Curious Imposters David Attenborough's Natural Curiosities

The natural world is full of extraordinary animals

with amazing life histories.

Yet certain stories are more intriguing than most.

The mysteries of a butterfly's life cycle

or the strange biology of the Emperor penguin.

Some of these creatures were surrounded by myth

and misunderstandings for a very long time

and some have only recently revealed their secrets.

These are the animals that stand out from the crowd.

The curiosities I find most fascinating of all.

Some animals have mastered the art of deception.

The cuckoo tricks other birds into raising its young,

while the death's-head hawkmoth infiltrates

a nest of bees to steal their precious honey.

They're cheats and impostors.

CUCKOO CALLS

The call of the cuckoo has long been regarded as a sign of spring,

but in fact it's the call of a killer and a cheat.

The cuckoo lays its egg in the nests of other birds and somehow

persuades them to treat it and its chick as if it were their own.

How does it get away with it?

It's a question that has puzzled people for centuries.

In Britain, the cuckoo arrives at a time

when most birds are nesting and laying eggs.

Early egg collectors noticed that the nests of some birds

had a slightly odd-looking egg in them.

These are the eggs laid by a number of different birds.

A marsh warbler, spotted flycatcher, a linnet and a whitethroat.

Amongst each of those clutches, there is a fraudster, a cuckoo egg,

which mimics that of its host.

Although cuckoos are long known to lay their eggs in the nests

of other birds, no-one had actually described it happening.

Then, in the 18th century, an English country doctor

with an interest in natural history decided to investigate.

Edward Jenner lived here in Berkeley, Gloucestershire,

and is best known for his work on the smallpox vaccine.

In fact, he is said to be the father of vaccination and that

his work has saved more human lives than that of any other man.

What is less known is that he first achieved scientific distinction

by his observations on the behaviour of the cuckoo.

At the time, it was believed that a cuckoo removes

all of the eggs in a nest and then lays its own.

By doing so, it would ensure its own chick gets all the food

brought in by the unwitting nest owners.

But Edward Jenner's detailed observations were to reveal

a rather darker tale.

Jenner's work on cuckoos was published in 1788

here in the Philosophical Transactions of the Royal Society,

the world's first scientific society.

It was entitled simply...

"Observations on the natural history of the cuckoo,

"by Mr Edward Jenner."

In it, he reported that it was not the parent cuckoo

but the newly hatched chick which pushes the eggs

and nestlings of the foster parents out of the nest.

As soon as it hatches,

the cuckoo chick's instinct is to kill anything else in the nest.

It's still blind and naked

but it has a cup-shaped depression on its back

into which an egg fits perfectly.

But sometimes the other eggs hatch earlier and Jenner's observations

of how the cuckoo chick deals with its nest mates

were quite shocking.

He writes...

"the mode of accomplishing this was very curious.

"The little animal, with the assistance of its rump and wings,

"contrived to get the bird on its back

"and, making a lodgement of the burden by elevating its elbows,

"clambered backwards with it up the side of the nest

"until it reached the top, where, resting for a moment,

"it threw off its load with a jerk

"and quite disengaged it from the nest."

The real villain had been uncovered.

Jenner's views were met with incredulity and some disbelief,

but nonetheless they earned him the Fellowship of the Royal Society.

It was the greatest honour that could be given to

a scientist at the time.

Jenner's observations had revealed

the true nature of the cuckoo's deception.

But it still wasn't clear why the cuckoos should opt for this

strange way of raising its young.

It wasn't until 100 years later that Charles Darwin finally

provided an explanation with his theory of evolution.

The cuckoo's behaviour has evolved

to increase its own breeding success.

By avoiding the task of raising chicks,

the cuckoo can lay more eggs than any other bird,

as many as 25 in a season.

While it makes evolutionary sense for the cuckoo

to lay its eggs in the nests of others, what about its victims?

Why do they put up with this trickery?

It seems that they sometimes don't.

This was revealed in an early natural history film in 1920.

The Cuckoo's Secret was made by Edgar Chance and Oliver Pike,

an egg collector and a wildlife film maker.

Chance was fascinated by cuckoos

and spent a great deal of time following them.

He was the first person known to see a cuckoo lay its egg.

The deception involves stealth and speed.

The female waits until a nest is unattended and then she strikes.

But if she is spotted, the owners fight back.

If she is successful,

the whole deception takes less than ten seconds.

She removes and eats just one egg and replaces it with her own.

The Chance and Pike film solved one mystery,

but there were still others.

How does the cuckoo choose its victim?

