Rajasthan, central India.
It's the middle of the morning, the day is beginning to warm up,
and the animal community is in a relaxed mood.
The sambar deer are cooling themselves in the shallows of the lake,
looking for a bit of greenery to nibble and tolerantly taking the egrets for a ride.
The egrets, too, are finding a little to eat - an insect, perhaps, picked out of the deer's coat.
Nature isn't always red in tooth and claw.
Different kinds of animals are often regular companions and get on well with one another.
In the trees, langur monkeys are finishing their morning meal of leaves.
They're fussy, untidy feeders, and drop a lot of the leaves,
either by accident or because they don't fancy those particular ones.
And that suits the spotted deer.
In the dry season, the ground is parched and greenery worth eating very scarce.
So even the smallest fragment of vegetation fallen from above is worth having.
The deer follow the monkeys from tree to tree,
picking up leaves that by themselves they couldn't reach.
The monkeys also benefit from the presence of the deer.
They sometimes come down to forage on the ground, and there they're vulnerable.
The deer have a keener sense of smell, and may detect dangers that the monkeys can't see.
And if they do, they will stamp a warning.
We ourselves have very few such relationships, voluntarily, with other species of animals.
Except, of course, with those animals that we have domesticated and enslaved.
But back in our evolutionary past, we doubtless had many.
Today, maybe we think we're so powerful or have become so detached from nature
that we think we no longer need them.
In the natural world, those relationships are widespread.
Sometimes they've been in existence for so long that they have transformed the bodies of animals.
Sometimes they are only just forming.
This species of goby, for example, that lives around coral reefs,
has, probably quite recently, struck up a relationship with a shrimp.
The two regularly live together, sharing the same hole.
But the goby plays no part in making it. It's dug entirely by the shrimp.
The shrimp, in fact, seems to be a compulsive excavator, never content with its home as it is,
always carrying out improvements and digging extensions.
And the goby doesn't help.
In fact, if anything, it gets in the way.
But it's an essential companion for the shrimp, for this species of shrimp is virtually blind.
The goby, on the other hand, has excellent eyesight, and is always on the alert.
The shrimp, as it works, keeps in touch - literally -
by continually flicking one of its long antennae over the fish to make sure it's still there.
If the goby is out of the burrow, the shrimp knows that it's safe to carry on working.
The goby is always on the lookout for something to eat
and may have to make little excursions to get it.
A tiny, edible morsel that floated by.
But even while it's feeding, the shrimp's antenna is still in touch with it.
Danger. And when the watchman retreats to safety, so does the shrimp.
The goby, having fed, seems content to remain in the hole.
Why expose yourself to danger unnecessarily?
But the shrimp is perpetually keen to work and often appears to be hustling the goby,
as though to persuade it to go out again.
The shrimp collects its food from a little patch of alga that grows beside the burrow entrance.
It knows just where that is,
so it can nip across quickly and snatch a few clawfuls with the minimum of risk.
All is well, as long as the shrimp keeps in touch with the goby.
But if it ventures away, then there can be trouble.
That was an anemone it blundered into, and it beats a swift retreat.
For a moment, it seems lost.
Then the goby comes over and contact is re-established.
The partners are together again and all is well.
So, two very different animals operate a partnership.
The blind landlord provides the accommodation, and the tenant provides a guidance service.
Hermit crabs live in a different kind of home.
Instead of a hole, they use an empty shell.
But they, too, can find themselves with lodgers.
This one's companion is a ragworm.
For the worm, this is a good place to be.
It has a home where it's safe from predators, curled up inside the shell alongside the crab.
And on its very doorstep, there's a regular supply of food brought there by the crab.
Nonetheless, collecting a share of that food seems a fairly risky business.
The crab's mandibles could easily chop the worm's head off.
But the worm has had a lot of practice at this sort of thing.
Whether the crab gets any benefit at all from the arrangement is rather doubtful,
but there's not much it can do to get rid of its lodger anyway.
A small octopus.
Hermit crabs are one of its favourite foods.
In the centre of those writhing arms it has a powerful beak
with which it can drag the crab from its shell.
And that's the end of both the hermit crab and its lodger.
But this species of hermit crab recruits a bodyguard.
Anemones have stings in their tentacles - stings that are strong enough to repel an octopus.
