0:00:02 > 0:00:06The natural world is full of extraordinary animals
0:00:06 > 0:00:08with amazing life histories.
0:00:09 > 0:00:13Yet certain stories are more intriguing than most.
0:00:15 > 0:00:19The mysteries of a butterfly's life cycle,
0:00:19 > 0:00:22or the strange biology of the emperor penguin.
0:00:23 > 0:00:25Some of these creatures
0:00:25 > 0:00:28were surrounded by myth and misunderstandings
0:00:28 > 0:00:29for a very long time.
0:00:30 > 0:00:33And some have only recently revealed their secrets.
0:00:35 > 0:00:38These are the animals that stand out from the crowd.
0:00:38 > 0:00:43The curiosities I find most fascinating of all.
0:00:51 > 0:00:55Animals are usually either male or female.
0:00:55 > 0:00:58And, usually, they behave in a way
0:00:58 > 0:01:00that is characteristic of their gender.
0:01:00 > 0:01:04But in nature, there are always curious exceptions.
0:01:05 > 0:01:09Female hyenas behave and look like males.
0:01:11 > 0:01:13And male seahorses play mother
0:01:13 > 0:01:15and physically give birth.
0:01:16 > 0:01:19Only now are we beginning to understand
0:01:19 > 0:01:24why these two animals seem to have swapped their sexual identities.
0:01:31 > 0:01:34Seahorses are fascinating.
0:01:34 > 0:01:38Some are tiny and blend perfectly with their surroundings.
0:01:38 > 0:01:43Others could grow to an impressive 35 centimetres in size.
0:01:43 > 0:01:46They live in shallow waters, both tropical and temperate,
0:01:46 > 0:01:48across much of the world,
0:01:48 > 0:01:51and have even been found in the Thames Estuary near London.
0:01:51 > 0:01:55Seeing one for the first time is a moment to remember.
0:01:56 > 0:02:00They're magical creatures, with a truly fantastic appearance.
0:02:00 > 0:02:04They have the head of a horse, eyes like a chameleon,
0:02:04 > 0:02:07the prehensile tail of a monkey,
0:02:07 > 0:02:08armour that can change colour
0:02:08 > 0:02:12and, perhaps most strangely of all, a pouch.
0:02:12 > 0:02:16Their unusual features inspired their name, Hippocampus,
0:02:16 > 0:02:18a combination of two Greek words -
0:02:18 > 0:02:22hippo, meaning "horse", and kampos, meaning "sea monster".
0:02:22 > 0:02:25For centuries, they've been considered
0:02:25 > 0:02:26animals of myth and legend,
0:02:26 > 0:02:30and only today are we unravelling the true story
0:02:30 > 0:02:33of males that give birth.
0:02:35 > 0:02:38Seahorses baffled early naturalists.
0:02:38 > 0:02:43Their unusual characteristics seemed to make them misfits.
0:02:43 > 0:02:44But after much debate,
0:02:44 > 0:02:47they were recognised as true bony fish.
0:02:48 > 0:02:52But their breeding habits were hardly fishy.
0:02:52 > 0:02:57Typically, female fish release large numbers of eggs into the sea
0:02:57 > 0:03:00that males must quickly fertilise.
0:03:00 > 0:03:04But a fish that kept its eggs in a pouch seemed scarcely believable.
0:03:06 > 0:03:08The seahorse's striking appearance
0:03:08 > 0:03:11has given it an almost magical status.
0:03:11 > 0:03:13Images and stories of a creature, part horse, part fish,
0:03:13 > 0:03:18have spanned the centuries across many cultures.
0:03:18 > 0:03:22Among the most famous are those belonging to Poseidon.
0:03:22 > 0:03:24This famous Greek god of the sea
0:03:24 > 0:03:26lived below the waves,
0:03:26 > 0:03:30and his golden chariot was pulled by a pair of giant hippocampi.
0:03:31 > 0:03:35The seahorse's odd behaviour appeared mysterious, too.
0:03:35 > 0:03:37As early as the third century BC,
0:03:37 > 0:03:41Aristotle noted in his book on the history of animals
0:03:41 > 0:03:44that pipefish, close relatives of the seahorse,
0:03:44 > 0:03:46had a pouch that burst into two
0:03:46 > 0:03:48to release the young.
