Finding Food The Trials of Life

A forest on the lower slopes of the Andes.

A spectacled bear is looking for a snack.

One of the problems that faces us and all animals is finding enough to eat.

Being animals and not plants, we feed on other organisms, and other organisms don't welcome that.

Animals run away or defend themselves,

and even plants have surprisingly effective methods of defence.

I'm in the South American rainforest, the richest proliferation of life on Earth,

so you might think that here of all places I'd have no difficulty finding food,

particularly if I was vegetarian. But it's not that simple.

The woolly spider monkeys up there are taking their first meal of the day.

This tranquil scene is in fact a battle. The tree defends itself by developing poison in its leaves,

but when newly sprouted, they are just about edible so the monkeys have to select leaves with care.

But they're bound to swallow a little poison,

and now they've had as much as they can tolerate. They move off to find another kind of tree.

This too has poison in its leaves, but it's a slightly different kind,

so the monkeys can take a second course,

providing they continue to be careful.

Also, leaves are not really very nutritious, and a monkey has to eat great quantities to sustain itself.

Huge meals require huge stomachs,

and that means that the monkeys, to be honest, are not the nimblest of movers up in the branches.

After an hour or so of feeding, the need to digest their vast meals

demands that they take time off and have a siesta.

Eating leaves is NOT easy.

The small red panda of the Himalayas

is one of the few animals that has beaten the defences of the bamboo.

Its leaves are not only fibrous but armed with tiny blades of silica so sharp that they can cut flesh.

But the panda's digestion can cope with them, and the reward is that it has all the bamboo it can eat,

as so few other animals can eat it.

Bamboo also grows in Madagascar,

and here the rare golden bamboo lemur feeds on it.

It scissors through the coarse outer leaves on the stem

to reach the marginally softer and more succulent ones within.

Its preferred choice is not so much leaves as the new shoots that come up through the ground like spears.

The plant values these shoots and it loads them with cyanide.

Eating one of these uncooked could kill a man.

The bamboo lemur can only eat them because its stomach produces juices which neutralise the poison.

But this ability has a price

the bamboo lemur can't eat much else. So if the bamboo disappears, so does the lemur.

The greatest plunderers of leaves, however, are insects.

They're the most numerous creatures on earth,

and a high proportion of them as caterpillars or adults eat leaves. The evidence is everywhere.

Insects are also fussy feeders.

A female lays her eggs on the kind of plant her caterpillars' digestion can cope with,

so when they hatch, they find the food they need close at hand.

They are little more than eating machines a pair of jaws attached to a bag-like gut.

No complicated wings or sex organs. Those come when they are adults.

Now it is just munch, munch, munch.

Plague beetles have a special way of beating a plant's poison.

Poison is costly to produce, so many plants keep only small stocks to deploy at the point of attack.

Plague beetles attack in such numbers that the poison is shared between many,

and each beetle only gets a tiny and tolerable amount.

It's true that each one only gets a small meal, but as soon as they finish with this plant,

they move on to another.

The milkweed invests much more in its defences.

Its abundant and very poisonous sap, latex,

is piped along special veins and is immediately available everywhere.

but this beetle deals with that by puncturing the pipeline that runs along the leaf rib,

so that the milky latex leaks out.

As a result, the poison never reaches the end of the leaf, and there the beetle can feed safely.

The latex also seals a plant's wounds, because it rapidly solidifies in air.

But since it can't reach here, the beetle has no problem with gummed-up jaws.

Marmosets also deliberately wound plants.

They repeatedly gouge grooves in the trunks of trees,

which then exude resin. Like the milkweed's latex,

resin seals off the tree's injuries, preventing loss of sap and the entry of infections.

But unlike latex, it's not poisonous. On the contrary, it's full of sugars,

and rather good to eat.

And the marmosets love it.

So the tree's measures to defend itself have actually resulted in encouraging its injury.

Not all plants are so uncooperative.

Some actually encourage animals to feed from them,

and advertise the fact that they've got food available with brilliant displays, as these poppies do.

This is not generosity but straightforward self-interest.

The food they offer is a bribe for the animals to act as couriers.

They need to have their pollen ferried across to other flowers.

Pollen, in itself, is edible,

and bumble bees have a great taste for it.

Bees have a complex arrangement of combs and brushes on their hind legs

with which they gather pollen to pack into baskets on their thighs.

As they move from flower to flower,

some pollen that was brushed onto their hairy bodies from one flower brushes off on another,

and the plant's purpose is accomplished.

Pollen, packed with genetic material, is complex and costly.

