0:01:09 > 0:01:12Rajasthan, central India.
0:01:12 > 0:01:19The day is warming up, and the animal community is in a relaxed mood.
0:01:19 > 0:01:24Sambar deer are cooling themselves in the shallows of the lake,
0:01:24 > 0:01:31looking for greenery to nibble, and tolerantly taking the egrets for a ride.
0:01:31 > 0:01:39The egrets, too, are finding a little to eat insects, perhaps, picked out of the deer's coat.
0:01:39 > 0:01:43Nature isn't always red in tooth and claw.
0:01:43 > 0:01:48Different kinds of animals are often regular companions and get on well with one another.
0:02:10 > 0:02:15Langur monkeys finish their morning meal of leaves.
0:02:21 > 0:02:24They are fussy, untidy eaters.
0:02:24 > 0:02:27They drop a lot of the leaves,
0:02:27 > 0:02:31either by accident, or because they don't fancy those particular ones.
0:02:33 > 0:02:36And that suits the spotted deer.
0:02:36 > 0:02:41At this time of the dry season, the ground is parched and greenery worth eating is very scarce.
0:02:41 > 0:02:47The smallest fragment of vegetation fallen from above is worth having.
0:02:47 > 0:02:51The deer follow the monkeys from tree to tree,
0:02:51 > 0:02:54picking up leaves that, by themselves, they couldn't reach.
0:03:15 > 0:03:20The monkeys also benefit from the presence of the deer.
0:03:20 > 0:03:23They sometimes come down to forage on the ground, and there, they are vulnerable.
0:03:23 > 0:03:27Deer have a keener sense of smell than the monkeys.
0:03:27 > 0:03:32They detect dangers that the monkeys can't see, and stamp a warning.
0:03:48 > 0:03:55We ourselves have very few such relationships, voluntarily, with other species of animals,
0:03:55 > 0:04:00except with those animals that we have domesticated and enslaved.
0:04:00 > 0:04:04But back in our evolutionary past, we had many.
0:04:04 > 0:04:12Today we think we are so powerful, or so detached from nature, that we think we no longer need them.
0:04:13 > 0:04:16But in the natural world at large those relationships are widespread.
0:04:16 > 0:04:22Some have existed for long enough to transform the animals' bodies.
0:04:22 > 0:04:26Some are only just forming.
0:04:26 > 0:04:32This species of goby, for example, that lives around coral reefs,
0:04:32 > 0:04:36has probably recently struck up a relationship with a shrimp.
0:04:38 > 0:04:42The two regularly live together, sharing the same hole.
0:04:42 > 0:04:45But the goby plays no part in making it.
0:04:45 > 0:04:47It's dug entirely by the shrimp.
0:04:54 > 0:04:58The shrimp, in fact, seems to be a compulsive excavator,
0:04:58 > 0:05:05never content with its home, always making improvements and digging extensions.
0:05:06 > 0:05:09And the goby doesn't help.
0:05:09 > 0:05:13In fact, if anything, it gets in the way.
0:05:13 > 0:05:16But it's an essential companion for the shrimp,
0:05:16 > 0:05:20for this species of shrimp is virtually blind.
0:05:23 > 0:05:27The goby, on the other hand, has excellent eyesight.
0:05:27 > 0:05:30It's always on the alert.
0:05:30 > 0:05:35The shrimp, as it works, keeps in touch literally
0:05:35 > 0:05:39by continually flicking one of its long antennae over the fish
0:05:39 > 0:05:42to make sure that it's still there.
0:05:42 > 0:05:44As long as the goby is out of the burrow,
0:05:44 > 0:05:48then the shrimp knows that it's safe to carry on working.
0:05:51 > 0:05:56The goby, naturally, is always on the lookout for something to eat,
0:05:56 > 0:05:59and may have to make little excursions to get it.
0:06:03 > 0:06:06A tiny edible morsel that floated by.
0:06:16 > 0:06:18But even while it's feeding,
0:06:18 > 0:06:21the shrimp's antenna is still in touch with it.
