Social Climbers The Life of Mammals

Monkeys, together with apes, are our closest relatives.

They have the richest social life of all mammals,

and much of their time is spent establishing relationships

and climbing the social ladder.

And that has had some remarkable consequences.

South America,

and a few capuchin monkeys are having a morning get-together.

Capuchins are very like the earliest of all monkeys, judging from fossils.

The basic pattern, you might say.

With grasping hands and gripping tails, they race through the forest

with astonishing speed and agility.

But it's their big brains that really give them an edge.

If there is one thing that is characteristic of monkeys,

it is their ability to spot an opportunity and then exploit it.

That's especially true of capuchins.

Of course, they'll grab a ripe fruit,

but they investigate everything in their path. Little escapes them.

But it's their inquisitiveness that really impresses you

when you follow them.

They look for food everywhere.

Their colour vision is excellent.

Their sense of smell, however, is no better than ours,

so to find food that is hidden from view, they use brains - not noses.

They can, clearly, imagine what might be lurking beneath dry leaves.

Out of sight is by no means out of mind.

A good memory must also help.

Recalling a previous experience

could make the difference between getting stung and getting honey.

The troop is just entering these mangroves.

Most mammals wouldn't find much to eat here,

but monkeys' big brains enable them to find things that others miss.

Only the bolder ones chance their hand with crabs.

It's quite brave to tackle sharp claws with delicate fingers.

A retreating tide exposes shellfish.

But eating those poses more problems.

The clams are full of meat, but it's all locked up behind closed doors.

However, grasping hands guided by a big brain can deal with that problem.

These clams are very tough.

I would have a lot of difficulty in opening them myself,

but these monkeys have learned that if you hit them hard enough,

the clam relaxes, the shells can be opened,

and they can get at the juicy meat inside.

The whole troop is in on the trick,

and now at least 20 of them are busy giving the clams the treatment.

Ten minutes on, and the shells of the clams are still tightly clenched.

This is hardly fast food.

The young watch attentively.

Most will eventually learn how to do it,

so the clam-cracking technique will pass to the next generation.

But it takes years for a young monkey to get it absolutely right.

Patience is important. Unfortunately, so is technique.

Capuchins, just like us,

have varied personalities and abilities,

and some never really get to grips with clam-cracking.

Those with a good technique AND perseverance

are now beginning to collect their rewards.

Although some aren't exactly thrilled by what they find.

It's not just food that monkeys collect from the forest.

The two bunches of leaves may look very much the same,

but this kind is used by the local people

as an antiseptic and insect repellent,

whereas this one has no medicinal qualities at all.

If I put them both down, I think I know which the monkeys will choose.

Those are pipa leaves - the ones with the insect-repelling sap.

Pipa is hard to find, so this big bunch will cause a lot of excitement.

Like many things in monkey life,

pipa rubbing is a major social event.

Those taking part disregard the differences

they might have had at other times and everyone joins in.

Do these capuchin really know that this behaviour

protects them against skin infections and mosquitoes?

Well, of course, it's impossible to say.

But they do it more frequently during the rainy season

when there are more skin infections and mosquitoes around

than at any other time during the year.

Not all capuchins use pipa. But once one discovers the plant's properties,

the knowledge quickly spreads,

so pipa-leaf rubbing becomes a group tradition.

Capuchins eat most things.

Some monkeys, however, have particular specialities.

These are uakaris, and they will eat fruit of an unusual kind.

The colour of their faces indicates their social position.

The brighter the scarlet, the more senior the animal.

The good colour vision that enables them to tell where they are socially,

also helps them to select fruit.

Unusually, they will eat fruits that are a vivid green and not yet ripe.

The monkeys in this forest can all eat slightly different foods,

allowing many species to live together side by side.

The saki has special teeth

which enable it to eat nuts that no other monkey can crack.

The spider monkey has long limbs and a grasping tail

which enable it to collect the very ripest fruit.

But there are foods in this forest

that not even a spider monkey can get to.

There is food for monkeys in every part of the rainforest,

but some of it is very difficult to reach.

I have only got up here into the canopy in a special crane,

and I am 120 feet above the ground.

And here, on the outermost twigs of the trees,

the stems are too thin to support a normal-sized monkey.

And yet, there is a lot of food up here.

Underneath these leaves -

a cocoon, a caterpillar, another leaf-eating insect.

