ATTENBOROUGH: 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 myths 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 1 find most fascinating of all.
Animals are usually either male or female
and usually they behave in a way that is characteristic of their gender.
But, in nature, there are always curious exceptions.
Female hyenas behave and look like males.
And male seahorses play mother and physically give birth.
Only now are we beginning to understand
why these two animals seemed to have swapped their sexual identities.
seahorses are fascinating.
some are tiny and blend perfectly with their surroundings.
Others can grow to an impressive 35 centimetres in size.
They live in shallow waters, both tropical and temperate,
across much of the world
and have even been found in the Thames estuary near London.
seeing one for the first time is a moment to remember.
They are magical creatures with a truly fantastic appearance.
They have the head of a horse, eyes like a chameleon,
the prehensile tail of a monkey, armour that can change colour,
and, perhaps most strangely of all, a pouch.
Their unusual features inspired their name, hippocampus,
a combination of two Greek words, hippo meaning ''horse''
and campus meaning ''sea monster.''
For centuries, they've been considered animals of myth and legend.
And only today are we unravelling the true story of males that give birth.
seahorses baffled early naturalists.
Their unusual characteristics seem to make them misfits.
But, after much debate, they were recognised as true bony fish.
But their breeding habits were hardly fishy.
Typically, female fish release large numbers of eggs into the sea
that males must quickly fertilise.
But a fish that kept its eggs in a pouch seemed scarcely believable.
The seahorse's striking appearance has given it an almost magical status.
Images and stories of a creature part horse, part fish
have spanned the centuries across many cultures.
Among the most famous are those belonging to Poseidon.
This famous Greek god of the sea lived below the waves
and his golden chariot was pulled by a pair of giant hippocampi.
The seahorses' odd behaviour appeared mysterious, too.
As early as the 3rd century BC,
Aristotle noted in his book on the History of Animals
that pipefish, close relatives to the seahorse,
had a pouch that burst into two to release the young.
These early observations of the pipefish's strange breeding behaviour
helped to reveal the true story of the male seahorse's mysterious pouch.
Just like seahorses, pipefish carry their eggs around with them.
some species simply stick the eggs to the outside of their bodies.
Others have a rudimentary pouch.
These simpler techniques provide some clues
as to how seahorses developed their more complex, closed pouch.
But what Aristotle didn't know when he spotted the pipefish giving birth
was that he was actually looking at a male.
And this important detail was to remain undiscovered for hundreds of years.
Although seahorses live in British waters,
until Victorian times, few people, apart from fishermen, had ever seen them.
In 1 859, a Mr Pinto brought four live seahorses back to London
from the mouth of the river Tagus in Portugal.
Pinto endured a sleepless, seven-day train journey through Europe,
waking himself frequently to aerate the seahorses' water with a syringe.
His seahorses survived and were installed in the new London Aquarium.
They were an instant hit.
seahorses were headline news.
Mr Pinto's journey and their arrival made the front pages.
Now, they could be seen in great detail
and the study of their mysterious breeding began.
1n that same year, what was described as ''a herd of baby seahorses''
was born in the British Midlands Aquarium.
This caused quite a stir,
as did the discovery that it was the male that gave birth to the young.
But why seahorses swapped parenting roles remained a mystery
and we're still searching for the answers today.
Here at the London Zoo's Aquarium,
over 1 50 years since the arrival of the first seahorses,
a detailed study is revealing more about their reproduction
and the unusual role of the male.
These tanks are set like a seahorse dating centre.
The first port of call is the courtship aquarium or ballroom tank.
Here, a number of adult seahorses spend time getting to know each other
as they look for compatible partners.
Breeding seahorses form lasting partnerships as mating pairs,
and their long, elaborate courtship dances
are a way of finding and securing a suitable mate.
Dances like those of this Australian species
can be complex and last several days.
They help the couple synchronise their bodies
so that the male's pouch is ready for the eggs.
They also help to establish the couple's joint territory.
seahorses were thought to be monogamous, but we now know that
some are only exclusive couples for the duration of the breeding season.
The female must choose the right male
because she's going to pass over her precious eggs to him.
Female seahorses do not have a pouch,
so a strong pair bond with a male is very important
as he will care for her eggs.
This is the honeymoon tank.
Seahorses that have shown an attraction for each other in the courtship tank
are removed as a couple and given their own private space.
In the wild, each pair has its own territory
and these smaller tanks make captive breeding more successful.