0:06:23 > 0:06:28Danger. When the watchman retreats to safety, so does the shrimp.
0:06:32 > 0:06:40The goby, having fed, seems content to remain in the hole. Why expose yourself to danger unnecessarily?
0:06:40 > 0:06:44But the shrimp is perpetually keen to work
0:06:44 > 0:06:47and appears to hustle the goby to persuade it to go out again.
0:06:51 > 0:06:56The shrimp collects its food from a little patch of alga that grows beside the burrow entrance.
0:07:05 > 0:07:08It knows just where that is,
0:07:08 > 0:07:13so it can nip across quickly and snatch a few clawfuls with the minimum of risk.
0:07:27 > 0:07:33All is well as long as the shrimp keeps in touch with the goby.
0:07:33 > 0:07:35If not, there can be trouble.
0:07:37 > 0:07:43That was an anemone it blundered into, and it beats a swift retreat.
0:07:43 > 0:07:46For a moment, it seems lost.
0:07:48 > 0:07:53Then the goby comes over and contact is re-established.
0:07:53 > 0:07:58The partners are together again and all is well.
0:08:08 > 0:08:13So two very different animals operate a partnership.
0:08:13 > 0:08:20The blind landlord provides accommodation. The tenant provides a guidance service.
0:08:22 > 0:08:27Hermit crabs live in a different kind of home.
0:08:27 > 0:08:30Instead of a hole, an empty shell.
0:08:30 > 0:08:34They, too, can find themselves with lodgers.
0:08:43 > 0:08:46This one's companion is a ragworm.
0:09:04 > 0:09:08For the worm, this is a good place.
0:09:08 > 0:09:13It has an excellent home, where it's safe from predators,
0:09:13 > 0:09:16curled up inside the shell alongside the crab's abdomen.
0:09:16 > 0:09:21And on its very doorstep there's a regular supply of food brought there by the crab.
0:09:22 > 0:09:28Nonetheless, collecting a share of that food seems a risky business!
0:09:28 > 0:09:31The crab's mandibles could easily chop the worm's head off.
0:09:31 > 0:09:36But the worm has had a lot of practice at this sort of thing.
0:09:36 > 0:09:41Whether the crab benefits from the arrangement is rather doubtful.
0:09:41 > 0:09:46But there's not much it can do to get rid of its lodger anyway.
0:09:59 > 0:10:05A small octopus. Hermit crabs are one of its favourite foods.
0:10:33 > 0:10:38In the centre of those writhing arms it has a powerful beak
0:10:38 > 0:10:43with which it can drag the crab from its shell.
0:10:43 > 0:10:48And that's the end of both the hermit crab AND its lodger.
0:10:53 > 0:10:58But this species of hermit crab recruits a bodyguard.
0:10:58 > 0:11:06Anemones have stings in their tentacles stings that are quite strong enough to repel an octopus.
0:11:06 > 0:11:14Since the crab wanders about a lot, its bodyguard to be any good has to travel with it.
0:11:21 > 0:11:27It's not easy to unstick an anemone from a rock. The crab knows how.
0:11:27 > 0:11:32You have to tickle it around the edge of its bottom.
0:12:11 > 0:12:16You can tell that the anemone isn't particularly alarmed by this
0:12:16 > 0:12:20because it hasn't closed up and is still waving its tentacles.
0:12:42 > 0:12:47That makes three guardian anemones on the crab's shell,
0:12:47 > 0:12:52but is that enough to give it protection?
0:13:17 > 0:13:20The octopus is not sure.
0:13:40 > 0:13:43No, it's not worth it.
0:13:53 > 0:13:56So the crab has its bodyguards,
0:13:56 > 0:14:03and its bodyguards, for wages, get bits and pieces that float by when the crab chews up its meals.
0:14:08 > 0:14:13It's not always easy to decide which partner is exploiting which.
0:14:13 > 0:14:18The balance of advantage is often very delicate.
0:14:18 > 0:14:21Take these ants, in Australia.