But there is one kind of monkey that can get up here and eat these things.

It's a pygmy marmoset - the smallest monkey in the world,

no bigger than my hand.

Its feet are too small to grasp anything but the thinnest twigs.

Their needle-sharp claws prevent them from slipping.

As it stalks its insects it has to move with great stealth,

lest any vibration should scare them off.

But these hunters are so small

that not even the thinnest branches tremble under their weight.

All monkeys have forward-pointing eyes so that they can judge distance.

That's invaluable when moving around in the trees.

But this tiny little hunter uses that ability also to pounce.

But their staple diet is altogether more extraordinary.

You might think that the lumps on this tree are a natural feature,

but, in fact, they are wounds inflicted by pygmy marmosets

that come to this tree trunk every day.

It's eating gum,

which it has forced the tree to produce by gnawing its bark.

And what's more, it is making sure that there will be gum tomorrow,

because it is also re-opening the wounds that the tree has healed up.

The marmosets keep opening these wounds

and the tree keeps closing them

until so much scar tissue builds up that circular mounds appear.

Generations of pygmy marmosets have done that on this particular tree,

resulting in these huge columns of lumps running right up the trunk.

A well-managed gum tree is an asset for a group of marmosets,

making it a tempting target for thieves.

And a rival group is preparing to invade.

The intruders charge and the owners flee,

leaving their precious gum undefended.

While the invaders steal the gum,

the displaced residents start scent-marking in the trees nearby.

This seems to reinforce the bonds between them

and stiffen their resolve to mount a counter-attack.

It works. The home team are back on their tree.

But you can share a food resource that is cropped by others

without having a face-to-face confrontation.

You might think that when the sun goes down,

all monkeys would be settling down to sleep.

That's almost true, but not quite.

There's only one monkey in the world that collects its food at night.

And precisely because it IS nocturnal,

it is very seldom seen and still little known.

But there is a family of them living in this tree behind me.

They are douroucoulis.

They are also known, for obvious reasons, as owl monkeys.

Their eyes may be huge,

but, in fact, their night vision is not particularly good,

suggesting that their ancestors lived by day

and that douroucoulis became nocturnal relatively recently.

During the day, the nectar-rich flowers on which they feed

are guarded aggressively by other monkeys.

By coming out only at night,

douroucoulis avoid having to fight for a share and can feed in peace.

Day breaks.

The douroucoulis retire

and a new set of fruit- and nectar-feeding monkeys awake.

Tamarins set off to collect their share.

But this forest is not rich in such food,

and though they live in small groups, finding all they need takes time.

The female always produces twins, which travel with the family.

But she has two male partners and she uses her tongue to signal

that it's time that one of them took over the twins.

Both are eager to do so, for both mated with her

and so each has cause to regard the babies as his.

But the twins have other ideas. They want to stay with Mum.

She gets rid of them at last, and it is important that she does so.

She has to provide them with milk, so she must eat much more than usual

and she would find that difficult

if she were burdened by her babies all the time.

The situation is made harder,

because there are other kinds of tamarins here as well.

This is one of them - a saddleback tamarin.

Monkeys usually guard their food ferociously,

but these two kinds of tamarin often feed side by side.

And when one leaves, the other follows.

The emperors go first, and the saddlebacks -

with their shorter, stumpier legs - try hard to keep up.

The travelling party shares a quick refuel.

Both kinds of tamarin, being small and energetic, need frequent snacks.

They share another thing, too. They have a common enemy.

They give alarm calls that both species understand.

TAMARIN SHRIEKS

A tayra - a kind of giant weasel.

Not the most dangerous predator in this forest,

but deadly to a bite-sized monkey.

It's certainly safer to travel in groups, but having to share food

is a high price to pay for a warning system.

There is, however, another rather more subtle advantage.

Each flower needs time to refill its reservoir of nectar

after a visitor has drunk from it.

If either kind of tamarin foraged separately,

they might make a hazardous journey,

only to reach a tree that had recently been emptied by the other.

That would be disastrous for an animal that has to feed so regularly.

It's far better for tamarins to travel together

and then share the food when they arrive.

Howler monkeys - ten times the size of a tamarin.

They also eat flowers, given the chance,

but these are not always available.

In tropical forest, at least leaves can be found all year round.

Howler monkeys specialise on these.