Since the crab wanders about a great deal,
its bodyguard, to be any good, has to travel with it.
It's not easy to unstick an anemone from a rock, but the crab knows the trick.
You have to tickle it around the edge of its bottom.
You can tell that the anemone isn't particularly alarmed by this procedure
because it hasn't closed up and is still confidently waving its tentacles.
That makes three guardian anemones on the crab's shell.
But is that enough to give it protection?
The octopus is not sure.
No, it's not worth it.
So the crab has its bodyguards,
and its bodyguards, for wages,
are likely to get little bits and pieces that float by when the crab chews up its meals.
It's not always easy to decide in these partnerships which is exploiting which.
The balance of advantage is often very delicate.
Take, for example, these ants in Australia.
They are extremely ferocious and normally they'll rip apart any caterpillar.
But see how they're treating this one.
The caterpillar has on its back a number of little nipples
which apparently fascinate the ants.
0ne near its back end, when stimulated by an ant,
produces a drop of liquid, honeydew, which the ant drinks.
As the caterpillar grazes on leaves, the ants keep continuous guard over it,
threatening anything that comes near it, so that even birds don't attack it.
The caterpillar has to make sure that the ants don't forget what kind of caterpillar it is.
If they think it's any other kind, they will tear it apart and eat it.
So the caterpillar every now and then makes a characteristic buzzing vibration.
Not only that, but on either side of the honeydew nipple there are two others.
From these sprout little tentacles which apparently release a pheromone,
a kind of perfume that keeps the ants happy and unaggressive.
Tree ants build nests almost as big as footballs from the growing leaves of the tree.
They feed on any small creature that happens to land in the tree.
This grasshopper stood little chance. As soon as it landed, they set upon it.
Now they are butchering it and carrying it back, piece by piece, to their nest.
As well as this nest, the workers also construct small shelters.
First, a team bridges two leaves and slowly pulls them together.
0thers arrive carrying grubs, which they gently squeeze,
so that the grubs are stimulated to produce a sticky silk.
By passing the grubs back and forth, they weave a fabric that holds the two leaves together.
They're making a shelter for their precious caterpillar.
When it's complete, they guide the caterpillar into it.
0nce in its shed, it'll be safe for the night.
The ants look after it like farmers looking after a dairy cow.
And their cow, in return, provides them with nourishing food.
At this stage in their relationship, neither ant nor caterpillar seems to have the advantage.
But this same species of ferocious stinging ant
has also got a partnership with a different species of caterpillar,
and there the result seems to be very different.
This one has a glossy, horny shield on its back,
and it, entirely of its own accord, marches right into the ants' nest.
It is in no way deterred by the ants' threatening postures and sprays of formic acid.
No matter what the ants do, they can't stop it.
Deeper and deeper it goes, through the corridors of sewn leaves, right into the heart of the nest.
It reaches the queen. If she is killed, the whole colony will die.
But she is not what the intruder is looking for.
The soldiers attack valiantly. Their jaws make little impression on the caterpillar's armour.
Neither can they get underneath it and reach the soft, vulnerable body.
0n it goes, until at last it reaches the nursery chambers
where the developing grubs lie.
Try as they might, they can't lift up the shield sufficiently
to enable other defenders to get beneath.
With the intruder actually within the nursery, the workers become totally confused.
Some try to carry off the grubs to safety elsewhere.
But they can't do it quickly enough.
The caterpillar snatches a grub, pulls it under the shield,
and then, secure beneath its armour, slowly eats it.
As the season progresses, several of these armoured intruders make their way into the nest
and there gorge themselves on ant grubs.
After several weeks, the caterpillars have eaten all the grubs they need to grow to full size.
Now, in the heart of the nest, they're ready to shed their armour and turn into butterflies.
But how can a butterfly get through the ranks of the ant soldiers?
Now, surely, they'll have a chance to get their revenge.
Slowly, the insect hauls itself out of its horny armour.
But it's a strange sort of butterfly that emerges.
It's covered in scales that are so slippery that the ants can't get a proper grip on them.
Those that do manage to bite
get their jaws covered with a sort of fluff that they clearly find intensely irritating.
So, at last, the murderous lodger goes free.
Ants and caterpillars, like crabs and anemones, are about the same size.
