through the jungle of fur to reach the ear.
There is one great danger in all this.
Blocking up an ear makes it useless to the moth,
and if the moth can't hear, it can't avoid the bats that hunt it.
That would be as disastrous for the mites as for the moth,
so the mites occupy only one ear
and always leave the other free.
Here they live, using one part of the ear for stacking droppings,
another for laying their eggs, and yet another for rearing their grubs.
How do their offspring find another of these highly-specialised homes?
By clambering down their host's tongue as it drinks
and waiting on the flower for another moth of the same species to turn up.
But parasites themselves may be preyed upon.
These finches in the Galapagos Islands are parasite hunters,
and their prey live on the giant tortoises.
You can hardly scratch yourself if you have legs like these!
Yet tortoises, like so many other animals,
are pestered by skin parasites, especially ticks.
The finches eat mainly seeds,
but ticks apparently make a welcome change.
When there's a tortoise nearby and the finches want a different meal,
they signal to the tortoise by jumping up and down in front of it.
The tortoise reacts to the finches' advances in a remarkable way.
It stiffens its legs so its huge body is lifted clear of the ground
and cranes up its neck.
The invitation is an unmistakeable one.
There's no way a tortoise could pick off parasites
from the places that these attendants manage to reach.
A few minutes' servicing by the finches
is enough to clear the tortoise of most of its pests.
Another satisfied customer.
Giraffe, like many other big game animals in Africa,
also have their own personal staff.
Oxpeckers live almost permanently on the bodies of their hosts,
scuttling about all over it.
On this spacious patterned stage, they eke out almost all their lives.
Here they argue and court; here, too, they feed their young.
They can't, it's true, nest here - they do that in holes in trees -
but they do line those holes with hair from the host's body,
so that presumably they'll still feel at home.
Their claws are so long they can cling in almost any position
and move in any direction.
Their beak is flattened so it slips easily between the long hairs of the giraffe's coat
as they scissor through it searching for ticks.
And they get everywhere, on young and on old.
Even when the animal moves off,
they will hang on with the skill and unconcern of accomplished jockeys.
But oxpeckers are a mixed blessing.
The ticks they eat are full of the giraffe's blood,
but sometimes they take that blood directly from an open wound.
By doing that, they're not improving their host's health, but damaging it,
keeping the wound open long after it would otherwise have healed.
Even so, without them, giraffes would be more seriously troubled
by their skin parasites than they are.
We ourselves can also get infested with ticks and fleas if we're not careful.
They're everywhere, particularly in the rainforest.
One has a reasonable chance of getting rid of animals that settle on your outside.
I can flick off these ticks.
If you can't do it for yourself,
you can get an oxpecker or a cleaner fish to do it for you.
But if the parasite settles not on the outside of your body,
but manages to get actually inside it,
that's a different matter.
The corridors and chambers of an animal's digestive system
offer great advantages to any creature that can dwell in them.
Inside here they are secure from enemies
and washed continuously by a nutritious soup
that their host has already chewed and partially digested for itself.
They just absorb it through their skin.
They don't even need a mouth.
The animals that are best suited to this interior life
are those long, spineless, legless creatures
we call collectively worms.
Flat, ribbon-shaped tapeworms hang on to the walls of the gut
with a crown of hooks that encircle their head.
In the long corridors of the intestines,
roundworms proliferate.
Every backboned animal that has been thoroughly examined,
whether fish or amphibian, reptile, bird or mammal,
has proved to be the host of a roundworm of some kind.
These living in a gut merely rob the host of some of its food.
But they may spread to cause severe damage to the liver and lungs.
Other roundworms, too, cause serious problems.
There are some as thin as threads that lodge in the heart valves.
The great problem faced by such internal parasites
is getting into their hosts in the first place,
and the complexity of some of their solutions verges on the unbelievable.
Denmark. A morning in summer.
There has been a shower of rain,
and the meadows and woodlands are drenched.