0:14:21 > 0:14:26They are ferocious, and normally they'll rip apart any caterpillar.
0:14:26 > 0:14:29But see how they treat THIS one.
0:14:31 > 0:14:36The caterpillar has on its back a number of little nipples
0:14:36 > 0:14:41which apparently fascinate the ants.
0:14:48 > 0:14:55One, near its back end, when stimulated by an ant, produces a drop of liquid
0:14:55 > 0:14:58honeydew, which the ant drinks.
0:15:05 > 0:15:09As the caterpillar grazes on leaves, the ants keep continuous guard over it,
0:15:09 > 0:15:14threatening anything that comes near it, so that even birds don't attack it.
0:15:30 > 0:15:36The caterpillar must make sure that the ants don't forget what kind of caterpillar they are dealing with.
0:15:36 > 0:15:41If they think it's any other kind, they will tear it apart and eat it.
0:15:41 > 0:15:46So the caterpillar regularly makes a characteristic buzzing vibration.
0:15:46 > 0:15:51And on either side of the honeydew nipple there are two others.
0:15:51 > 0:15:56From these sprout tentacles which apparently release a pheromone
0:15:56 > 0:16:01a kind of perfume that keeps the ants unaggressive.
0:16:01 > 0:16:07Tree ants build nests almost as big as footballs from the leaves of the tree.
0:16:07 > 0:16:13And they feed on any small creature that happens to land in the tree.
0:16:16 > 0:16:19This grasshopper had little chance.
0:16:19 > 0:16:23As soon as it landed, they set upon it.
0:16:23 > 0:16:28Now they are butchering it and carrying it back, piece by piece, to their nest.
0:16:30 > 0:16:35As well as this nest, the workers construct small shelters.
0:16:35 > 0:16:38First, a team bridges two leaves
0:16:38 > 0:16:41and slowly pulls them together.
0:16:41 > 0:16:47Others bring grubs which they squeeze, to produce a sticky silk.
0:16:47 > 0:16:55By passing the grubs back and forth, they weave a fabric that holds the two leaves together.
0:16:55 > 0:16:58It's a shelter for the caterpillar.
0:16:58 > 0:17:03When it's complete, they guide the caterpillar into it.
0:17:09 > 0:17:13Once in its shed, it will be safe for the night.
0:17:13 > 0:17:18The ants look after it like farmers looking after a dairy cow.
0:17:18 > 0:17:23Their cow, in return, provides them with food.
0:17:33 > 0:17:40So, at this stage, neither ant nor caterpillar seems to have the advantage.
0:17:40 > 0:17:48But this same species of ferocious stinging ant also has a partnership with another species of caterpillar
0:17:48 > 0:17:51and there the result is different.
0:17:54 > 0:17:58This one has a glossy, horny shield on its back.
0:17:58 > 0:18:04It, entirely of its own accord, marches right into the ants' nest,
0:18:04 > 0:18:10undeterred by the ants' threatening postures and sprays of formic acid.
0:18:10 > 0:18:15No matter what the ants do, they can't stop it.
0:18:15 > 0:18:21Deeper and deeper it goes, through the corridors of sewn leaves, right into the heart of the nest.
0:18:44 > 0:18:49It reaches the queen. If she is killed, the whole colony will die.
0:18:49 > 0:18:53But she is not what it is looking for.
0:18:53 > 0:18:58The soldiers make little impression on the caterpillar's armour.
0:18:58 > 0:19:04Neither can they get underneath it to reach the soft, vulnerable body.
0:19:04 > 0:19:08On it goes, until it reaches the nursery chambers,
0:19:08 > 0:19:11where the developing grubs lie.
0:19:14 > 0:19:21Try as they might, they can't lift the shield sufficiently to enable other defenders to get beneath.
0:19:29 > 0:19:36With the intruder actually within the nursery, the workers become totally confused.
0:19:36 > 0:19:40Some try to carry off the grubs to safety elsewhere.
0:19:49 > 0:19:53They can't do it quickly enough.
