back to health.
They're tested for TB, and if all's well
they'll be released back into the wild later in the year.
We can give this year's young cubs the chance to grow up
in a place that should feel just like home.
Sarah, I'm not normally one for cooing over cute animals,
but I'm going to give these 9.9 out of 10 for cuteness.
Who are they, who have I got here? Who's this boisterous one here?
The boisterous one you've got is Luna,
that's a female that has been hand-reared.
She was found in Cornwall, crying and really cold, in a field.
She's lovely. They haven't got that badgery smell yet, have they?
No, because they're on milk they're a bit like babies,
they just have that milky, sweet smell,
but it won't be long before they start being musty.
Wow. Look at that. Still got all the pelage patterns,
and her fur is already characteristically badger, isn't it?
A nice, long tail, as well. Look at that female tail there.
A nice long female tail. Let's have a look at her front feet.
Claws already growing, big, spatulate front foot.
All soft at the moment, no coarse. No. No digging's been done.
No, but they are just like great big shovels, aren't they?
Yeah, look at that, it's fantastic, isn't it?
(You've got lovely feet. Yes, look, you've got lovely feet. You have.)
What about this little one here? This little one is called Truffle.
He was found in Wiltshire in the back of someone's garden.
How old is he, do you think?
Probably about 8-10 weeks and she's about 12-14.
Yeah, and look at the growth difference there.
They grow very quickly at this stage, don't they?
'Now, on its own, a badger cub will struggle to survive.
'As a group, they all stand a much better chance.
'Luna and Truffle will be joined by five other orphans,
'creating a group similar in size to a wild badger family.'
They will all be released together, and it's just important that
that bonding and socialisation happens right from an early age.
This one's trying to excavate an early sett in the back of the chair.
Already using her little claws. Their instincts kick in.
Yeah, look at that.
'It's going to be really nice to be able to provide Luna,
'Truffle and all of the others with a new home
'to play an important role in their rehabilitation.
'But to get that right we need to get the design of the sett right'
to maximise their chances of survival and recovery.
So, what does a badger sett look like underground?
Easy question, but difficult to answer.
You see, we can't do what we've done at Bicton,
investigate their sett that way,
and we can't dig it up to understand it, either.
No, what we need to do here is to employ some good,
old-fashioned detective work involving the skills and services
of our badger expert, Dr Chris Cheeseman.
In a Somerset Valley, we can unearth the key features of a sett,
and as badgers are protected, we can only look for clues -
we can't disturb anything.
This sett looks like it's been here a long time.
This is part of the landscape.
This sett could have been here hundreds of years, a couple of hundred.
I expect it predates all of these trees. Yeah?
I should think it goes back a few hundred, if not more.
'Like 60% of badger setts in Britain, this one's in a wood,
'and the trees here not only provide shelter,
'but, below ground, their roots support the tunnel network.'
I wouldn't be surprised if it's quite a deep sett,
probably a couple of metres down.
'But how do the tunnels and the chambers compare to the rabbits'?'
That's a perfect hole, a classic badger hole.
The oval shape, and an S-chamber is generally 60 centimetres long,
45-50 centimetres wide.
Look at the groove here with the digging.
That furrow is where they back out of the sett,
pumping soil, pushing it out onto the spoil heap behind.
And that's been going on for generations of badgers.
'Like most of us, badgers like to be comfortable when they're sleeping.'
Of course, there's their bedding trail leading out there,
where they're dragging the material back in.
Quite a busy highway. Yes, it is.
'When we build our sett, we're going to need to replicate
'these distinct oval shaped tunnels,
'and build much larger chambers -
'not only are badgers physically bigger,
'but these are social animals,
'they're going to need extra space, too.
'And, to make them feel really at home,
'we'll design their sett with roots, as if it's under a tree.'
It's late winter,
and now, after months of intensive research,
construction... and one or two tea breaks,
the big day has arrived,
because the first of our burrows is ready.
We can finally go down the rabbit hole into the wonderland below.
Our rabbits have had two weeks to acclimatise.
There are 10 of them
to reflect the winter population of a small warren in the wild.
By summer, there could be five times as many,
so we've allowed plenty of space for serious expansion.
Look at that, I wasn't convinced that we could pull it off,
but to me that looks absolutely fantastic.
And this has to be the first time that any human being has had
this sort of view of a rabbit warren from underground.