But if a lodger is very much smaller than its landlord,
then it tends to live not so much with it as on it.
Those monkeys over there, for example.
They've got a number of tiny passengers.
Like most mammals with hairy coats, they've got fleas.
And when fleas bite and start sucking blood, they itch.
It may be necessary to get a friend to help pick them out from parts that you can't reach.
This, however, is not fur. This is the fabric of a bird's nest, and fleas live here, too.
A young starling within two days of hatching is likely to have several dozen fleas.
Fleas have six legs, just like any other insects, but they've lost their wings.
Those would be an encumbrance to an animal crawling around among fur and feathers.
Instead, they have powerful hind legs that enable them to jump onto their host.
Their jaws have become specialised for sucking blood, and they feed on nothing else.
They have to live on another animal, and they contribute nothing to its welfare.
This is not a partnership. It's parasitism.
Nor are fleas the only parasites in a bird's nest.
Lice are there, eating feathers.
They, too, are insects, and any one bird may have up to a dozen different kinds,
each living on and eating a different kind of feather on the neck, the wings or the head.
Insects seem to have a particular flair for parasitism.
Every one of their main families has some members who've taken it up.
But insects themselves can also be parasitised.
This nest of bumble bees has been invaded by mites, diminutive cousins of spiders.
They're so tiny that several hundred of them can sit on the leg of a bee.
And they, too, itch.
They get everywhere,
and once they've found their way into a colony, they spread to every member of it.
Mites are just as specialised as feather lice.
These bee mites live nowhere else but on this particular species of bumble bee.
And this flower, milkweed, is a staging post for one of the most specialised mites of all.
Moths come to feed on the milkweed at night,
dipping their long, threadlike tongues deep into the heart of the flowers to sip the nectar.
But this moth is already infested with mites.
Its ear, a tiny hole in the side of its head,
has become the home of a whole colony of them.
And a new colonist awaits on the flower itself.
While the moth drinks, the mite crawls up its tongue.
0nce on the moth's head, it knows, mysteriously,
just which direction it must take 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.
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 obligingly occupy only one ear and always leave the other free.
Here they live and breed, using one part of the ear tube for stacking their droppings,
another for laying their eggs, and yet another for rearing their grubs.
And how do their offspring find another of these highly specialised homes?
Why, of course, 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 are themselves preyed on.
This mouse that lives in Central America
regularly carries a dozen or so passengers wriggling around in its fur.
They're beetles, and they were once thought to be parasites that sucked the mouse's blood,
for they have large and powerful jaws.
But, oddly, the mice that carry the most beetles are not the most anaemic, as you might expect.
0n the contrary, they seem to be the most healthy.
It turns out that the mouse's most serious parasites are here in the lining of the nest.
Fleas and ticks that D0 suck its blood.
Each mouse has several holes in the forest, and all are likely to be infested with these fleas.
When a mouse settles down to rest in one,
the beetles drop off and go hunting for the fleas in the nest lining.
So the beetles, far from injuring the mouse, actually aid it.
Got one.
As far as a beetle is concerned,
the mouse is a convenient transport system for getting from one hunting ground to another.
The mouse that carries the most beetle passengers has the most flea-free life.
These birds, too, are hunters of parasites.
They're finches that live in the Galapagos Islands.
And the creatures they help - 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 meal with a difference,
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 that its body is lifted clear of the ground, and cranes up its neck.
The invitation is an unmistakeable one.
There's no way that the tortoise could pick off parasites
from the places that these attendants manage to reach.
A few minutes' servicing by the finches
is quite enough to clear the tortoise of most of its pests.
Another satisfied customer.
Fish have the same sort of problem, and the same sort of solution.
The huge manta ray is troubled by sea lice and barnacles that burrow into its skin.
But it has other company, an attendant fleet of small fish that travel with it.
When the opportunity arises, they swim over their host's body, even inside its gaping mouth,
picking off the passengers.
Like giant tortoises, fish with skin problems patronise regular cleaning establishments.
This grouper hangs in the water at this special place on the reef,
and small wrasse that have been waiting amongst the coral start fussing around it,
even daring to swim inside the huge jaws.
It's not only fish that work as cleaners.
This moray eel is being tended by a shrimp.
0pen wide, please.