0:19:53 > 0:19:59The caterpillar snatches a grub and pulls it under the shield.
0:19:59 > 0:20:02Then, secure beneath its armour,
0:20:02 > 0:20:04it slowly eats it.
0:20:15 > 0:20:20As the season progresses, several of these armoured intruders make their way into the nest,
0:20:20 > 0:20:24and there gorge themselves on ant grubs.
0:20:57 > 0:21:04After some weeks, the caterpillars have eaten all the grubs they need to grow to their full size.
0:21:04 > 0:21:10Now, in the heart of the nest, they are ready to turn into butterflies.
0:21:10 > 0:21:17But how can a butterfly get past the ants? Surely they have a chance for revenge.
0:21:17 > 0:21:23Slowly, the insect hauls itself out of its horny armour.
0:21:39 > 0:21:44But it's a strange sort of butterfly that emerges.
0:21:48 > 0:21:55It's covered in scales that are so slippery that the ants can't get a proper grip on them.
0:22:02 > 0:22:05Those that DO manage to bite
0:22:05 > 0:22:11get their jaws covered with a sort of fluff that they clearly find intensely irritating.
0:22:14 > 0:22:19So, at last, the murderous lodger goes free.
0:22:25 > 0:22:32Ants and caterpillars, like crabs and anemones, are about the same size.
0:22:32 > 0:22:40If a lodger is much smaller than its landlord, it tends to live, not so much WITH it, as ON it.
0:22:40 > 0:22:43Those monkeys over there, for example.
0:22:43 > 0:22:46They've got a number of tiny passengers.
0:22:59 > 0:23:03Like most mammals with hairy coats, they've got fleas.
0:23:03 > 0:23:08When fleas bite and suck blood, they itch.
0:23:12 > 0:23:20It may be necessary to get a friend to help pick them out from parts that you yourself cannot reach.
0:23:23 > 0:23:29This is not fur, but the fabric of a bird's nest. Fleas live here too.
0:23:32 > 0:23:37A young starling, within two days of hatching, is likely to have several dozen fleas.
0:23:41 > 0:23:45Fleas have six legs, like all insects, but no wings.
0:23:45 > 0:23:51Those would be an encumbrance to crawling about in fur and feathers.
0:23:51 > 0:23:55Instead, they have powerful hind legs that enable them to jump onto their host.
0:23:55 > 0:23:59Their jaws have become specialised for sucking blood.
0:23:59 > 0:24:02They feed on nothing else. They HAVE to live on another animal.
0:24:02 > 0:24:09They contribute nothing to its welfare. This is not a partnership, it's parasitism.
0:24:09 > 0:24:16Nor are fleas the only parasites in a bird's nest. Lice are there, eating feathers.
0:24:16 > 0:24:23They, too, are insects, and any one bird may have up to a dozen different kinds,
0:24:23 > 0:24:29each living on, and eating, a different kind of feather on the neck, the wings or the head.
0:24:32 > 0:24:37Insects seem to have a particular flair for parasitism.
0:24:37 > 0:24:42Each of their main families has some members who have taken it up.
0:24:42 > 0:24:46But insects themselves can also be parasitised.
0:24:46 > 0:24:52This nest of bees has been invaded by mites tiny cousins of spiders.
0:24:52 > 0:24:59They are so tiny that several hundred of them can sit on the leg of a bee.
0:24:59 > 0:25:02And they, too, itch.
0:25:10 > 0:25:18They get everywhere, and once they have found their way into a colony, they spread to every member of it.
0:25:30 > 0:25:33Mites are just as specialised as feather lice.
0:25:33 > 0:25:39These bee mites live only on this particular species of bumble bee.
0:25:39 > 0:25:46And this flower, milkweed, is a staging-post for one of the most specialised mites of all.
0:25:49 > 0:25:52Moths come to feed on the milkweed at night,
0:25:52 > 0:25:57dipping their long, threadlike tongues deep into the heart of the flowers to sip the nectar.
0:26:00 > 0:26:06But this moth is already infested with mites.