And, aside from looking great, of course,
this is a magnificent opportunity
to do some really good science, a unique chance
to find out exactly how these burrowing animals live underground.
How will our rabbits use the warren?
How do they organise their underground community?
And, crucially, how do they breed?
Our burrow incorporates everything we've learned
from the abandoned warren at Bicton park.
We built a network of tunnels and chambers,
condensed it down,
and then sliced it in half
to make this cross-section through the burrow.
Toughened glass protects our rabbits and allows us to observe them.
They're connected by tunnels, which are rabbit-sized,
and wide enough in places for our rabbits to pass.
Of course, our warren has its outside component.
It is a bit different than a wild warren, though.
Typically, they will have been built underneath some brambles,
in a hedgerow, anything to stop predators seeing the rabbits
when they first emerge from that burrow.
We've gone for a covering of mesh here
to protect our rabbits from things like foxes and buzzards.
We've also changed the flooring, too.
Rather than natural soil, we've gone for sand
because it reduces the chances of them getting infections of parasites.
We've also given some of our rabbits an ear tag.
It's a painless addition and it helps us keep track.
Look at this.
It's the picture of a lazing, dozy rabbit.
That might be what you think life's like in the warren
throughout the winter - the rabbits just taking it easy,
basically, avoiding the inclement weather up top,
conserving their energy, there's not much food about, so they're not as active.
But then, at a certain point in the winter, everything changes.
There's a massive hormonal change and it all kicks off.
In the wild, the ratio of males to females
during the hormone surge is typically low.
So, in our warren, there are only two males, or bucks.
Why there are so few males becomes apparent within minutes.
This is a battle for supremacy...
..just as it would happen in the wild.
And this is the male with the upper hand - Thumper.
He regularly chases his rival out of the warren
and won't tolerate any other males.
So, what's it all about?
The girls.
Thumper wants them all to himself.
Of course, we're only able to see this, get this insight
into their lives, because we can watch them beneath the ground.
Already, our warren is paying dividends
when it comes to understanding their behaviour.
The life of our eight does is no less competitive.
They also fight for dominance.
But how do they display their power?
They do it with urine.
This is the biggest perpetrator - Hazel.
Think about it.
If this were a wild warren,
it would be completely dark,
so pungent urine is the perfect way
for Hazel to mark her territory.
As the dominant female,
Hazel gets the pick of the chambers.
And she's gone with this one.
Then less powerful females
make the choice from what's left.
It's the does that pick their rooms.
And what they're doing inside them
is cause for real excitement
because they're building nests.
What's really encouraging is just how settled these rabbits are.
Ultimately, what we want them to do is give birth in here
so that we can reveal some of the intimate secrets of that process.
Whether or not we will - that's just a matter of patience.
This river is perfect water vole habitat
and it's the inspiration for our replica riverbank.
The second of our burrows for our water voles is nearly ready.
Vole expert Derek Gow adds a few finishing touches -
aquatic plants that the voles will both eat and use as bedding.
This is a real favourite.
These are the roots of flag iris
and we're going to plant quite a lot of flag iris here.
And here at the waters edge are the tunnels that connect it
to our riverbank burrow system.
Inside, our sandbox experiment has been scaled up
to become water vole nirvana.
Because the females are fiercely territorial,
just one pair of voles from Derek's breeding programme is living here.
This is our water vole burrow system.
This is our idea of a slice through a riverbank
so that we can reveal the subterranean lives of this species.
Now this animal here is a typical brown water vole. That's our male.
Of course, first task is to give them a name.
So I thought I'd bring this along.
This is The Wind In The Willows by Kenneth Grahame.
And if any of you have read it,
you'll know that the vole in here was misnamed.
Yeah. They called him Mr Ratty.
Not a rat at all, of course, but a water vole.
So that is Mr Ratty.
There's a female in here as well, actually,
but she's slightly different in colour.
She's a black water vole.
These are typically found in the north of Scotland
and on that account, we thought we might call her Bonnie.
We want Mr Ratty and Bonnie to show us
whatever water voles get up to at home.
We also want to discover how water voles mate and raise their young.
And if the pair do breed, their young will be released
back into Britain's waterways to help boost numbers in the wild.
But for the moment,
mating seems to be the last thing
on their minds.
This is fascinating, there's an altercation going on
between Mr Ratty and Bonnie here.
She seems to be the dominant animal,
although it's him that is occasionally
slapping her in the face with his paws.