Snails are slowly crawling around through the wet leaves, grazing.
They're feeding on algae and rotting vegetable matter.
Early morning is the best time for them.
The sun won't dry them out,
and they can explore the vegetation they can't reach at other times.
But this one is different from the others.
Its left tentacle is swollen and pulsating.
It has a parasite.
A few months ago, the snail digested some bird droppings.
Within them were the tiny eggs of a fluke living in the bird's gut.
Those hatched and the parasite developed,
taking over much of the snail's body.
As the sun shines brighter, the parasite extends a muscular bag
packed with tiny larvae into the snail's tentacle.
If it has the choice, it nearly always picks the left one.
Birds rarely eat whole snails - they're far too big, and few can extract them from their shells.
Nonetheless, the larvae must reach the body of another bird to develop further.
For some reason, the presence of the parasite changes the snail's behaviour.
It does not, like uninfected snails,
crawl into the cool undergrowth out of harm's way.
Instead, it remains exposed, out in the open.
Dangerously so.
Now there is a parasite in each tentacle.
Perhaps they look like caterpillars or tasty worms.
Maybe they just look odd.
But certainly the flycatcher finds them interesting.
The connection has been made, the circle is complete.
Another bird has become infected.
Inside the bird, the striped bag releases its multitudes of larvae.
They move through the bird's body, taking up residence in its gut,
and the whole cycle starts all over again.
All internal parasites, however, do not necessarily injure their hosts.
Some indeed help them.
These organisms are undoubtedly alive and arguably animals,
since they don't have chlorophyll from which to manufacture their food.
They can break down the cellulose which forms most plant tissues,
something the digestive juices of most plant-eating animals can't do.
Their ancestors swam in ponds,
as some of their relatives still do today.
These are members of the family that have found a warmer, darker pond,
and one that is extraordinarily rich in edible material: a stomach.
So a buffalo, like most wild animals,
is not, as it might appear, a single individual.
It's a walking zoo.
Its oxpecker companions are obvious enough,
but if we look closer we would find ticks boring into its skin,
in its mouth leeches picked up when it drank from the river.
Tapeworms are trailing through its guts,
flukes are moored in the veins of its liver,
and protozoans are swilling around in its blood and stomach.
It's a whole community of different kinds of animals
that have been committed by evolution, for better or for worse,
in sickness and in health, to live together.
20,000 years ago, this cave in Wales was home for human beings.
We know because their bones have been found here.
It's a good home, too.
You're out of the wind and the rain, you can make a fire to keep warm,
and you can defend yourself against enemies,
whether they're other human beings or animals like bears or wolves.
In short, here you've got some control over your surroundings,
and can make yourself comfortable.
Animals find homes for themselves for much the same sort of reasons.
Some use ready-made ones like this cave. Bats, for example.
Others build the most ingenious and elaborate structures
to shield themselves from a world that can be so very hostile.
A rocky coast is one of the most difficult of all places to live.
Continuously pounded by waves, submerged twice every 24 hours,
and twice exposed to the air.
To survive, small molluscs secrete stout, almost unbreakable shells.
And when they're dead and gone, their vacated homes are much sought-after.
And in this pool, cut off by the falling tide,
there's a large housing market full of anxious tenants.
This could what they're waiting for.
Hermit crabs have found the easiest solution to the housing problem -
using somebody else's.
There is a major difficulty:
as a householder grows, he needs a bigger house.
This might be one.
A check has to be made on the dimensions.
Too big is almost as bad as too small,
because then enemies could pull the occupier out.
It's just the thing.
Now a slightly smaller home is on the market.
And another one, smaller still.
And another tenant.
Nobody wants to vacate a perfect residence,
but they can be forced to leave by strong-arm tactics.
The open grasslands of the American west are exposed places to live.
A small animal sitting around is very vulnerable.
So prairie dogs dig holes as refuges.
When lookouts sound the alarm, everyone can scamper to safety.