0:26:06 > 0:26:14Its ear, a tiny hole in the side of its head, has become the home of a whole colony of them.
0:26:19 > 0:26:24And a new colonist awaits on the flower itself.
0:26:29 > 0:26:34While the moth drinks, the mite crawls up its tongue.
0:27:12 > 0:27:20Once on the moth's head, it knows which direction to take through the jungle of fur to reach the ear.
0:27:20 > 0:27:27There is a danger in this. Blocking up an ear makes it useless to the moth.
0:27:27 > 0:27:32If the moth can't hear, it can't avoid the bats that hunt it.
0:27:32 > 0:27:36That would be disastrous for moth AND mites.
0:27:36 > 0:27:42So the mites occupy only one ear, and always leave the other free.
0:27:42 > 0:27:47They use one part of the ear-tube for stacking their droppings,
0:27:47 > 0:27:53another for laying their eggs, and yet another for rearing grubs.
0:27:53 > 0:27:57How do their offspring find another of these homes?
0:27:57 > 0:28:02Why, of course, by clambering down their host's tongue as it drinks,
0:28:02 > 0:28:09and waiting on the flower for another moth of the same species to turn up.
0:28:17 > 0:28:21But parasites are themselves preyed on.
0:28:21 > 0:28:23This little mouse lives in Central America
0:28:23 > 0:28:28and regularly carries a dozen or so passengers wriggling around in its fur.
0:28:36 > 0:28:42They are beetles, and they were once thought to be parasites that sucked the mouse's blood,
0:28:42 > 0:28:46for they have large and powerful jaws.
0:28:46 > 0:28:53But the mice carrying most beetles are not the most anaemic, as you might expect.
0:28:53 > 0:28:55They seem to be the most healthy.
0:28:55 > 0:29:00The mouse's most serious parasites
0:29:00 > 0:29:03are, in fact, here in the lining of the nest
0:29:03 > 0:29:06fleas and ticks that DO suck its blood.
0:29:11 > 0:29:16Each mouse has several holes in the forest,
0:29:16 > 0:29:20and ALL are likely to be infested.
0:29:20 > 0:29:23When a mouse settles down in one,
0:29:23 > 0:29:27the beetles drop off and go hunting for the fleas in the nest lining.
0:29:27 > 0:29:33So the beetles, far from injuring a mouse, actually aid it.
0:29:40 > 0:29:43Got one!
0:29:50 > 0:29:52As far as a beetle is concerned,
0:29:52 > 0:29:58the mouse is a convenient transport system for getting from one rich hunting-ground to another.
0:29:58 > 0:30:04The mouse that carries most beetles has the most comfortable life.
0:30:05 > 0:30:11These birds hunt parasites. They're finches from the Galapagos Islands.
0:30:11 > 0:30:16And the creatures they help the giant tortoises.
0:30:24 > 0:30:27You can hardly scratch yourself
0:30:27 > 0:30:30if you have legs like these!
0:30:30 > 0:30:38Yet tortoises, like so many other animals, are pestered by skin parasites, especially ticks.
0:30:38 > 0:30:40The finches eat mainly seeds.
0:30:40 > 0:30:45But ticks, apparently, make a welcome change.
0:30:50 > 0:30:53When there's a tortoise nearby and the finches want a meal with a difference,
0:30:53 > 0:30:59they signal to the tortoise by jumping up and down in front of it.
0:30:59 > 0:31:04The tortoise reacts to the finches' advances in a remarkable way.
0:31:04 > 0:31:12It stiffens its legs so that its huge body is lifted clear of the ground, and cranes up its neck.
0:31:14 > 0:31:18The invitation is unmistakable.
0:31:25 > 0:31:30There's no way that the tortoise could pick off parasites
0:31:30 > 0:31:35from the places that these attendants manage to reach.
0:31:48 > 0:31:52A few minutes' servicing by the finches is enough
0:31:52 > 0:31:56to clear the tortoise of most of its pests.
0:31:56 > 0:31:59Another satisfied customer!