But just listen, there's a huge amount of squeaking going on.
Judging by the cheek movements,
it's Mr Ratty that's doing most of that.
And now, look, she's picked up a piece of leaf and is eating it.
That is what we call classic displacement behaviour.
She's stressed, so she does something that's completely
unconnected to the conflict that's going on.
But he is cowering on this side.
But you don't need to be a genius to see that Bonnie is the boss.
Things go from bad to worse.
Mr Ratty and Bonnie are leading very separate lives in the burrow.
They've divided it in half.
Separate bedrooms.
With separate en-suite latrines.
Frankly, it looks like our water vole story
has ended before it's even begun.
Back at Secret World Wildlife Rescue Centre in Somerset,
our six badger cubs are joined by a late arrival.
Hello, you two. Here's a little friend.
Tiny Bramble was found fighting for life in a Wiltshire field.
Who's that?
She's just seven weeks old and she's the weakest.
Will the other cubs accept this new arrival?
Because if they don't, then Bramble won't survive.
The big moment has arrived.
Time to introduce the cubs to their new home.
They're stepping into a sett that should feel just like the wild.
After weeks of being hand-reared in a cosy kitchen...
..this is going to be their home
until they're ready to be released into the wild.
We've recreated a sett based on the wild one
we investigated in Somerset.
Chambers large enough to give our family plenty of space
to bond as a group.
A network of oval-shaped tunnels
and tree roots to help them feel
like they're living in a woodland.
In the wild, badger cubs spend their first three months underground.
But now, for the first time, we're down here with them
to witness the most formative period of their lives.
To be able to crouch here, this far away,
from a little badger like this
and observe what it's up to...
..it's beyond my dreams.
How will they develop from cubs to adults?
Will they form a social structure?
And without parents to teach them, will they have the instinct
to develop the skills they'll need to survive as adults?
But how will or badgers take to their new home?
Boisterous female Lunar wastes no time in exploring the whole sett.
Like all badgers, her eyesight is poor,
but her sense of smell is around 700 times stronger than ours.
And in this new environment, she's like a kid in a sweet shop.
I'm intrigued, though, by the male cub Truffle.
He's doing something that I know to be instinctively badger.
These little animals have only been in here a matter of minutes,
but given the right material, their instinct is to make a nest
so that they can stay warm, which is essential to them.
So we've learned something already -
that they're not entirely dependent on the females, the sows,
when it comes to looking after the nest down under the ground.
'Truffle is already doing something
'that I've only glimpsed in adult badgers above the ground
'when they move bedding in and out of their sett to give it an airing.'
What about that?!
And they do it - look at this - in exactly the same way.
Grabbing it with their forepaws,
putting it between them on their belly
and then pulling it backwards.
Amazing.
Watching our young badgers, we can already see
how well suited their body structure is
to this underground environment.
They're the perfect shape to fit through tunnels
and to create them.
Looking at the structure of this animal from its skeletal form, here,
it says only one thing to me, Chris. It's a digger.
It's a burrower.
Low slung, short limbs, long spine, very rigid skeletal structure here.
And again, if you look at it end on, like this,
look down the nose of this badger, it is round, it is compact,
it is perfect for passing down through a tunnel, isn't it?
You can see the shape of the tunnel there.
Because most badger tunnels are that dome shape, with a flat bottom.
Exactly. And that profile is exactly fitted to that shape of the tunnel.
This animal is a perfect example of a digging animal. Massive bones.
Very short, but look at the thickness of that humerus.
That is as thick as my index finger. Yes, very powerful bone.
Together with solid scapulas.
Big, powerful muscles, on thick strong bones.
And look at these four feet. And the length of the claws.
It is an animal that evolved to be able to live the fossorial life,
the burrowing life.
Our badgers have plenty of time to earn their stripes as diggers.
But there is a more immediate concern.
You see, they must form a strong social group to survive in the wild.
And nearly all of them share just one chamber.
There is, however, one absentee. It is the latecomer to the group.
The youngest female, Bramble.
She is spending most of her time hiding in another room.
That's not a good sign.
If she does not integrate, then she has got no chance in the wild.
She must bond with the others,
she has got to become part of the family.
Winter continues. And life above ground is still dormant.
But below ground it is a month since our rabbits moved in. They are busy.
Hazel has been observed doing something strange.
She has been plucking the fur from her belly, to line the nest.
Why becomes all too apparent in the next few astonishing moments.