And why don't the nest owners reject the alien egg?

Reed warblers are one of the cuckoo's main targets

and the pair has a nest just in here.

The female warbler has laid four speckled eggs,

and, using a model egg, I can illustrate the cuckoo's trickery.

This is the sort of egg that a cuckoo would lay

in the reed warbler's nest.

It matches the reed warbler's actual egg very closely in colour.

Experiments with model eggs have shown that reed warblers

have become very good at recognising an alien egg

and either throw it out or desert their nest to start afresh.

So the cuckoo has to make sure that it produces an egg

that is a very good match.

The cuckoo and its victims are evolving competitively.

With each generation, cuckoos improve their mimicry,

while the nest owners become better at spotting a foreign egg.

While many birds are very good

at detecting a strange egg in their nest,

they seem incapable of recognising

the monstrous cuckoo chick as an impostor.

But the deception is not complete.

The young cuckoo is much larger than the reed warbler chick,

so it also needs a lot more food.

How does it get enough?

The cuckoo has a solution.

It now uses vocal deception to trick its foster parents

into providing more food.

This is a sonogram of the sound waves produced by a single

reed warbler chick begging for food.

Below it is the call of a cuckoo chick,

and, as you can see, it looks very different.

In fact, it more closely resembles

the calls of a whole nestful of reed warbler chicks.

So, the cuckoo chick's call is a super stimulus

that sounds like a whole nestful of chicks.

And it appears to work.

The adult birds rush back and forth,

providing the impostor with the same amount of food

as they would for an entire brood of their own.

At three weeks old, the cuckoo chick

has spilled out of the nest.

It's now almost eight times the size of its foster parent.

It was over 200 years ago

that Edward Jenner first shocked us

with his revelation of the cuckoo's extraordinary lifestyle.

Now we know that its unusual behaviour

is due to an extraordinary arms race

that has resulted in one of the most fascinating

specialisations in nature.

The cuckoo's success relies on deceiving just two parent birds.

But our second subject is a moth

that is able to deceive hundreds of bees.

How does it infiltrate

one of the most heavily guarded nests in nature?

This wonderful creature was once

one of the most feared insects in Europe.

It's a death's-head hawkmoth,

and it's easy enough to see how it got its name.

It has this mark on its back

that looks just like a human skull.

This gave it a bad reputation that lasted for centuries,

but now there are new ideas about this moth's strange appearance

that may help explain its extraordinary ability

to rob hives without being stung.

Death's-head hawkmoths are a rare sight in Britain,

for they spend most of their lives in Africa and Asia.

But every summer, a small number of migrants

arrive in northern Europe

and if the weather is warm enough, they breed.

Their caterpillars, unlike the drab adult moths,

are beautifully coloured.

After feeding for several weeks,

they can grow to a length of 13 centimetres.

Once ready to become adults, they pupate in the soil

and emerge as the sinister, strangely patterned moths.

In the early 19th century,

a region of northern France was hit by a terrible pestilence

and at the same time, a large number of hawkmoths

were seen in the area.

The local people linked the deaths to these night-flying insects.

But there was another even more disturbing side to this moth.

It could make an unusual noise.

SQUEAKING

There.

A strange squeak.

And that only added to its chilling reputation.

Moths don't usually squeak.

Tiger moths sometimes produce ultrasonic warning clicks

that tell bats that they're poisonous and not good to eat,

but this is not a noise we can generally hear.

Perhaps the death's-head hawkmoth squeaks

to scare predators like birds.

However, other large migratory moths

don't make such a sound.

This makes the death's-head hawkmoth's squeak

all the more surprising,

and it has intrigued people for centuries.

These moths are more than 200 years old.

We know that because the handwritten label there tells us

they were collected in 1801 by a Robert Darling Willis,

the personal physician to King George III.

George III is well-known as the king

who suffered from bouts of madness

and on a visit to see the king during one of them,

Dr Willis discovered these large moths in the monarch's bedchamber.

Unable to identify them, the doctor sent them to his grandson,

who was at that time superintendent

at the Museum of Zoology in Cambridge.

He confirmed that they were death's-head hawkmoths

and, unusually for an insect,

this moth produces a loud call

that has been likened to the mournful cry

of a grief-stricken child.

Did the disturbed king hear the plaintive calls of a hawkmoth?

That we don't know.

But certainly many of the ordinary people of the 19th century

were struck with a sense of terror whenever this moth appeared.

The moths' unusual appearance and strange behaviour baffled people.

But in nature, such traits usually have a purpose.