Here, the pair can synchronise their courtship.
Timing is crucial.
The female's eggs must be fully developed at exactly the same time
that the male's pouch is ready to receive them.
Once the female's eggs are ready, she hydrates them with sea water.
They must then be laid within 24 hours.
she transfers her eggs to her partner
by inserting her egg-laying tube, or ovipositor, into the male's pouch.
Once pregnant, the male attaches himself to one spot
and the female visits him every day.
she checks to see when he'll be ready for her next batch of eggs.
One theory suggests that, because the male is incubating the eggs,
the female has more time to feed,
and can put energy into making new eggs more quickly.
swapping roles may be a smart way to use their resources more efficiently.
What goes on inside the pouch is still a mystery.
The male may simply provide a closed incubator.
Or the inner skin may develop extra blood vessels
to give a more placenta-like connection.
1t's not clear.
During pregnancy and birth, the male's metabolism increases,
but that's little wonder, for he may have up to 1,500 eggs in his pouch.
The male seahorse gives birth to dozens of miniature babies,
perfect in every detail.
The free swimming young are put into separate crash tanks
where they can be fed and cared for.
The parent seahorses in this biological hotel
remain in the honeymoon suite, ready to mate again.
These are some of last year's youngsters and they've grown enormously.
Next year, they'll be breeding themselves.
swapping the parental roles seems to work well for seahorses.
1n warm conditions, a male can give birth every 28 to 30 days.
But of the thousands of fry produced each year, only a few survive.
There is no safe creche in the open sea.
To succeed, seahorse parents must work well together.
Yet, in this partnership,
the female seems to have the freedom to swim, feed and patrol her territory
which is normally the prerogative of the male.
So, is the male seahorse a slave to a gallivanting female?
Well, latest research suggests not
and shows that some males may have more control over breeding
than first thought.
If small or poor quality eggs are deposited into their pouches,
some males will absorb them.
Such males appear to be choosy about how they invest their time and energy.
And some females, in entrusting their eggs to males, are being cheated.
But the male seahorse can't be duped,
as having a pouch means that he can always be certain
that all the baby seahorses he gives birth to are his own.
so, male and female seahorses have swapped their roles.
The male is the mother and he gives birth to the babies.
Another animal with unusual parenting habits is the hyena.
Here it's the female that looks and behaves more like a male.
Why have female hyenas become so masculine?
These are African spotted hyenas,
creatures that have an undeservedly bad reputation
and a very strange biology.
In the wild, they live in clans of up to 80 individuals
and the females dominate the males.
The females are big, aggressive and look physically almost exactly like males.
Unravelling why the female is like this has not been easy
as it's difficult to tell the difference between the sexes.
The females male appearance is made all the more convincing
by her reproductive organs.
They're external and very similar to a male's.
Understanding hyena biology has helped to explain the female's masculinity
and the species' reputation as aggressive scavengers.
But in the past, these strange traits gave hyenas a very bad image.
In the first century, Pliny the Elder
described the hyenas and did them a great disservice.
This is what he wrote.
''Hyenas are like a cross between a dog and a wolf.
''They break everything with their teeth, swallow it with a gulp,
''and masticate it in the belly.
''They are believed to become male and female in alternate years.
''They can imitate the human voice, calling a shepherd by name
''so that he comes outside where they tear him to pieces.
''Any animal that a hyena looks at three times will be unable to move.''
That tainted image of hyenas was perpetuated for many years to come
and they were branded as evil, dangerous creatures.
Hyenas are not, of course, evil.
But their competitive nature and unusual eating habits make them appear fearsome.
They're specialist feeders.
They crush, eat and digest bones
that other creatures can't tackle and so leave behind.
And this diet has a significant effect on the female's appearance
and her family relationships,
especially those with her cubs.
1n the early 1 9th century, an unusual discovery in Britain
excited one man to look more closely at the hyena's diet.
In 1 822,
a rather eccentric but very eminent geologist called William Buckland
made a significant discovery
that was to further the modern understanding of hyenas.
Quarry workers in Kirkdale, Yorkshire
had come across a cave that contained a large number of bones.
Buckland was very excited
and rushed to see the remains before they were disturbed any further.
And he found that mud deposits in the cave
had preserved the bones of over 22 different species of animals
including tiger, bear, wolf, elephant and, significantly, hyenas,
which Buckland described as littering the cave like the bones in a dog kennel.