Now it is quite hard to see what is happening. But there - a tiny nose.
A baby rabbit has just been born. One in a litter of five.
They are tiny. They are bald. They are vulnerable.
They weigh just 30 grams. And they are completely blind.
And now, something we didn't expect.
Just minutes after giving birth, Hazel leaves the chamber,
heading outside to feed.
Can this be normal?
One week later, I return to Burrower Central.
There has been a baby boom in the warren.
Hazel's kids are only one of several litters.
And amazingly, they have all been left to fend for themselves.
So far today I haven't seen any of the rabbits in here.
How are such tiny, vulnerable kids able to survive
underground on their own even in winter?
Heat sensitive cameras can reveal
just how effective a fur-lined nest is.
The brighter the image, the warmer it is.
The key thing is that they are born without fur. They are naked.
And that is why they need this luxurious nest to keep them warm.
And also why they have to huddle like that.
Yes, I mean, they are very altricial, they are undeveloped young,
they cannot see, they cannot hear, and they've got no fur.
You can see how good it is at insulating them.
Because the heat is just percolating through here.
Most of it is dark, which means that it's... Yes, very dark.
That movement is actually indicative of
quite a clever way of keeping themselves warm.
Which is, they will jostle for position,
and try to get to be the sort of inside rabbit, the warmest one.
Then another one says, "Hang on a minute, I am feeling a bit cold!"
And they have got very sensitive temperature gauges in the skin.
They will shift back inside and so on, keep changing position.
If we speed up the footage
we can see something even more extraordinary.
Each kid does indeed take a turn in the middle where it is the warmest.
The constant rotation pulls the warm nesting material all around them,
like a blanket.
This natural incubator works so well that the mother can spend
the whole day out of the burrow, foraging for food.
And, amazingly, the mothers only have to return to the nest
once every 24 hours to suckle the young.
Here she comes. There she is.
Look at that one. He is out straightaway.
There is a fury of activity when she is over the nest
and they are all trying to latch on.
And they are all kneading at her belly to get the milk to flow.
The milk doesn't flow immediately.
But when it starts to flow, there is a high-pressure jet of milk
straight into their mouths.
And this is really the sum total of her maternal care
while they are in the nest.
So the milk that she is giving them has got to last them for 24 hours.
So that is extremely rich milk.
Yes, one of the richest milks among mammals.
And there she goes, she is gone. Two or three minutes, at most. Yes.
That is the last you're going to see of your mum for probably 24 hours.
We set out to understand how it was
that warrens are ideal for rabbits to breed...like rabbits.
And now we have our first clue.
It is the warren itself that gives rabbits such an advantage.
In such warmth and security they can breed as early as January.
Well ahead of other mammals.
And, in the sanctuary of the warren, warm and safe from predators,
our kids are already growing fast.
This litter is around 12 days old.
But already much bigger, stronger, and furrier.
Over the next few months
we'll follow them as they grow and develop.
We left our water voles, Mr Ratty and Bonnie, living separate lives.
They may not be natural partners,
but they are certainly natural swimmers.
But unlike other aquatic animals, like this marsh frog,
they haven't evolved webbed feet.
Now, you might think that is because they use their feet for digging.
But as vole expert Derek can reveal,
they use a completely different tool.
It surprises me, Derek, because we know that badgers,
for instance, are phenomenal digging machines.
Their whole skeleton, their musculature, their feet,
is all about shifting soil.
And here we have got a fairly conventional rodent
which is doing a great job of burrowing. Yes.
Actually they are better adapted for it than we realise.
They have forward-pointing teeth, and of course,
they use it to basically chop out mouthfuls of soil, which they
then drop in front of them and kick back out of the tunnel.
So they can actually burrow incredibly quickly.
Mr Ratty is certainly shifting the soil.
He is digging for Britain, isn't he? Very tough, very versatile species.
We may be well impressed by his teeth and his digging prowess,
but someone isn't. Bonnie.
Mr Ratty and Bonnie have been having a torrid time.
A bit of outright aggression between them in the burrow.
They have got separate bedrooms.
Is it going to happen? That is the question.
Well, I mean, I haven't seen anything like this before.
This is again down to the fact that we have got this facility
and we are looking at behaviour which is just new to us.
I think it is very likely that what they are doing
is just getting used to the idea of being together in the burrow system.
They are establishing their own space.