(CHIRPING)
Below, there's a warm, safe refuge.
The animals are so successful that they can proliferate in huge numbers
and form great settlements with hundreds of tunnels.
I'm in the middle of one of these towns,
and there are a couple of dozen burrows within yards of me.
But why does the prairie dog build so many burrows,
when digging's very hard work?
There's one way to get a clue.
This is a candle which produces a perfectly harmless smoke.
If I light it...
..and put it in the burrow, we can see what happens.
The smoke doesn't all blow away, but a wisp is carried down
and eventually emerges at another entrance mound 20 yards away.
The smoke has been carried between these two openings.
It's useful to have an escape hole if you're pursued by a wild ferret.
But there's more to it than that.
One of the problems of having a long burrow like this
is that it can get very stuffy.
The prairie dog deals with that problem
by building two differently-shaped entrances.
One is low to the prairie surface,
and the other has this mud tower built around it.
Wind blowing over the prairie moves faster a foot above ground
that it does at ground level.
So a breeze moving across this entrance will suck out the stale air.
The prairie dog, in fact, has a home with air-conditioning!
So no matter how long the prairie dog remains underground,
the air of its home remains fresh.
This refuge was not dug, it was woven from silk.
Only spiders and insects have the ability to produce silk.
This strange insect, a web-spinner,
has spinnerets on the end of its forelegs like boxing gloves.
Silk is such a useful building material
that others who can't make it...steal it.
A hermit hummingbird uses sticky spider silk
to bind her nest to the edge of a leaf.
Flying round as only a hummingbird can, with a strand in her beak,
she creates a suspended nest sheltered under the roof of the leaf
and far more difficult for predators to even find
than one placed on a branch or on the ground.
The Indian tailor bird also uses silk,
not to bind but to stitch.
She searches all the bushes around for her silk threads.
With this spectacular feat of craftsmanship,
she converts two floppy leaves into a single firm cup.
It may not look strong,
but it's quite secure enough to hold a lining of fibre and hair,
and mother and chicks sitting on top.
Leaves form excellent protection from the rain.
There's nowhere you need it more than the tropical rainforest.
Many animals that live here feed on fruit.
If their shelters are permanent ones, like a cave or hollow tree,
they may have to travel long distances to find a fruiting tree.
It's more convenient to have temporary encampments,
and no creature uses leaves more elegantly for that purpose
than those living under this leaf.
They've cut through these side ribs so the leaf flops down
and forms a perfect watertight tent.
And there they are.
Tent-making bats, the size of golfballs and pure white,
though the light filtering through the leaf makes them look green.
They'll only use it for a few nights,
then they'll be off to another leaf near another fruiting tree.
But you can use leaves in a much more radical way to build a home.
Weaver birds are the great experts.
It's the males who do the building.
The first step is to tie a leaf strip onto a twig.
It's not easy, for the fibre is very springy.
The trick is to keep a firm hold on it with one foot.
Then you can tie the knot with your beak.
It's a simple half hitch but a firm one,
and all subsequent work will literally depend on it.
Knotting takes practice, and this young bird is having trouble.
The next step is to weave a ring.
But you can't do that until you've got the first knot right.
The ring must be big enough to allow its maker to slip through
and not so big that it will allow larger animals to do so.
But it certainly mustn't be too small.
The strips must be fresh and supple,
and the birds get supplies from a patch of grass nearby.
They tear up the strips by gripping the grass blade close to the ground
and then simply flying straight up.
Over a thousand strips are needed for a single nest.
Once the ring is complete and firm, work starts on the roof.
The novice still has his problems.
Their technique in essence is the same as human weavers.
A strip is threaded alternately above and below other strips
that run at right angles to it.
In the beak of a master craftsman, this produces beautiful results.
In less skilful beaks...well, not so good.
But he's learning!
Many critical eyes are watching progress.
A female selects her mate largely on his ability as a weaver,
and he calls attention to it with his fluttering wings.