The best leaves are up in the canopy,

so that's the best place to go if you want to watch howlers close-up.

Leaves are not all the same.

Some are poisonous, even to a howler monkey.

Of those that are edible, some are nicer than others.

But, once again, that monkey ability to see in colour is a great help.

These red ones are young and succulent,

and would be very good to eat, if they weren't protected by poisons.

These mature ones are green and the poison in them has faded,

but they're fibrous and woody and NOT very good to eat.

A leaf-eater wants something in between, like this.

Imagine how difficult it would be to pick the right one

if you didn't have colour vision.

Even the best leaves are not easy to digest,

so, although howlers spend a short time collecting food,

they spend a long time digesting it.

In fact, they spend half of every day just lying around,

like cows, chewing the cud.

Even after all that digestion, leaves are not very nutritious,

so howlers can't waste energy chasing rivals through the treetops.

They use a rather more labour-saving way of doing that.

GRUNTING

Howlers have a specially enlarged bone in the throat, the hyoid,

that enables them to make this extraordinary song.

It's one of the loudest noises made by any animal

and the whole family joins in almost every evening.

They work themselves into a frenzy,

but since neither they, nor their rivals actually move anywhere,

it's very hard to say who's winning.

It may be that this howling serves to strengthen family solidarity

and deter intruders.

RASPING GRUNTS

That may sound a bit like a howler monkey,

but I've left South America and I'm in Africa.

The howler's African equivalent is the black and white colobus.

Many different kinds of monkeys are able to share the same forest,

because they find their food in different ways.

Red colobus' stomachs allows them to eat unripe fruit and leaves,

whereas sooty mangabeys, like saki monkeys,

have powerful jaws that can break open really tough foods.

The rest of the monkeys have a more general diet, like capuchins,

and belong to a group called the guenons.

All guenons eat fruit and insects,

but each looks for them at its own level in the forest.

There are 17 different kinds of guenon in Africa.

They all have similar-shaped bodies,

but each has its own colours and patterns

that serve as signals allowing them to recognise friends and rivals.

They are the most colourful group of monkeys in the world.

But colour signals are not a good way of sending urgent messages

in places where the visibility is poor,

and that's certainly the case here in the Tai Forest in West Africa.

Failing to spot another monkey is one thing, but if you miss a warning,

the consequences could be fatal.

So how do monkeys keep out of harm's way?

I'm travelling with one of the most extraordinary

anti-predator alliances in the world.

Around me is a troop of sooty mangabeys and mongooses.

They're keeping an eye out for danger on the ground.

In the branches above me, there are at least five species of monkey,

all ready to detect a threat from the sky or from within the canopy.

In this alliance, monkeys of many kinds travel side by side.

It's a huge safety network

that extends from the canopy right down to the forest floor.

With so many pairs of eyes at every level,

someone is sure to spot a threat, no matter where it comes from.

There's little squabbling over food between different species

because each finds it in a different way.

Visibility is so poor here that they communicate almost entirely by sound.

Unlike visual signs, a sound reaches every monkey in a split second.

It's the best means of staying in contact or warning of danger.

Diana monkeys forage near the top of the canopy,

so they're usually first to spot danger from the air.

Crowned eagles are never far away here, but if one comes too close,

a specific alarm will be sounded, telling monkeys to drop for cover.

Danger above! Monkeys plummet down without even looking up.

The alliance has saved another member.

All these monkeys have different alarm calls for different predators.

Watch this!

SQUEALING

GRUNTING

That's the leopard alarm call of the Diana monkey.

GRUNTING That different one is the spot-nose.

But the important point is that all species of monkey in the alliance

recognise one another's call and know the nature of the enemy.

That's important

because each predator requires its own particular response.

All these monkeys know there's a leopard around

and you might think, therefore, that they would flee,

but they don't - they're advancing towards it,

and that's because the leopard is an ambush predator.

Once it's spotted, it usually gives up and goes away.

Monkeys are drawn by the call from a large area.

They even drop lower, so they can keep the cat in full view.

And here they will stay, on maximum alert,

until the leopard finally gives up and leaves them in peace.

The ancient city of Polonnaruwa in Sri Lanka.

The civilisation that built it 1,000 years ago has long since crumbled,

but one society that lived here then is still here now and thriving.

Toque macaques have walked these walls for centuries,

but only in the last few decades

have we started to understand their complex society.