0:03:50 > 0:03:51These early observations
0:03:51 > 0:03:54of the pipefish's strange breeding behaviour
0:03:54 > 0:03:56help to reveal the true story
0:03:56 > 0:03:59of the male seahorse's mysterious pouch.
0:04:00 > 0:04:04Just like seahorses, pipefish carry their eggs around with them.
0:04:05 > 0:04:10Some species simply stick the eggs to the outside of their bodies.
0:04:10 > 0:04:12Others have a rudimentary pouch.
0:04:14 > 0:04:18These simpler techniques provide some clues
0:04:18 > 0:04:23as to how seahorses developed their more complex closed pouch.
0:04:26 > 0:04:27But what Aristotle didn't know
0:04:27 > 0:04:30when he spotted the pipefish giving birth,
0:04:30 > 0:04:33was that he was actually looking at a male.
0:04:33 > 0:04:35And this important detail
0:04:35 > 0:04:38was to remain undiscovered for hundreds of years.
0:04:43 > 0:04:45Although seahorses live in British waters,
0:04:45 > 0:04:48until Victorian times few people apart from fishermen
0:04:48 > 0:04:50had ever seen them.
0:04:50 > 0:04:55In 1859, a Mr Pinto brought four live seahorses back to London
0:04:55 > 0:04:59from the mouth of the River Tagus in Portugal.
0:04:59 > 0:05:04Pinto endured a sleepless seven-day train journey through Europe,
0:05:04 > 0:05:06waking himself frequently
0:05:06 > 0:05:08to aerate the seahorse's water with a syringe.
0:05:10 > 0:05:11His seahorses survived
0:05:11 > 0:05:15and were installed in the new London Aquarium.
0:05:15 > 0:05:17They were an instant hit.
0:05:18 > 0:05:21Seahorses were headline news.
0:05:21 > 0:05:26Mr Pinto's journey and their arrival made the front pages.
0:05:26 > 0:05:28Now they could be seen in great detail,
0:05:28 > 0:05:31and the study of their mysterious breeding began.
0:05:33 > 0:05:34In that same year,
0:05:34 > 0:05:37what was described as a "herd" of baby seahorses
0:05:37 > 0:05:41was born in the British Midlands Aquarium.
0:05:41 > 0:05:45This caused quite a stir, as did the discovery
0:05:45 > 0:05:48that it was the male that gave birth to the young.
0:05:50 > 0:05:54But why seahorses swapped parenting roles remained a mystery,
0:05:54 > 0:05:57and we're still searching for the answers today.
0:05:59 > 0:06:01Here at the London Zoo's aquarium,
0:06:01 > 0:06:05over 150 years since the arrival of the first seahorses,
0:06:05 > 0:06:09a detailed study is revealing more about their reproduction
0:06:09 > 0:06:11and the usual role of the male.
0:06:11 > 0:06:15These tanks are set like a seahorse dating centre,
0:06:15 > 0:06:18the first port of call
0:06:18 > 0:06:21is the courtship aquarium, or ballroom tank.
0:06:21 > 0:06:23Here, a number of adult seahorses
0:06:23 > 0:06:26spend time getting to know each other
0:06:26 > 0:06:28as they look for compatible partners.
0:06:28 > 0:06:32Breeding seahorses form lasting partnerships as mating pairs,
0:06:32 > 0:06:35and their long, elaborate courtship dances
0:06:35 > 0:06:38are a way of finding and securing a suitable mate.
0:06:41 > 0:06:45Dances like those of this Australian species
0:06:45 > 0:06:48can be complex and last several days.
0:06:50 > 0:06:53They help the couple synchronise their bodies
0:06:53 > 0:06:55so that the male's pouch is ready for the eggs.
0:06:58 > 0:07:02They also help to establish the couple's joint territory.
0:07:07 > 0:07:09Seahorses were thought to be monogamous,
0:07:09 > 0:07:13but we now know that some are only exclusive couples
0:07:13 > 0:07:15for the duration of the breeding season.
0:07:15 > 0:07:18The female must choose the right male
0:07:18 > 0:07:21because she's going to pass over her precious eggs to him.