0:32:03 > 0:32:06Fish have the same sort of problem, and the same sort of solution.
0:32:06 > 0:32:14The huge manta ray is troubled by sea lice and parasitic barnacles that burrow into its skin.
0:32:15 > 0:32:23But it has other company an attendant fleet of small fish that travel with it.
0:32:23 > 0:32:31When the opportunity arises, they swim over their host's body, even inside its gaping mouth,
0:32:31 > 0:32:34picking off the passengers.
0:32:49 > 0:32:56Like the giant tortoises, fish with skin problems patronise regular cleaning establishments.
0:32:59 > 0:33:02This grouper hangs in the water at this special place on the reef,
0:33:02 > 0:33:10and small wrasse that have been waiting amongst the coral swim out and start fussing around it.
0:33:12 > 0:33:16They even dare to swim inside the huge jaws.
0:33:22 > 0:33:25It's not only fish that work as cleaners.
0:33:25 > 0:33:28This moray eel is being tended by a shrimp.
0:33:31 > 0:33:34Open wide, please!
0:33:49 > 0:33:54Amazingly, the cleaners are never harmed...
0:33:55 > 0:33:58..even though they tickle!
0:34:05 > 0:34:11These shrimps are quite large big enough to make a good meal
0:34:11 > 0:34:14but they're never injured, either.
0:34:21 > 0:34:26Regular customers return to these cleaning-stations every few days.
0:34:26 > 0:34:32Although the resident staff of wrasse and shrimps can deal with as many as fifty an hour,
0:34:32 > 0:34:37there are often queues of itchy fish waiting their turn.
0:34:42 > 0:34:47Some fish, however, have their own personal valets.
0:34:49 > 0:34:56Sucker-fish, or remoras, have got a fin on their back that has been modified into a sucker so powerful
0:34:56 > 0:35:01that it's almost impossible to pull one off if it wants to stay on.
0:35:01 > 0:35:04They travel with their host wherever it goes,
0:35:04 > 0:35:09picking off parasites whenever there's an opportunity to do so.
0:35:27 > 0:35:31Giraffe, like many other big game animals in Africa,
0:35:31 > 0:35:34also have their own personal staff.
0:35:40 > 0:35:45Ox-peckers live almost permanently on the bodies of their hosts,
0:35:45 > 0:35:47scuttling about all over it.
0:35:55 > 0:35:58On this spacious, patterned stage
0:35:58 > 0:36:00they act out most of their lives.
0:36:00 > 0:36:06Here, they argue, court, and feed their newly-fledged young.
0:36:06 > 0:36:09True, they can't nest here. They do that in holes in trees.
0:36:09 > 0:36:16But they line those holes with giraffe hair, so they'll feel at home.
0:36:21 > 0:36:24Their claws are so long
0:36:24 > 0:36:28that they can cling in almost any position and move in any direction.
0:36:28 > 0:36:33Their flat beak slips easily between the hairs,
0:36:33 > 0:36:37as they scissor through it, searching for ticks.
0:36:40 > 0:36:45And they get everywhere, on young and on old.
0:36:50 > 0:36:55Even when the animal moves off, they will hang on
0:36:55 > 0:37:00with the skill and unconcern of accomplished jockeys.
0:37:07 > 0:37:10Ox-peckers are a mixed blessing.
0:37:10 > 0:37:14The ticks they eat are full of blood,
0:37:14 > 0:37:19but sometimes they take that blood directly from an open wound.
0:37:19 > 0:37:24By doing that, they don't improve the host's health, but damage it.
0:37:24 > 0:37:27They keep the wound open long after it would otherwise have healed.
0:37:29 > 0:37:37Even so, without them, giraffes would be more seriously troubled by skin parasites than they are.
0:37:41 > 0:37:49We ourselves, of course, can also get infested with ticks and fleas if we're not careful.
0:37:49 > 0:37:51They're everywhere, particularly in the rainforest.
0:37:58 > 0:38:04One has a reasonable chance of getting rid of animals that settle on your outside.
0:38:04 > 0:38:07I can flick off these ticks.