Many flowers offer a cheaper bribe,

nothing more than sweetened water.

Nectar is produced from nectaries, usually deep in the heart of the flower.

Thirsty insects, to reach it,

have to brush past the stamens, collecting a dusting of pollen on the way.

In temperate lands, flowers can only be found in spring and summer, when there's no frost.

So insects that shelter from winter in nests

have to build up stocks as quickly as they can. They have, in fact, to be "busy as bees."

In the tropics, on the other hand,

there are always plants of one kind or another in bloom,

so nectar is available all year. This vine, Combretum, is particularly generous.

Almost any animal can get nectar easily when the plant is in flower.

Many birds which feed mostly on fruit, berries or even insects come to drink from it.

And so do monkeys. The smaller kinds squirrel monkeys, tamarins, and marmosets

can clamber right out on to the thinner branches.

Even the much bigger capuchins which eat fruit, nestling birds, lizards and even small monkeys

enjoy a sweet drink.

As the monkeys feed, the stamens brush their fur, and the pollen is on its way to another flower.

Nectar feeding has its problems.

This Heliconia flower produces only a little nectar at a time.

So if a hummingbird comes to feed from one of these blossoms, it has to go elsewhere to get more,

bringing about the plant's purpose of cross-pollination.

It takes a little time to produce more nectar, so if the bird returns too soon it may waste a journey,

but if it's too late, another bird may get the nectar.

So hummingbirds patrol a whole group of plants,

visiting each flower in rotation to an accurately timed schedule.

And he was right on time!

Hummingbirds are among the very few animals that live almost entirely on nectar.

Small insects are their only other food, and they have developed special equipment to collect it.

The wings have joints that enable the bird to beat them with a whirling motion,

giving it perfect control in the air.

So it can hover by a flower and insert its beak with absolute precision.

The long, thread-like tongue flicks in and out 13 times a second.

But this specialisation means that hummingbirds can feed on almost nothing else

so they are in the plant's power.

It may SEEM like the bird is deciding which flower to drink from,

but you could equally argue that the plant, by controlling the rate at which it produces nectar,

is dictating the bird's movements.

Many hummingbirds have a bill which, in its length and curvature,

exactly matches the shape and dimensions of the particular flower on which they mostly feed.

The violet sabre-wing's beak fits into the Columnia like a dagger in a scabbard.

The flower has stamens in precisely the position needed to put a dab of pollen on the bird's forehead.

This partnership suits Columnia

because its pollen is not taken by birds who feed on other kinds of flowers and so it isn't wasted.

It suits the sabre-wing since, because of its unique bill, it has all the Columnia nectar to itself.

But not quite.

The mountain gem hummer is waiting for the flower's legal partner to leave.

Its bill is far too short to reach the nectar as the sabre-wing does.

With a thrust from its wings, it tries to pierce the flower.

This time, it holds the flower with its feet.

It's broken in! Columnia has been burgled!

Another thief, only too eager to take advantage of a flower.

Indian langur monkeys.

The flowers of flame-of-the-forest are protected by being placed at the ends of long, thorny twigs,

so they're reserved for their particular pollinators, birds.

But langurs find them very tempting.

This, of course, is disastrous for the tree.

Its complex, subtle mechanisms for getting its seeds fertilised evolved over millions of years

are being chewed to pieces.

Even the remotest flowers aren't safe,

for the young babies can clamber right out on to the thinnest branches.

In the continuous struggle between animals and plants, this round has certainly been lost by the plant.

In the lush forests of the tropics where flowers bloom all year long,

animals that feed on nectar can always find a drink somewhere.

But in other places, where perhaps the winters are bitterly cold,

or here in the desert where flowers only bloom after brief rains,

animals that rely on nectar need a way of storing it to last them through the hard times.

These mulga trees produce nectar on which ants feed and they have the most extraordinary larders.

The galleries of their nests lie four feet or so below the surface of the ground.

These golden globes hanging from the roof

are their storage pots, full of honey.

Each one is alive an ant with an abdomen expanded to the size of a grape.

The dark flecks are the plates you see on any ant. The membrane between them is stretched.

These bloated individuals are almost totally inactive, so they consume little of the honey they hold.

It is drunk by the busy workers, who, when there is little food above ground,

come down here and induce the honey-pots to regurgitate it.

The workers also tend the swollen bodies, keeping them clean.

During good times, the workers collect nectar

and take it down to the larders

to top up the colony's storage jars by feeding it to them, drop by drop.

The Aborigines who have roamed these deserts for millennia

have always valued these ants as one of their few sources of sugar.

