David Attenborough's Natural Curiosities

The natural world is full of extraordinary animals

with amazing life histories.

Yet certain stories are more intriguing than others.

The mysteries of a butterfly's life cycle,

or the strange biology of the emperor penguin.

Some of these creatures were surrounded by

fantastic myths and misunderstandings.

Others have only recently revealed their secrets.

These are the creatures that stand out from the crowd,

the curiosities that I find particularly fascinating.

In this programme, we investigate the stories of two animals

that owe their existence to human interference.

Killer bees that were created accidentally

when a well-meaning breeding experiment went wrong.

And pizzly bears, the result of polar bears

and brown bears interbreeding.

How were these strange animals created,

and are they unnatural mutants, or valuable new hybrids?

This is a grizzly bear.

And that white one there is, of course, a polar bear.

But in between, there's a different kind of bear.

It's got the white coat of a polar bear, except that

around the eyes, it's rather brown, and its front legs are very brown.

It is, in fact, a hybrid,

the result of a mating between a polar bear and a grizzly bear,

and they're sometimes called a pizzly.

The first-ever pizzly bears were born in a German zoo in 1876,

after a polar bear and a grizzly - a type of brown bear -

were housed together.

Hybrid animals have frequently been born in captivity,

both intentionally and by accident.

Tigers and lions produce offspring called ligers,

and donkeys and zebra, babies known as zonkeys.

But usually, these hybrid creatures are sterile

and not well adapted for surviving in the wild.

Animals, of course, usually mate with their own kind.

If different species are to interbreed

and produce fertile young,

they have to be extremely closely related.

Grizzly bears and polar bears are certainly somewhat

similar in appearance.

But are they, in fact, closely related?

In the bear family, the black species came first,

and then came the brown bear, and finally, the white polar bear.

This was thought to have happened four to five million years ago,

but recent fossil evidence suggests that it may have

happened as recently as only half a million years ago.

So the polar bear is the relatively recent species of bear,

one that branched off late in the bear family tree.

Polar and brown bears are, in evolutionary terms, close cousins.

They share some characteristics,

but there are also many physical differences.

The most obvious is the colour of their fur.

Colour acts as camouflage, so that's not surprising,

since they live in very different habitats.

Grizzly bears have rounded heads and prominent shoulder humps

that have evolved for digging.

Polar bears, on the other hand, have more pointed heads,

but no shoulder humps.

Their feet are large and flat

so that they can act as paddles when swimming.

They also have hairy pads and short claws

which helps to prevent them slipping on ice.

The grizzly has more obvious footpads

and much larger, curved claws.

We know that polar and grizzly bears can mate successfully.

But what sort of offspring do they produce?

For many years, zoos discouraged the breeding of pizzly bears.

But recently, there was a chance to study them in Germany.

In 2004, at Osnabruck Zoo, a brown bear called Susi

and a polar bear called Elvis,

who had shared an enclosure for 24 years, unexpectedly produced twins.

The small brown cubs are bigger now and have changed colour.

The male, named Taps, is brown,

and the female, Tips, has a lighter coat.

But otherwise, they have traits inherited from both parents.

Long necks and visible tails that are more typical of polar bears,

but also small shoulder humps

that are reminiscent of those of brown bears.

Their feet are intermediate in form.

And their size is between the two -

smaller than a polar bear, but larger than a brown bear.

We know from Osnabruck Zoo that when polar bears and grizzlies mate,

they can produce pizzly cubs.

But what are the chances of these very different bears

meeting in the wild?

This is the grizzly bear's home range in North America.

Polar bears live higher up in the Arctic.

So the two species are neighbours, but ones with very different

lifestyles and feeding habits that restrict their ranges.

But their habitats are changing as the climate is warming.

And that is beginning to have an effect on the behaviour

of the two species in the wild.

In 2003, a researcher working on a remote island between Churchill

and the North Pole discovered strange bear footprints in the snow

together with brown hairs.

The hair came from a grizzly. This was extraordinary.

The grizzly must have strayed hundreds of miles from its home

and travelled deep into polar bear territory.

So it was clear that the chances of these cousins meeting

and mating in the wild was becoming a real possibility.

But would their offspring survive?

Research on captive pizzly bears suggested that they could.

The hair of brown bear, polar bear

and pizzly bear are quite different -

not only in colour, but in structure.

Here are cross-sections of a hair from each of them.

The brown bear has a central canal

which is filled with a honeycomb structure.

The polar bear, that central canal is almost empty, making that hair

good for insulation - just what you need in a cold climate.

And the pizzly bear is a sort of compromise between the two.

The central canal has just a little infilling, so you might

say that it is not bad for cold temperatures and not bad for warm.

Possessing a mix of characteristics of both parent bears

could actually help these hybrids to survive in a rapidly changing world.

Their hunting skills have also become more variable.

Tips and Taps sometimes fish like brown bears,

but at other times behave like polar bears.

Stomping, for instance.

Polar bears push down on ice to break it

during their search for seals.

The Osnabruck pizzlies perform a similar action.

This mixture of physical and behavioural characteristics

suggest that pizzlies may be well-equipped to

survive in the wild if conditions in the Arctic continue to change.

And, in 2006, this notion became reality.

An odd-looking bear was shot during a polar bear hunt

in northern Canada.

It was small, hunched and had dark smudges around its eyes and muzzle.

DNA testing showed that its mother was a polar bear

and its father, a grizzly, that had travelled further north

and was hundreds of miles beyond its normal range.

This was the first proof of a hybrid pizzly bear in the wild.

Several have been reliably identified in the wild,

though none has yet been caught alive.

But as the climate warms, so brown bears are moving north

and polar bears coming south,

and their close genetic relationship means that not only can

they interbreed, but the offspring are likely to be fertile.

The ranges of these bears are now increasingly overlapping.

It's now not uncommon to see polar and grizzly bears feasting together

when there's plenty of meat around, as there is after a whale hunt.

So are we seeing a new development

in the evolutionary history of bears?

In April 2010, biologists in the Northwest Territories

of Canada shot a dangerous polar bear.

It was strange in appearance,

and DNA analysis showed something very significant -

this was a second-generation pizzly bear,

the result of a female pizzly mating with a polar bear.

As global warming continues to diminish the Arctic sea ice habitat,

climate scientists believe that the polar bear will struggle

to survive as a species.

But at least some of the polar bear traits will be