0:38:08 > 0:38:15If YOU can't do it, an ox-pecker or cleaner-fish may do it for you.
0:38:15 > 0:38:23But if the parasite manages to get actually inside your body, that's a very different matter.
0:38:24 > 0:38:29The corridors and chambers of an animal's digestive system
0:38:29 > 0:38:34offer great advantages to any creature that can dwell in them.
0:38:34 > 0:38:39Inside here they are secure from enemies,
0:38:39 > 0:38:46and washed by a nutritious soup that their host has already chewed, mashed and partially digested.
0:38:46 > 0:38:51All they have to do is to absorb it through their skin.
0:38:51 > 0:38:53They don't even need a mouth.
0:38:53 > 0:38:58The animals that are best suited to this interior life
0:38:58 > 0:39:03are those long, spineless, legless creatures we call worms.
0:39:03 > 0:39:08Flat, ribbon-shaped tapeworms hang onto the walls of the gut
0:39:08 > 0:39:11with a crown of hooks that encircles their head.
0:39:16 > 0:39:21In the corridors of the intestines, roundworms proliferate.
0:39:21 > 0:39:29Every backboned animal that has been examined, whether fish, amphibian, reptile, bird or mammal,
0:39:29 > 0:39:32proves to be the host of a roundworm.
0:39:32 > 0:39:37These, living in a gut, merely rob the host of some of its food.
0:39:37 > 0:39:43But they may spread to cause severe damage to the liver and the lungs.
0:39:43 > 0:39:47Other roundworms, too, cause serious problems.
0:39:47 > 0:39:55Some, as thin as threads of cotton, swim along the blood-vessels and collect in the heart-valves,
0:39:55 > 0:40:00blocking them so seriously that their host dies.
0:40:02 > 0:40:06The young of such threadworms, swimming around in the bloodstream,
0:40:06 > 0:40:10depend on biting insects to transfer them to another host.
0:40:10 > 0:40:15During the day they swim in blood-vessels deep within the body,
0:40:15 > 0:40:20but at night they move up into the capilliaries beneath the skin.
0:40:20 > 0:40:25So when a mosquito sucks their host's blood, they are taken up.
0:40:25 > 0:40:30They grow inside the mosquito and when it bites some other animal
0:40:30 > 0:40:35they are transferred into a new host, a new home.
0:40:35 > 0:40:40Others, smaller still, that wriggle among the blood corpuscles,
0:40:40 > 0:40:45belong to the most ancient of all animal groups the protozoons.
0:40:45 > 0:40:53They first got inside animals so long ago that most of their hosts have developed an immunity to them.
0:40:53 > 0:40:58Human beings have not yet done so, and in THEM they cause sleeping sickness and death.
0:41:00 > 0:41:05Internal parasites have a problem getting their offspring into another host.
0:41:05 > 0:41:09Tiny ones may use biting insects.
0:41:09 > 0:41:15Bigger ones, like this roundworm, have to use other methods.
0:41:15 > 0:41:21The first stage getting their eggs to the outside world is easy.
0:41:26 > 0:41:31This roundworm, full of eggs, sheds them into its host's gut,
0:41:31 > 0:41:35so that they fall out with its droppings.
0:41:35 > 0:41:41Once in the soil, they may lie dormant for some considerable time.
0:41:41 > 0:41:48When conditions are suitable temperature just right and moisture reasonable they begin to hatch.
0:42:06 > 0:42:11The tiny worms crawl up leaves of grass,
0:42:11 > 0:42:16and await the moment when a hungry mouth will crop the grass,
0:42:16 > 0:42:19carrying them into another stomach.
0:42:19 > 0:42:27Transfers are not always simple. The complexities of some routes are almost beyond imagining.
0:42:28 > 0:42:31Denmark. A morning in summer.
0:42:33 > 0:42:36There has been a shower of rain.
0:42:36 > 0:42:39Meadows and woodlands are drenched.
0:42:47 > 0:42:53Snails are slowly crawling around through the wet leaves, grazing.