0:07:23 > 0:07:26Female seahorses do not have a pouch,
0:07:26 > 0:07:29so a strong pair-bond with a male is very important,
0:07:29 > 0:07:31as he will care for her eggs.
0:07:31 > 0:07:35This is the honeymoon tank.
0:07:35 > 0:07:38Seahorses that have shown an attraction for each other
0:07:38 > 0:07:39in the courtship tank
0:07:39 > 0:07:40are removed as a couple
0:07:40 > 0:07:43and given their own private space.
0:07:43 > 0:07:46In the wild, each pair has its own territory
0:07:46 > 0:07:50and these smaller tanks make captive breeding more successful.
0:07:50 > 0:07:53Here, the pair can synchronise their courtship.
0:07:53 > 0:07:55Timing is crucial.
0:07:55 > 0:07:58The female's eggs must be fully developed
0:07:58 > 0:07:59at exactly the same time
0:07:59 > 0:08:02that the male's pouch is ready to receive them.
0:08:05 > 0:08:07Once the female's eggs are ready,
0:08:07 > 0:08:10she hydrates them with seawater.
0:08:10 > 0:08:13They must then be laid within 24 hours.
0:08:14 > 0:08:17She transfers her eggs to her partner
0:08:17 > 0:08:21by inserting her egg-laying tube, or ovipositor,
0:08:21 > 0:08:22into the male's pouch.
0:08:25 > 0:08:29Once pregnant, the male attaches himself to one spot
0:08:29 > 0:08:31and the female visits him every day.
0:08:33 > 0:08:37She checks to see when he'll be ready for her next batch of eggs.
0:08:39 > 0:08:43One theory suggests that because the male is incubating the eggs,
0:08:43 > 0:08:45the female has more time to feed
0:08:45 > 0:08:49and can put energy into making new eggs more quickly.
0:08:51 > 0:08:54Swapping roles may be a smart way
0:08:54 > 0:08:56to use their resources more efficiently.
0:09:01 > 0:09:05What goes on inside the pouch is still a mystery.
0:09:07 > 0:09:10The male may simply provide a closed incubator.
0:09:11 > 0:09:14Or the inner skin may develop extra blood vessels
0:09:14 > 0:09:17to give a more placenta-like connection.
0:09:17 > 0:09:18It's not clear.
0:09:19 > 0:09:21During pregnancy and birth
0:09:21 > 0:09:23the male's metabolism increases,
0:09:23 > 0:09:25but that's little wonder,
0:09:25 > 0:09:28for he may have up to 1,500 eggs in his pouch.
0:09:34 > 0:09:38The male seahorse gives birth to dozens of miniature babies,
0:09:38 > 0:09:40perfect in every detail.
0:09:40 > 0:09:44The free-swimming young are put into separate creche tanks
0:09:44 > 0:09:47where they can be fed and cared for.
0:09:47 > 0:09:50The parent seahorses in this biological hotel
0:09:50 > 0:09:52remain in the honeymoon suite
0:09:52 > 0:09:54ready to mate again.
0:09:55 > 0:09:58These are some of last year's youngsters
0:09:58 > 0:10:00and they've grown enormously.
0:10:00 > 0:10:03Next year, they'll be breeding themselves.
0:10:06 > 0:10:08Swapping the parental roles
0:10:08 > 0:10:10seems to work well for seahorses.
0:10:12 > 0:10:17In warm conditions, a male can give birth every 28-30 days.
0:10:17 > 0:10:21But of the thousands of fry produced each year, only a few survive.
0:10:22 > 0:10:25There is no safe creche in the open sea.
0:10:28 > 0:10:31To succeed, seahorse parents must work well together,
0:10:31 > 0:10:33yet in this partnership,
0:10:33 > 0:10:36the female seems to have the freedom
0:10:36 > 0:10:38to swim, feed, and patrol the territory,
0:10:38 > 0:10:42which is normally the prerogative of the male.
0:10:44 > 0:10:49So, is the male seahorse a slave to a gallivanting female?
0:10:49 > 0:10:52Well, latest research suggests not,
0:10:52 > 0:10:56and shows that some males may have more control over breeding
0:10:56 > 0:10:58than first thought.
0:10:58 > 0:11:02If small or poor-quality eggs are deposited into their pouches,
0:11:02 > 0:11:05some males will absorb them.