Amazingly, the cleaners are never harmed...
..even though they tickle.
These shrimps are really quite large, big enough to make a reasonable meal,
but they're never injured, either.
Regular customers come back to these cleaning stations every few days.
Although the staff of wrasse and shrimps can deal with as many as 50 an hour,
there are often queues of itchy fish waiting their turn.
Some fish, however, have their own personal valets.
Suckerfish, or remoras, have a fin on their back that has been modified into a sucker so powerful
that it's almost impossible to pull off a remora if it wants to stay on.
They travel with their host wherever it goes
and slip around its body, picking off parasites, whenever there's an opportunity to do so.
Giraffe, like many other big game animals in Africa,
also have their own personal staff.
0xpeckers live almost permanently on their hosts' bodies, scuttling about all over it.
0n this spacious, patterned stage, they act out almost all their lives.
Here they argue and court. Here, too, they feed their newly fledged 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 their host's body,
so that, presumably, they'll still feel at home.
Their claws are so long that they can cling in almost any position and move in any direction.
Their beak is flattened, so that 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.
And 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, of course, can also get infested with ticks and fleas if we're not careful.
They're everywhere, particularly in the rainforest.
0ne 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, maybe 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 inside it,
that's a very 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, mashed and partially digested for itself.
All they have to do is to 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 onto 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.
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 the lungs.
0ther roundworms, too, cause serious problems.
There are some as thin as threads of cotton that collect in the valves of the heart,
blocking them so seriously that their host dies.
The young of such threadworms, swimming around in the bloodstream,
depend on biting insects to transfer them to another host.
During the day, they swim in blood vessels.
At night, when mosquitoes are biting, they move up into the capillaries just beneath the skin,
so that when a mosquito does start to suck their host's blood, they are taken up.
They continue to grow inside the mosquito.
When, in due course, it bites some other animal, they're transferred into a new host, a new home.
0thers, smaller still, that wriggle among the blood corpuscles,
belong to the most ancient of all animal groups, the protozoans.
They first got inside animals so long ago
that most of their hosts have developed an immunity to them.
But human beings, those most recent of mammals, have not yet done so.
In them, they cause sleeping sickness and death.
The problem that faces all internal parasites is how to get their offspring into another host.
Tiny ones, like the protozoans, may be transferred by biting insects.
Bigger ones, like this roundworm, have to use other methods.
The first stage, getting their eggs to the outside world, is easy.
This roundworm, full of eggs, simply sheds them into its host's gut
so that they fall out with its droppings.
0nce in the soil, they may lie dormant for some considerable time.
But eventually, when conditions are suitable, the temperature just right and moisture around,
they begin to hatch.
The tiny worms crawl up leaves of grass,
and there await the moment when a hungry mouth will crop the grass
and they will get carried into another stomach.
Such transfers are not always straightforward.
Sometimes the complexities of the route they follow are almost beyond imagining.
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 of one kind and another.
Early morning is the best time for them.
The sun is not yet hot enough to dry them out
and they can explore parts of 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 took in, along with its normal food, some bird droppings.
Within them were the tiny eggs of a fluke that was living within 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 striped, muscular bag packed with 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.
As the day wears on, it does not, like uninfected snails,
crawl back 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 soon move through the bird's body and take up residence in its gut,
and the whole cycle starts all over again.
Flukes are related to the flatworms that live independent lives in ponds and swamps.
But they've found their greatest success as internal parasites.
Some reside in the liver. 0ther kinds anchor themselves in the bladder, the lungs or the gut.
Most are capable of causing serious disease.
All internal parasites, however, do not necessarily injure their hosts.
Some, indeed, actually help them.
These microscopic organisms, undoubtedly alive and arguably animals
since they don't have chlorophyll, like plants, with which to manufacture their food,
live in the stomachs of most large animals.
They are able, chemically, to break down the cellulose which forms most plant tissues,
something the digestive juices of most large, plant-eating animals can't do.
Their free-living ancestors swam in ponds, as some of their relatives still do today.
These are members of the family that have simply 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 looked closer we would find ticks boring into its skin,
in its mouth, leeches that it 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 swimming in its blood and swilling around in its stomach.
It's a whole community of different animals that have been committed by evolution,
for better or for worse, in sickness and in health,
to live together.