But others have the same idea.
She flies over to have a look.
The novice is being a little optimistic.
No one is taking any notice.
And eventually she makes her choice.
This looks neat enough, but it's not yet won him a mate.
But this bird, luckier or more skilful, has,
and now he can get on with finishing the job.
The whole construction is completed by adding with a looser weave
a long, downward-pointing entrance tube.
This will deter unwelcome visitors.
If a weaver hasn't managed to attract a partner,
then all his work is wasted.
For a female never chooses a nest that's so old it's turned brown.
There's only one thing to be done.
He'll have to start all over again.
That means dismantling his first attempt.
This is the only place he can build, as all the other sites are occupied,
and dismantling it is almost as hard work as weaving it.
On rocky cliffs, the problem is not so much shortage of sites for nests
but of material with which to build it.
Shags like to cushion their breakable eggs to stop them rolling about.
But there's only what the sea brings, and that's a very mixed bag indeed.
There's an easier way of getting stuff than carrying it all the way from the seashore.
When material is in such short supply, almost anything will do.
Even the beak and bones of another bird, a tern,
or the dried corpse of a rabbit.
There's no shortage of building material here.
Particularly if you build in wood,
which is the lifelong preoccupation of this animal.
It's a beaver.
And beavers are always busy.
This is one of the most massive of all animal constructions.
It's a dam that's blocked this valley and built up behind it a sizeable lake.
Its foundations are sticks rammed vertically into the bed of the stream.
Horizontal poles are laid across those, then boulders are dumped on it to give weight.
The downstream side slopes gently, buttressed with poles.
The upstream is more vertical and faced with mud.
It was designed, built and maintained by a family of beavers over decades,
and handed down through generations.
If ever there was an ancestral pile, this is it!
Such large properties need constant care and attention.
Dams must have a spillway to carry away the flow of the stream.
After heavy rains, it has to be enlarged
to allow the rising water to escape before it bursts the dam.
When the flood subsides, the dam has to be built up again.
Beavers have very clear ideas about the exact position of each piece
and labour away until they get it just right.
Large beams are needed for structural strength.
Small twigs, leaves and mud are essential to plug the gaps.
The purpose of all this labour is to create a lake.
In summer, they sink branches in it.
In winter, when the lake is frozen over, they dive beneath the ice
to retrieve the still green leaves from cold storage.
But it has another purpose: it makes their home virtually impregnable.
This is the lodge, the family residence.
There seems to be no way into it.
That's because the entrance is underwater, just about here.
So you have to be a skilled swimmer and underwater diver to get in.
In fact, it's a pretty well burglar-proof home.
Impressive though it is, and ingenious though the beavers are,
they do little more than cut it into convenient lengths.
However, there are some animals that can process wood
and turn it into a more malleable material.
Wasps chew up wood,
mix it with their saliva and make it into a fine paste,
which dries into a material that is both lightweight and strong: paper.
The common European wasp produces a very high-quality paper,
and with it builds nests of great perfection.
Within these identical cells,
a huge workforce is raised to serve the queen and maintain the nest.
Bees make defences with a substance no other animal produces: wax.
They secrete it from abdomen glands
and derive it from honey combined with fat.
Tropical stingless bees mix it with resin
and build entrance tubes to their nests within tree trunks.
The narrow entrance to the tube is heavily guarded by sentries,
who vet every arrival.
Inside, the workers labour, building interconnecting struts and plates
to support the brood combs.
Special pots are constructed for storing honey.
They will be filled to the brim, as reserves for times
when there's little or no food outside.
Wax is certainly a superb building material,
malleable, strong and capable of holding liquids,
but it's very expensive to produce.
Mud is much cheaper.
These pea-sized vessels are the work of another wasp:
a potter wasp.
Once again, saliva is an important ingredient.
It prevents the mud from crumbling when it dries.