This is one of the most studied monkey groups of all time.

For 30 years, their daily existence has been recorded in detail.

That has given us great insight into their private lives.

These youngsters are playing as equals,

but they have very different futures ahead of them.

There's a ruthless class system in monkey society.

Those born in high-ranking families have a massive head-start in life.

Emelda is one of them. She is highborn.

Others have been feasting on berries, but she's come too late to get any.

The berries are stashed in the cheek-pouches of others.

Emelda spots Poppin. She is far larger and older than Emelda,

but she is from a lower-ranking family,

entitling Emelda to take anything of Poppin's that she wants,

including the food right out of her mouth.

Poppin makes it as hard for Emelda as she dare,

but if she resists too strongly, she risks a beating from the rest.

In monkey society, it helps to be highborn.

It also helps to be intelligent,

and that is particularly apparent during the mating season.

Toque macaques try to attract one another's attention

by fluttering their mauve eyelids and flashing their teeth.

But although there are willing males everywhere you look,

there is a shortage of females ready to mate.

Females are only fertile for a few weeks in the year.

During this short period, lots of males want to mate with them,

but hierarchy does not allow that.

This male in front of me,

after years of political manoeuvring and a good deal of muscle,

has become alpha male, and he has first pick,

but there are always some young males who try to beat the system.

Booster has a lesser rank

and that prevents him from openly mating with a fertile female.

His best chance is a secret liaison,

but the alpha male has recruited many allies,

and these males will stop any female who tries to sneak away

and reprimand her, if necessary.

This security team allows the alpha to concentrate

on the privileges of his position.

But Booster's chance has come.

The alpha male's allies don't have eyes in the backs of their heads,

and females, like Shanti here, see Booster as a future alpha.

Allowing him to father their young could be a shrewd investment.

First, she needs to attract Booster's attention.

Shanti is flirting.

Her glances are subtle, because she doesn't want to get caught,

but they're unmistakable, if you know what you're looking for,

and Booster certainly does.

He needs to signal his intentions to Shanti,

but feign disinterest should the alpha male look in his direction.

But the alpha has other females to attend to

and his allies are nowhere to be seen.

Shanti gives Booster the eye.

They must be discreet and quick. Shanti's absence may be noticed.

If Booster DOES take over, he may look favourably on Shanti's young.

They might, after all, be his.

A big brain certainly helps you to take advantage of others,

and the larger the group you live in, the bigger your brain needs to be.

But it was a dramatic change to the Earth itself

that allowed monkeys to climb up to the next rung of the social ladder.

10 million years ago, the climate of the world changed.

Many places became much drier.

The rainforests shrank

and were replaced by scrub and open grassland.

But there was a lot to eat in this new terrain.

Some of the monkeys came down from the branches

and went out to find it in the open.

The first monkeys to walk out here would have found some familiar foods,

flowers and seasonal berries,

but today baboons collect all kinds of other things -

bulbs, cactus,

even rabbits.

The grasslands are a great place for an opportunist.

Baboons use their fingers and brains

to collect an extraordinary variety of foods.

Insect grubs can be picked up

from the edge of a soda lake in Kenya's Rift Valley.

Flamingos gather in vast numbers and generate a rich layer of compost

that contains plenty of food for those patient enough

to pick through it.

But a few baboons have become more ambitious.

Only this troop of baboons catch flamingos.

It's just five years since the first of them discovered this new food

and worked out how to get it,

but the skill has spread quickly

and now the whole troop are dedicated flamingo hunters.

Life in the open has its disadvantages, too.

There are more big predators around,

so baboons protect themselves by assembling in large groups.

As among macaques, living in big groups causes social problems.

And here there's an even more complex social structure,

within which males compete for females.

When a female becomes sexually available,

her bottom swells and turns bright pink.

All the males can read the message, but she only offers herself

to males that have looked after her during the rest of the year.

This male is attempting to join the troop,

but it may be some time before he can father any offspring.

First, he has to learn the social order in this complex community

and a good way to start is to befriend one of the senior females.

The ledge can get very crowded,

and the presence of the alpha male is intimidating.

Even deciding where to sit is a political decision.

The new male inevitably invades the personal space of others.

Tempers flare.

SHRIEKING AND CHATTERING

Life for a new arrival can be very stressful.

Established males rely on a network of allies for support.

The most successful of them invested heavily in such social relationships.