But they are quite a bolshie species anyway, so they are very territorial,
and what we do know, from above-ground observation,
is that males have been seen chasing females and pursuing them right into
the water, then jumping on top of them, to mate with them in the water.
And that is quite a vigorous, seemingly aggressive-type process.
Derek gives us reason to hope that not all is lost.
Our water voles may be British,
perhaps it's just that their relationship has a fiery Latin edge.
Is this the first sign of a beautiful new beginning
for Mr Ratty and Bonnie?
We'll have to wait and see.
Back in Somerset, our autumn badgers are getting used to their new home.
They are making full use of all of the chambers and all of the tunnels.
But are they starting to bond?
Well, the good sign is that every three hours
the whole group hunkers down into one giant huddle.
All except one.
The young female, Bramble, has joined the cubs in the main chamber,
but she is still on the edge of the group.
We are hoping that she is going to be fully accepted soon.
For expert, Chris Cheeseman,
this is his first chance to see what is going on.
For me, after 37 years of studying badgers professionally,
seeing them interact is a fascinating insight into their behaviour.
Whilst our badgers have already formed a social group,
will they also create a social structure?
A hierarchy, without parents to guide them?
When they are not asleep, they bite, they scratch, and they wrestle.
It looks like the opposite of bonding.
In fact, this is how they work out the pecking order.
And boisterous female, Luna, still the biggest of all of them,
is throwing her weight about.
It is going to be interesting to see how things develop, because
these are very early days in the formation of this group of badgers.
At the moment we have got a female badger which seems to be
taking the dominant role, but I think through time
that will probably change, and maybe a male,
especially as they become sexually mature,
will probably more likely take over the dominant lead in this group.
But who knows? We will see.
As winter draws to an end,
there has been an intriguing development at our riverside.
Water vole pair Mr Ratty and Bonnie have blocked out
all but one of the burrow entrances.
So the question is, why have they done that?
Quite deliberately, why have they blocked this burrow?
It could be they are worried about predators
getting into the burrow system.
It could be that they don't want the smell of the latrines inside
wafting out to attract predators.
But why have they done it at this particular moment?
It may be that there is a parallel between our voles
and the behaviour that we see in wild animals.
Because they typically do this when they are about to,
or have, just given birth.
So maybe there is a chance that somewhere in here
we have got some baby water voles.
We can't disturb them at this sensitive time.
So it will be a few days before we know
if our voles have swelled the ranks of this endangered animal.
But across the farm, with spring in the air, breeding is in full swing.
There are now four litters of rabbits.
And with each born just a few days apart,
we can see how rapidly they change in the first two weeks of life.
We designed the warren so we could reach our rabbits if needed.
And this gives me a rare chance to check on our kids' growth.
These youngsters are about 10 days old.
So we are quite confident that we won't disturb them
or the mother by investigating the nest.
I am wearing the gloves not to protect me
from anything nasty that these kids may have
but to make sure that I don't give them anything nasty of course.
Look at that. See the ears are very much larger.
The eyes are openable, although this one is squinting at the moment.
The fur is much more developed.
It has even got the dark guard hairs visible.
I am just going to cup my hand in and lift this one out.
So unbelievably warm. There. Look at that.
And within a couple of days these youngsters will be
emerging from this nest.
You can see they are quite active.
Their claws have developed here. Their rear legs.
And there is the little tail at the back. It is keen to get back in.
So I'm going to huddle around.
There is no doubt that our warren is off to a great start.
We studied wild burrowers in intricate detail
so that we could get the design of our replicas here exactly right.
And these are the largest structures of this kind ever built.
And they are working too.
Our animals have moved in and they are
so comfortable that they are behaving perfectly naturally.
We really have had a very exciting start to our project.
But there is so much more to learn.
Next time, in spring, will the smallest cub, Bramble,
bond properly with the rest of our orphaned badgers?
For the first time, we see the scale of a wild rabbit warren
when we cast an abandoned one in concrete.
This is an absolute revelation.
And we meet the ultimate burrower.
It rarely ever surfaces and can shift an amazing amount of soil.
Up to 540 times its own bodyweight.
If you are enjoying the world of our burrowers
and you want to see more, there is plenty available
on our website, where we have put together the most
fascinating behaviour that we have discovered throughout the series.
I have also produced a few guides that will show you how to spot
burrowers living where you live.
Learn how to spot badgers, water voles,
rabbits, foxes and moles.
That is all on...