With jaws working on the inside to keep the mud mixed and fluid,
and front legs checking the thickness of the wall,
she lays the mud in a strip,
using a technique human potters call coiling.
When the body of the pot is finished,
this greatly accomplished potter brings another ball of mud
and adds a final and most elegant flourish.
In goes an egg.
Now the cells must be provisioned.
A potter wasp doesn't feed her larvae on chewed bodies or honey.
She gives it living food: a caterpillar paralysed by her sting.
The lip built around the entrance helps to guide it in.
The mouth of the pot will be sealed with a pellet.
In a week, the larva hatches, eats its caterpillar, turns into an adult,
and breaks through the side of the pot and flies away,
leaving behind its beautifully-crafted nursery.
But you must go to West Africa
to see the most magnificent mud mansions built by insects -
homes that are indeed the most complex and sophisticated
constructed by any animal: the mounds of termites.
This immense fortress, towering 15 feet above me,
is the work of a Nigerian termite.
But what could be in these towers? They sound hollow...
There's an easy way to find out.
Very little. This long chimney is virtually empty.
To find the inhabitants, you must penetrate much further into the nest.
The workers are continually constructing magnificent arches,
vaults and corridors.
Among them are the bigger soldiers,
their huge heads filled with muscles needed to power their jaws.
Each worker places its mud in a position demanded by a master plan.
How they're able to do so, we don't begin to understand.
They store their food - dead wood - in special chambers throughout the nest.
Wood is hard to digest, but they extract the most from it
by first eating it and then cultivating a fungus on their dung,
which extracts more of the nutriment.
They then eat the fungus.
This fungus grows nowhere else but inside termite hills,
where the temperature is exactly right for it.
In the very heart of the fortress lives the queen.
She produces a thousand eggs a day,
to provide fresh recruits for the teams of gardeners, masons,
and the ranks of the army.
She resides in a special chamber which the workers renovate
to accommodate her growing bulk.
After a year or two she is, in effect, a prisoner,
for she's far too big to squeeze through the corridors of her residence.
But that is of no consequence.
She's so bloated producing eggs that she couldn't move if she wanted to.
And her eggs, as she produces them,
are carried to the nurseries by the attentive, indefatigable workers.
There are a million and a half insects in this one colony.
They and their gardens generate a lot of heat.
Within this enclosed building the air could become foul and hot.
The fungus and therefore the colony itself would die,
if the temperature varies more than two degrees from 31 degrees centigrade.
But the colony has a solution, and it's an architectural one.
This...6 feet beneath the surface of the earth,
is the cellar of the colony.
Its floor is studded with shafts that go down 12-14 feet,
down to the water table, where the workers can gather moist mud
to carry on building.
Its ceiling is a great plate
which carries the entire weight of the colony.
But on its underside is, I think,
really the most remarkable animal structure I've ever seen.
Lines of concentric veins.
They are made of mud and absorb moisture from the colony above,
and as it evaporates, it leaves this encrustation of white salts.
But more important than that, as it evaporates, it cools,
so that this cellar is much the coolest part of the colony.
And it's this that drives the air-conditioning.
The air heated by all the activity in the middle of the building
rises up into the upper storeys.
But this basement, thanks to these veins, is many degrees colder.
It draws down the stale warm air from the colony above,
down long chimneys which go round the edge of the cellar.
As it does so, there's a seepage of gas
through porous dimples in the walls.
Oxygen flows in and carbon dioxide out,
so that the mixture approximates to fresh air.
So these spires and turrets are key elements
in an air-conditioning system of a near-perfect mansion.
It has stout walls as protection from the elements and enemies.
Deep dungeons for gathering moisture.
Space inside for barns to store their food,
and gardens to grow their crops.
And yet all this was built by tiny insects with minute brains,
working in total cooperation in the complete darkness.
We might think we are the world's most accomplished architects,
but if this was built in human terms with every termite the size of me,
then it would stand a mile high.
And we haven't done that yet!