0:42:53 > 0:42:58They are feeding on algae and rotting vegetable matter.
0:42:58 > 0:43:06Early morning is the best time for them. The sun is not yet hot enough to dry them out.
0:43:06 > 0:43:10They can explore parts of the vegetation they can't reach at other times.
0:43:28 > 0:43:32But this one is different from the others.
0:43:35 > 0:43:41Its left tentacle is swollen and pulsating. It has a parasite.
0:43:41 > 0:43:49A few months ago, the snail took in, along with normal food, some bird droppings.
0:43:49 > 0:43:54They contained the eggs of a fluke that was living in the bird's gut.
0:43:54 > 0:43:59They hatched and the parasite grew, taking over the snail's body.
0:43:59 > 0:44:01As the sun shines brighter,
0:44:01 > 0:44:09the parasite extends a striped muscular bag packed with tiny larvae into the snail's tentacle.
0:44:09 > 0:44:13It nearly always picks the left one.
0:44:13 > 0:44:21Birds rarely eat whole snails. They are too big, and few can extract them from their shells.
0:44:21 > 0:44:26But the larvae must reach the body of another bird to develop further.
0:44:26 > 0:44:31The presence of the parasite changes the snail's behaviour.
0:44:31 > 0:44:39As the day wears on, it does not, like uninfected snails, crawl into the undergrowth, out of harm's way.
0:44:39 > 0:44:44Instead, it remains exposed, out in the open dangerously so.
0:44:48 > 0:44:52Now there is a parasite in each tentacle.
0:45:02 > 0:45:08Perhaps they look like caterpillars or tasty worms. Maybe they just look odd.
0:45:08 > 0:45:12But certainly, the fly-catcher finds them interesting.
0:45:29 > 0:45:34The connection has been made. The circle is complete.
0:45:34 > 0:45:37Another bird has become infected.
0:45:38 > 0:45:44Inside the bird, the striped bag releases its multitudes of larvae.
0:45:44 > 0:45:49They move through the bird's body and take up residence in its gut.
0:45:49 > 0:45:54And the whole cycle starts all over again.
0:45:56 > 0:46:01Flukes are related to the flatworms that live independently in ponds.
0:46:01 > 0:46:06But they found their greatest success as internal parasites.
0:46:06 > 0:46:14Some reside in the liver. Other kinds anchor themselves in the bladder, lungs or gut.
0:46:14 > 0:46:18Most are capable of causing serious disease.
0:46:18 > 0:46:25But not all internal parasites injure their hosts. Some help them.
0:46:25 > 0:46:28These microscopic organisms
0:46:28 > 0:46:35undoubtedly alive and arguably animals, since they don't have chlorophyll to manufacture food
0:46:35 > 0:46:39live in the stomachs of most animals.
0:46:39 > 0:46:44They can break down cellulose the substance of most plant tissue.
0:46:44 > 0:46:49That's something the digestive juices of most animals can't do.
0:46:49 > 0:46:57Their free-living ancestors swam in ponds, as some of their relatives still do today.
0:46:57 > 0:47:04These are members of the family that have simply found a warmer, darker pond.
0:47:04 > 0:47:09This pond is extraordinarily rich in edible material. It's a stomach.
0:47:11 > 0:47:19So a buffalo, like most wild animals, is not, as it might appear, a single individual.
0:47:19 > 0:47:21It's a walking zoo.
0:47:21 > 0:47:29Its ox-pecker friends are obvious, but if we looked closer, we would find ticks boring into its skin.
0:47:29 > 0:47:34In its mouth, leeches it picked up when it drank from the river.
0:47:34 > 0:47:37Tapeworms trail through its guts.
0:47:37 > 0:47:40Flukes are moored in its liver.
0:47:40 > 0:47:45Protozoons swim in its blood and swill around in its stomach.
0:47:45 > 0:47:50It's a community of animals that have been committed by evolution,
0:47:50 > 0:47:55for better or for worse, in sickness and in health,
0:47:55 > 0:47:57to live together.