But knowing who to build them with takes intelligence.

Grooming is a good way to maintain friendships.

Looking after babies will also score you points.

Child-caring brings another benefit.

Babies are protected in baboon society,

so no-one will hit you if you're carrying one.

That allows smart baboons to use infants like shields

when they feel threatened.

It takes time and brain power

to rise to the top in a large group.

It's easy to imagine how a big brain could help you in baboon society.

But if social complexity IS the driving force

behind the growth of the primate brain,

then it SHOULD be that brain size in primates

corresponds closely to group size.

This is the skull of a baboon

and this lump of Plasticine

represents the brain which the skull once contained.

This represents the brain of a bush baby.

It's essentially a solitary animal,

but even allowing for its small body size, its brain is tiny.

This, slightly larger, is the brain of a colobus - group size 15.

This, bigger still, a guenon - group size 25.

And back to our baboon - group size 50.

The relationship is so close,

that were you to give a skull to a researcher who works with monkeys

even though they didn't know what kind of monkey it belonged to,

they could accurately predict the size of group in which it lived.

500 miles away in the highlands of Ethiopia

live the largest monkey groups of all.

Geladas - the world's only grazing monkeys.

There's a lot of grass up here

so geladas flourish and gather into enormous herds.

They don't have nipping teeth like sheep or rabbits

so they have to collect their grass by hand.

This is done best by sitting down

and they spend much of their lives shuffling along on their backsides.

Because they sit down so much,

their sexual displays have moved to their chests,

where other geladas can see them.

These indicate strength and virility

and allow individuals to assess each other's status at a glance,

so avoiding unnecessary confrontations.

Within these herds are smaller groups,

like those formed by other monkeys.

Related females are the core and a male harem leader watches over them.

And he has to be very vigilant.

A pack of bachelor males with one goal in mind -

to displace the holder of a harem.

The bachelors avoid males with the brightest chests -

they are in their prime and will have the full support of their family.

But less impressive males will have to prove their worth in battle.

A lip-flip from one of the bachelors is a threat. Flip answers flip.

There's going to be trouble.

These fights are largely for show and rarely result in injury,

but they do waste valuable grazing time.

Eating grass is time-consuming and also occupies your hands,

so it's hard to find time for grooming.

So geladas maintain their friendships in another way -

they chatter to one another.

CHIRPS AND MURMURS

These noises are, in effect, VOCAL grooming

and enable geladas to communicate with many individuals at once.

And this is the secret of their ability to live in huge groups.

Their continuous chatter SOUNDS like a language,

but we have no idea what sort of information they are exchanging.

One thing, however, IS likely -

it was the need to communicate detailed social information

between many individuals

that led to the evolution of language in our own species.

So although monkeys living in the tree tops have rich and varied lives,

it's the ones that came down to the ground and formed large groups

that have the most complex and communicative societies of all -

a fact not without significance for our own ancestry.

Those monkeys cracking shellfish and anointing themselves

with a natural insecticide

were showing a kind of behaviour that, not so long ago,

was thought to be exclusively human.

But over the past 40 years,

intensive studies have shown that monkeys have a surprising complexity

in their tool use, in their language, and in their societies.

Primatologist Jane Goodall

is famous for her ground-breaking studies of wild chimpanzees,

but in 1960, she also began one of the first field studies of monkeys -

the olive baboons of Tanzania's Gombe Stream National Park.

Many monkey species have large, heavily armed males.

They seem to be constantly fighting

and soon the researchers identified the rank of different males

in a turbulent hierarchy of threats and challenges.

It is the young, prime male, entering a new troop

and eventually confronting the existing leader

who becomes the new, undisputed ruler of a troop.

Here, a male's rank depends on his dominance,

with the alpha male at the top.

Across Africa, biologists revealed baboon troops as complex,

highly-ordered and stable societies, containing long-term relationships.

This includes a key role for the females in the species

studied by Chadden Hunter -

the gelada baboon.

We have about 600 here filling up this valley,

so just this ocean of fur.

It's really hard to see just how many monkeys are here in one place.

Family groups are basically sisters who stick together -

say, four or five sisters.

They'll pick a male, he comes into the family,

has to toe the line with them -

basically protect them, protect the kids whenever they want him to.

On the side of the group

is this tight-knit kind of street gang of mean-looking bachelors

and if the sisters aren't happy with their male

then they'll chuck him out and get a new guy.