And it may be for the death's-head hawkmoth

that they enable it to break into beehives and steal their honey.

These are the giant honey bees of south-east Asia,

and they form some of the largest bee colonies in the world.

I once got up close to one in order to demonstrate

their response to a predator.

I had a model of a large hornet,

which produced a kind of Mexican wave,

and that makes it very difficult for an aggressor to land.

BUZZING

This covering of bees looks impossible to penetrate.

But at night,

a thief can break through their ranks.

A death's-head hawkmoth lands on the carpet of bees

and pushes its way through without being attacked.

In just a few seconds, it takes some sips of honey

and emerges unharmed.

Getting past the guard bees is quite a feat,

but surviving inside is even more astounding.

Death's-head hawkmoths raid domestic beehives too,

and can be quite a pest.

Somehow, the moth slips past the guards and, as if invisible,

walks through the hive, heading straight for the honeycomb.

It then feeds unnoticed.

How does it do this?

One theory proposes that its spooky appearance

may help it avoid being attacked.

BUZZING

Miriam Rothschild, a great entomologist

and expert on fleas and butterflies,

suggested that the moth's skull pattern looks like

the head of a worker bee,

and that this could play a role in the moth's deception.

Well, this is a photograph

of a worker bee face taken through a microscope.

Let's see how it looks next to a close-up photo

of the skull pattern of the moth.

There.

Well, I suppose there's a slight resemblance,

but given the fact that most moths raid beehives and nests

during the night, it's unlikely the bees

could see that much detail.

The most likely answer lies in the scent the moth gives off.

In America in the 1950s,

a German entomologist called Thomas Eisner

studied chemical ecology - in particular,

the chemical defences of insects.

Most famously, he illustrated how bombardier beetles

fire hot acid onto a predator.

He also studied moths,

and showed that the feathery projections on their abdomens

and their large antennae were used to produce and pick up scent.

It seemed that many insects were using scent in surprising ways.

Tests on the chemical scents produced by hawkmoths

reveal a remarkable similarity to those produced by the worker bees

in the hives that they raid.

Their scent is not identical,

but it contains several key chemicals

that exactly match those produced by bees.

So the death's-head hawkmoth's scent

acts as an invisibility cloak

that makes it undetectable to the worker bees in the nest.

With thick scales on its body,

clawed feet that grip the honeycomb,

and a short, pointed proboscis to pierce the honey cells,

the moth has evolved into an effective hive robber.

But there is another, even more impressive impostor

that can also penetrate the protective defences

of an insect colony.

Its victims are not bees but ants.

The impostor that invades this ant nest

doesn't get in there by flying.

Nothing as blatant as that.

Instead, the caterpillars of some species of blue butterfly,

like this one, wait for red ants to collect them.

Remarkably, passing ants don't kill them.

They pick them up and take them back into their nest.

The cuckoo caterpillar will stay inside the nest

for up to ten months.

Just like the death's-head hawkmoths,

it produces a chemical scent that deceives the ants.

This pink caterpillar, which belongs to the alcon blue butterfly,

has been collected because, to them,

it smells just like the young of their own nest.

They become controlled by the impostor's intoxicating scent,

and feed the butterfly larva even more regularly

than they do their own.

There's another way this impostor pulls off its deceptive trick.

When it's inside the nest,

the butterfly larva makes a strange chattering noise.

To our ears, it's very faint,

but it's clear enough to other insects. This is it.

CHATTERING NOISE

And this is the sound that's made by a queen ant.

SIMILAR CHATTERING NOISE

To worker ants, these calls are very similar,

and they react by treating the butterfly larva

as if it's one of their own.

Caterpillars of the blue butterfly are impressive impostors.

Not only do they mimic the scent of the ants,

but their queen's calls too.

This seems to trump the death's-head hawkmoth's ability

as a nest invader.

But the hawkmoth may also be using sound to trick its victims.

Remember the eerie squeak that was thought to be so frightening?

SQUEAKING

There.

The hawkmoth makes this sound inside the beehive

when it enters to steal honey.

It's been suggested that this might calm the bees,

because the squeak is thought to sound like the piping call

that the queen honey bee makes to pacify her workers.

We can't be sure if the call and the strange skull marking

evolved to deceive bees, but we can be certain

that the death's-head hawkmoth's life as an impostor

is more curious than the superstitions

that have surrounded it for hundreds of years.

The cuckoo and the hawkmoth are both audacious impostors,

but the cuckoo's ability to make its victim raise its young

is perhaps the most accomplished deception of all.