And they eat them just as they are.

Mmm!

Mmm! It's liquid, warm, and marvellously sweet!

A few weeks after flowering, many plants tempt animals with another food fruit.

They have another problem.

Their seeds are formed, and need to be distributed,

and by wrapping them in sweet, edible pulp, they recruit lots of animals to do the job.

The trees dissuade animals from collecting the fruit before the seeds are fully developed

by not producing the sugars in it until the last moment so unripe fruit tastes bitter

and is really not worth picking.

To indicate when it IS, the fruit often changes colour.

Squirrel monkeys are primarily fruit eaters. They move about in groups of up to 40 or so,

and they have to wander over a great area to find all the fruit they need.

Capuchins live in small families, each with its own familiar patch of forest. They eat lots of things.

But if there's a fruiting tree in their area, they know about it.

So although squirrel monkeys are frightened of the bigger capuchins, they follow them as they forage.

There may be food for a capuchin here, a lizard maybe. But no fruit.

So the squirrel monkey is not interested,

and must wait.

The capuchin moves on.

And the squirrel monkeys follow.

As the capuchins get near the fruiting tree,

the squirrel monkeys, perhaps smelling the fruit, scamper ahead to try to get to it first.

Now they must grab as much as they can

as quickly as they can.

The capuchins arrive.

The squirrel monkeys depart with the tree's seeds inside their stomachs.

These pass through the monkeys unharmed until, some distance away,

they are deposited with a convenient dollop of fertiliser.

Seeds themselves, of course, are packed with nourishment,

so plants enclose them in shells strong enough to defeat even a mangaby. Victory to the plant.

But the chimp is so clever, it can crack them.

Victory to the animal.

Sharp teeth enable an agouti to chisel into the acorn of a tropical oak.

In spite of the acorn's armour, it seems the oak has lost the contest.

But not totally. The oak produces many more acorns than the agouti can eat immediately.

The rest it carries away and buries for later.

But an agouti's memory is not infallible.

Occasionally it forgets about an acorn, which then grows into a new oak. Victory to the plant.

Perhaps the most extraordinary tool for nut-eating is wielded by a strange Madagascan lemur,

the aye-aye.

First it gnaws a hole,

and then it scoops up the contents with this long, bony probe

which is, in fact, its finger, but one quite unlike the rest on its hand.

This curious digit serves equally well for eating a grub.

The aye-aye uses it to mash up the body hidden in its burrow, and then flicks out the puree.

The spiny pocket mouse has a double problem. The seed it is gnawing is not only hard-shelled,

but full of poison. The mouse simply punctures the shell.

It then tucks it into its cheek and carries it back to its burrow.

The hole in the shell stimulates the seed to germinate,

and the tender white shoot which emerges is poison-free!

The macaw has probably the most powerful nut-cracker of all.

It can demolish even the most resistant of nuts.

But many seeds that macaws eat are also filled with poison.

Yet it doesn't seem to upset them. How do they survive?

Every day, they fly through the forest

to dose themselves with a special antidote.

Macaws usually fly in pairs. Only in such places as this do they assemble in flocks.

They've come to collect their medicine.

The regular gathering of birds attracts eagles and other predators.

So before the macaws come out of the trees,

they wait for one bird, braver than the rest of them,

to make the first move.

There it goes. And this what they're after.

Kaolin. The soil in this riverbank is rich in it.

Several kinds of parrots and macaws come here daily from miles around to take the treatment.

Kaolin combats acidity in the stomach, absorbs and neutralises poisons.

As a bonus, this clay is rich in calcium and sodium, which is lacking in diets of fruits and nuts.

So eating plants poses more problems than one might think.

But eating other animals, even small, defenceless ones, also has its difficulties.

It's dawn on the east coast of England the middle of winter, and food is very scarce.

But behind me is a huge and abundantly stocked larder.

Its doors have been shut for the past three hours, but now the tide is on the turn.

A horde of hungry animals awaits.

Tens of thousands of knot and dunlin have assembled on a lagoon

on the other side of the sand dunes.

They have sensed that the tide has exposed a mud-bank. Breakfast is served.

Millions of tiny molluscs lie just below the surface of this mud, and the birds are feeling for them

with their bills.

Abundant though the food is out there, collecting it is a very dangerous business.

It's very exposed on the mud flats, there's nowhere to hide,

so the birds stick together in tight flocks.

That way each bird has a thousand eyes ready to spot danger.

But if that is such a good idea,

why is this redshank out on its own?