Even when there's relative stability,

males are constantly under pressure.

The alpha male can never relax.

By measuring hormone levels,

Professor Bob Sapolsky has shown that, just like humans,

baboons suffer from stress and stress-related diseases.

Baboons and us are very similar.

WE don't get ulcers cos we have to wrestle somebody for a parking spot.

We don't have the physical stressors most animals do.

We're psychologically sophisticated enough

to invent nonsense in our heads and we get sick with it.

And largely that's what baboon society is about.

They've got enough free time and enough food

that they can spend a lot of time making each other miserable

with social stress.

Here's one really stressful thing you can do if you're dominant -

some lower-ranking guy is in a sexual consortship with a female.

She's in heat, she's reproductively accessible, they're hanging out.

What does the dominant guy do?

He doesn't attack. He's just AROUND.

He's six feet away, just being on the scene harassing the pair.

No fights, no threats, no nothing.

The guy who's being harassed picks up and walks away voluntarily.

This is psychological stress, straight out of the dictionary.

So we know that monkeys live complex, even stressful, social lives.

To maintain cohesion, they constantly send and receive messages.

Monkeys are excellent communicators.

If it is possible to get inside the mind of a monkey,

then the squeals and chatters that accompany almost everything they do

perhaps hold the key.

Right from the beginning of these studies,

it was clear monkeys had something important to say to one another.

These vervet monkeys are warning each other of a snake.

RAPID CLICKING

The first detailed study of primate communication,

by Robert Seyfarth and Barbara Cheney,

revealed the true meaning of vervet alarm calls.

We did a series of experiments in 1977 and '78

where we'd wait until the monkeys were foraging in a certain area

and then we would hide a loudspeaker in a bush nearby.

We'd recorded the alarm calls in natural encounters.

We took one of these alarm calls and played it from the loudspeaker

when there were no predators around.

The monkeys responded by running up into the trees at a leopard alarm,

looking up or running into bushes at an eagle alarm,

or looking on the ground around them in the case of a snake alarm.

This means that the alarm call almost functions like a word.

Vervet infants have to learn these words.

They start with broad classes of danger.

The eagle alarm covers all birds and flying objects.

As they grow, they become more precise

and, quite simply, those that can't learn don't survive.

So we now know that vervet monkeys use words

and have the rudiments of language.

Different vocalisations mean different things

and must be learned by the young members of the troop.

Recent studies have shown that that applies

to other species of monkeys as well.

And monkeys don't just call out warnings of danger.

Professor Mark Hauser's experiments with these rhesus macaques

show that they're constantly discussing food.

COUGH-LIKE GRUNTS

They tell each other about mundane food items

using a grunt that Mark refers to as "the boiled-potato call".

But they use "the caviar call" when the food is more interesting.

WHISTLES

Experiments show some monkeys even recognise the calls of other species.

They've become bilingual.

The monkeys of the Tai forest are hunted by eagles and leopards,

but in the wet season the greatest threat is a primate - the chimpanzee.

They are expert climbers that can follow monkeys into the trees

and they hunt cooperatively.

So their prey - red colobus monkeys -

form a partnership with Diana monkeys.

Klaus Zuberbuhler plays chimp calls

and the red colobus immediately look for Diana monkeys.

THEY are the most likely to spot the chimps and raise the alarm.

What is interesting when looking at inter-species communication,

between one monkey species to the next,

you find that each species possesses their own repertoires.

A Campbell's monkey has a particular alarm call for a leopard

which is completely different from the leopard alarm call of, say,

a Diana monkey

and it's quite obvious they do understand each other's alarm calls.

That tells us they must learn it at some point.

They aren't born with the knowledge of all these other species' calls.

So as it grows up, a young monkey learns the calls of other species.

It's a bit like learning a second language.

It also seems that GROUPS of calls

may carry a greater meaning than single ones

with call-groups that are equivalent to adding an adjective to a noun.

For example, Campbell's monkeys also have multifier calls

that seem to affect subsequent meaning of alarm calls

and it's this way of combining calls that has caught our attention.

And that's an important step.

If adding one call to another creates a new meaning,

there's a fascinating parallel with human language.

One thing is for sure - the more we listen to monkeys,

the more they'll tell us about their complex, highly social lives.

Subtitles by Red Bee Media Ltd