0:11:05 > 0:11:07Such males appear to be choosy
0:11:07 > 0:11:11about how they invest their time and energy.
0:11:11 > 0:11:15And some females, in entrusting their eggs to males,
0:11:15 > 0:11:16are being cheated.
0:11:16 > 0:11:19But the male seahorse can't be duped,
0:11:19 > 0:11:22as having a pouch means that he can always be certain
0:11:22 > 0:11:26that all the baby seahorses he gives birth to are his own.
0:11:28 > 0:11:31So male and female seahorses
0:11:31 > 0:11:33have swapped their roles.
0:11:33 > 0:11:36The male is the mother and he gives birth to the babies.
0:11:39 > 0:11:42Another animal with unusual parenting habits is the hyena.
0:11:42 > 0:11:47Here, it's the female that looks and behaves more like a male.
0:11:47 > 0:11:51Why have female hyenas becomes so masculine?
0:11:55 > 0:11:58These are African spotted hyenas,
0:11:58 > 0:12:01creatures that have an undeservedly bad reputation
0:12:01 > 0:12:04and a very strange biology.
0:12:04 > 0:12:07In the wild, they live in clans of up to 80 individuals
0:12:07 > 0:12:10and the females dominate the males.
0:12:10 > 0:12:12The females are big, aggressive,
0:12:12 > 0:12:16and look physically almost exactly like males.
0:12:18 > 0:12:22Unravelling why the female is like this has not been easy,
0:12:22 > 0:12:25as it's difficult to tell the difference between the sexes.
0:12:26 > 0:12:28The female's male appearance
0:12:28 > 0:12:32is made all the more convincing by her reproductive organs -
0:12:32 > 0:12:36they're external and very similar to a male's.
0:12:36 > 0:12:38HYENAS SQUEAK
0:12:38 > 0:12:40Understanding hyena biology
0:12:40 > 0:12:44has helped to explain the female's masculinity
0:12:44 > 0:12:48and the species' reputation as aggressive scavengers.
0:12:49 > 0:12:52But in the past, these strange traits
0:12:52 > 0:12:54gave hyenas a very bad image.
0:12:56 > 0:13:00In the first century, Pliny the Elder described the hyenas
0:13:00 > 0:13:02and did them a great disservice.
0:13:02 > 0:13:04This is what he wrote.
0:13:04 > 0:13:08"Hyenas are like a cross between a dog and a wolf.
0:13:08 > 0:13:11"They break everything with their teeth,
0:13:11 > 0:13:12"swallow it as a gulp
0:13:12 > 0:13:15"and masticate it in the belly.
0:13:15 > 0:13:20"They are believed to become male and female in alternate years.
0:13:20 > 0:13:23"They can imitate the human voice,
0:13:23 > 0:13:25"calling a shepherd by name
0:13:25 > 0:13:29"so that he comes outside, where they tear him to pieces.
0:13:29 > 0:13:33"Any animal that a hyena looks at three times
0:13:33 > 0:13:35"will be unable to move."
0:13:35 > 0:13:37That tainted image of hyenas
0:13:37 > 0:13:40was perpetuated for many years to come,
0:13:40 > 0:13:44and they were branded as evil, dangerous creatures.
0:13:46 > 0:13:48Hyenas are not, of course, evil,
0:13:48 > 0:13:52but their competitive nature and unusual eating habits
0:13:52 > 0:13:54make them appear fearsome.
0:13:55 > 0:13:57They're specialist feeders.
0:13:57 > 0:13:59They crush, eat and digest bones
0:13:59 > 0:14:01that other creatures can't tackle
0:14:01 > 0:14:03and so leave behind.
0:14:04 > 0:14:07And this diet has a significant effect
0:14:07 > 0:14:11on the female's appearance and her family relationships,
0:14:11 > 0:14:13especially those with her cubs.
0:14:15 > 0:14:17In the early 19th century,
0:14:17 > 0:14:19an unusual discovery in Britain
0:14:19 > 0:14:23excited one man to look more closely at the hyena's diet.
0:14:25 > 0:14:29In 1822, a rather eccentric but very eminent geologist
0:14:29 > 0:14:31called William Buckland
0:14:31 > 0:14:33made a significant discovery
0:14:33 > 0:14:36that was to further the modern understanding of hyenas.