It's hunting not by touch but by sight.

It's searching for its favourite food, small shrimp-like crustaceans.

If they are alarmed by vibrations produced by many moving feet,

they will disappear into the mud where the redshank can't see them.

So if a redshank wants to catch this more swiftly moving food, it has to forage by itself,

despite the risks.

And if it does become a little alarmed, it just squats.

Some waters are so rich in food, there is plenty for everybody.

On this Indian lagoon, there are storks, herons and egrets, openbills and spoonbills.

Each has its own beak-technique

with which to catch its favoured prey probing and sieving, scything and stabbing.

Even parts of the open sea, like this bay in the West Indies,

have, at certain times, enough fish to attract great flocks of birds.

Barracuda, among the most ferocious hunters in the sea.

And they regularly drive shoals of small fish into the bay.

The pelicans can even tackle shoals which are a foot or two beneath the surface.

But a pelican can only swallow a fish in its bill

after draining the water out by opening its beak very slightly.

And that is the moment the gulls are waiting for.

Few places on land offer quite the density and richness of animal food

that can be found in parts of the sea like this.

But one land animal does swarm in vast numbers, and this tenrec from Madagascar is hunting them.

They can be sniffed for, but the tenrec's huge ears also help,

for they make a rustling noise scurrying along their pathways.

Termites. The juicy soft-bodied workers are largely defenceless.

But with them come soldiers. This kind squirt noxious chemical sprays from nozzles on their heads.

The tenrec, with is sensitive nose, can tolerate a certain amount of chemical spray

but after a while, it just has to come up for air.

Termites are hugely abundant in the tropics, and many animals collect them whenever possible.

A small gecko in the deserts of Australia eats little else.

It's such a fastidious and accurate feeder,

it can avoid the soldier termites, picking out the workers one by one.

Of all food-collecting devices, the most ingenious and elegant

must be the webs of orb spiders, nearly always built by the females.

One starts by rigging filaments of silk across a flyway used by insects.

Around the spokes,

with another kind of silk, she sets a spiral mesh.

As she secures each section, she twangs it,

so the glue that coats the silk

breaks up into a line of sticky beads.

One of the biggest of these webs, which may be two yards across, is that of the the Nephila spider.

She is huge. Her legs can span six inches. And she is virtually blind.

A fly, caught in her web, is quickly seized.

She rapidly injects it with a venom that will liquify the contents of its body.

She then wraps it up in silk and parks it on the web to allow the venom to take effect.

But Nephila's not alone on her web.

Argyrodes is tiny much smaller even than the fly.

Nephila could well eat her too.

She too is blind, but she too feels the vibrations of the struggling fly.

With what seems like suicidal recklessness, she approaches Nephila feasting on her prey.

And she too begins to eat food that Nephila not only caught but has conveniently predigested.

Another capture calls Nephila away.

Once again she stabs the fly, trusses it up,

and carries it away to hang on the web. She'll eat that later.

Argyrodes seems well aware of what's going on.

As soon as Nephila has finished hanging up her latest catch,

Argyrodes starts trying to discover its precise position by pulling the web filaments.

Nephila has returned to finish her first meal.

Meanwhile, Argyrodes has run a line from the top of the web to the fly, which she is now cutting loose.

Once the fly is free of the web, she lowers it down.

The stolen fly is now hanging entirely free. Nephila won't be able to reclaim it now.

Even so, Argyrodes must get it away to a place

where she can feed on it in safety.

Step by step, she heaves it up. Her theft is complete.

Tropic birds nest on this cliff in Tobago in the West Indies.

They are magnificent flyers, able to exploit all the air currents with spectacular ease.

They fish out at sea and every day the parents return to the nest

with crops full of food for the family.

Frigates were swift, armed ships which plundered merchantmen and these are frigate-birds.

The fishing fleet is returning.

The frigate wasn't trying to kill the tropic bird, only to make it surrender its cargo of fish.

But it'll have to find another victim.

The usual tactic of these pirates is to come up astern of a victim and grab its leg or tail.

It surrenders! There goes its fish.

And the frigate catches it.

Frigates aren't always successful, and you can't help wondering

if it wouldn't be easier to catch fish for themselves, and indeed they often do.

But they seem positively to enjoy a life of piracy!

As for the tropic birds well, most of them escape, and at worst they only lose a few fish.

But it's only a short step between robbing a victim and killing him for the pirate to become a hunter.

That raises a completely new set of problems, and it's those we'll be looking at in our next programme.

Subtitles by Anne Morgan BBC Scotland 1990