This is one of the actual hyena jaws that Buckland found.
It belonged to a young but ancient hyena.
There are also a lot of these on the cave floor.
They are coprolites, or fossilized faeces, from hyenas.
They contain bone fragments
that have passed through the hyena's digestive tract
and so showed that they were successful bone crushers.
Buckland's discovery of so many bones in what he believed to be a hyena's den
indicated that they were very successful hunters.
Contrary to popular belief, they scavenge very little
and kill over 80% of their own food.
A lone hyena can easily kill a wildebeest or a topi
and, with teamwork, they will tackle bigger animals, like zebra and giraffe.
They do scavenge as well,
but it's more usual for lions to steal from hyenas
rather than the other way around.
Female hyenas have become big and strong
and compete for food with other members of their clan.
Nothing goes to waste. They can eat even the thickest of bones.
Buckland was fascinated by the marks on the bones from the cave
but found it hard to believe that hyenas had made them.
He wanted to be sure of his findings
and understand how their jaws,
with their strange, massive teeth, actually worked.
Hyenas are African or Asiatic animals.
So, Buckland's discovery of hyena bones in an English cave
was strange, to put it mildly.
As a man of science, he wanted to confirm
that the skull he had collected from Kirkdale
was definitely from a hyena
and that it had made the marks on the many fractured bones.
To try and prove his case,
he asked a friend, William Burchell, an African traveller,
to send a young hyena back to England from the Cape.
He planned to kill it and compare its skull and teeth
with the specimens in the cave.
The young hyena that arrived at the docks was already tame
and had become a great favourite with the sailors, who christened him Billy.
Billy became quite a celebrity and was as tame as a pet dog.
No one could bring themselves to sacrifice him for the sake of science.
1nstead, a search of British museums produced a hyena skull
and Billy's life was spared.
Buckland was then able to compare the new and old skulls
and they matched.
Billy also helped to clarify the fractures on the bones.
He was fed ox bones.
This was one.
And Buckland compared it with one that was found in the cave
and they closely match.
This ability to crack massive bones
explains why female hyenas look like bristling males.
1t's also tied up intricately with the relationship they have with their cubs.
Cubs are born underground and are fed on their mother's rich milk.
At about three months of age, they emerge from the den
and continue to suckle for almost another two years.
Their mother helps feed the youngsters
as they can't yet crack and crush bones for themselves.
Even at almost a year in age, when they're big enough to join the kill,
their teeth and jaws are still not sufficiently developed
to tackle big bones.
The skull of a young hyena is quite different from that of an adult.
It's got a flat top, narrow cheeks and relatively small teeth.
An animal with a skull like this would not be able to crush and eat big bones.
It takes almost three years for a young hyena's skull to grow to full size
and reach mechanical maturity.
And this is the result.
This skull has a large, vaulted forehead
that dissipates biting stress, carrying it away from the face.
It's also got wide arches at its sides
for the attachment of powerful jaw muscles
and robust pre-molars that have specialised crack-resistant enamel.
jaws like these can crack the dense bones of zebra and even giraffe.
Developing this substantial specialised eating equipment takes time.
so it may be several years before a young hyena can feed independently.
This puts pressure on their mothers to become dominant and aggressive.
They need to fight to get enough food for their cubs.
The female's status in the clan's hierarchy
will directly affect the survival of her young.
The biggest, oldest, most established females are the most dominant
and take a bigger share of the kill.
so food and the need to fight for it
has made females look and behave like aggressive males.
But it has also had a strange side effect.
Female hyenas have large amounts of the male hormone testosterone
and consequently develop male-like reproductive organs.
This can be a problem.
Having a long, thin birth canal makes mating very difficult
and both mothers and cubs sometimes die during birth.
The female's strange gender swap
is one of the most unusual in the animal kingdom
and new science has now made sense of the old clues
and solved this mystery.
Hyenas are very intriguing animals.
William Buckland's early observations of their bones and his hyena experiments
started a study of these creatures that was to reveal their fascinating biology.
Hyenas may have a frightening reputation
but their odd characteristics all have a reason.
The story of their aggression and bizarre bodies
is intimately tied up with their food and the survival of their cubs.
They've evolved a perfectly formed bone-breaking jaw,
but the time it takes to grow has resulted
in one of the most unusual but dedicated mothers in the animal kingdom.
so, to become the best parents
female hyenas have become more male and male seahorses more motherly.