0:14:38 > 0:14:40Quarry workers in Kirkdale, Yorkshire,
0:14:40 > 0:14:45had come across a cave that contained a large number of bones.
0:14:45 > 0:14:47Buckland was very excited
0:14:47 > 0:14:51and rushed to see the remains before they were disturbed any further.
0:14:51 > 0:14:53And he found that mud deposits in the cave
0:14:53 > 0:14:57had preserved the bones of over 22 different species of animals,
0:14:57 > 0:15:01including tiger, bear, wolf, elephant
0:15:01 > 0:15:04and, significantly, hyenas,
0:15:04 > 0:15:07which Buckland described as "littering the cave
0:15:07 > 0:15:09"like the bones in a dog kennel".
0:15:10 > 0:15:15This is one of the actual hyena jaws that Buckland found.
0:15:15 > 0:15:18It belonged to a young but ancient hyena.
0:15:18 > 0:15:23There were also a lot of these on the cave floor.
0:15:23 > 0:15:25They are coprolites,
0:15:25 > 0:15:29or fossilised faeces from hyenas.
0:15:29 > 0:15:30They contain bone fragments
0:15:30 > 0:15:33that have passed through the hyena's digestive tract
0:15:33 > 0:15:37and so showed that they were successful bone-crushers.
0:15:40 > 0:15:42Buckland's discovery of so many bones
0:15:42 > 0:15:45in what he believed to be a hyena's den
0:15:45 > 0:15:48indicated that they were very successful hunters.
0:15:48 > 0:15:50Contrary to popular belief,
0:15:50 > 0:15:52they scavenge very little
0:15:52 > 0:15:55and kill over 80% of their own food.
0:15:58 > 0:16:02A lone hyena can easily kill a wildebeest or a topi,
0:16:02 > 0:16:05and with teamwork they will tackle bigger animals,
0:16:05 > 0:16:08like zebra and giraffe.
0:16:08 > 0:16:10They do scavenge as well,
0:16:10 > 0:16:13but it's more usual for lions to steal from hyenas
0:16:13 > 0:16:15rather than the other way around.
0:16:16 > 0:16:20Female hyenas have become big and strong
0:16:20 > 0:16:23and compete for food with other members of their clan.
0:16:23 > 0:16:24Nothing goes to waste -
0:16:24 > 0:16:27they can eat even the thickest of bones.
0:16:28 > 0:16:32Buckland was fascinated by the marks on the bones from the cave,
0:16:32 > 0:16:35but found it hard to believe that hyenas had made them.
0:16:35 > 0:16:38He wanted to be sure of his findings
0:16:38 > 0:16:40and understand how their jaws,
0:16:40 > 0:16:43with their strange, massive teeth, actually worked.
0:16:45 > 0:16:48Hyenas are African or Asiatic animals,
0:16:48 > 0:16:53so Buckland's discovery of hyena bones in an English cave
0:16:53 > 0:16:55was strange, to put it mildly.
0:16:55 > 0:16:57As a man of science, he wanted to confirm
0:16:57 > 0:17:02that the skull he had collected from Kirkdale was definitely from a hyena
0:17:02 > 0:17:06and that it had made the marks on the many fractured bones.
0:17:07 > 0:17:08To try and prove his case,
0:17:08 > 0:17:12he asked a friend, William Burchill, an African traveller,
0:17:12 > 0:17:16to send a young hyena back to England from the Cape.
0:17:16 > 0:17:17He planned to kill it
0:17:17 > 0:17:21and compare its skull and teeth with the specimens in the cave.
0:17:21 > 0:17:25The young hyena that arrived at the docks was already tame
0:17:25 > 0:17:28and had become a great favourite with the sailors,
0:17:28 > 0:17:29who christened him "Billy".
0:17:29 > 0:17:33Billy became quite a celebrity
0:17:33 > 0:17:35and was as tame as a pet dog.
0:17:35 > 0:17:38No-one could bring themselves to sacrifice him
0:17:38 > 0:17:40for the sake of science.
0:17:40 > 0:17:44Instead, a search of British museums produced a hyena skull
0:17:44 > 0:17:46and Billy's life was spared.
0:17:47 > 0:17:51Buckland was then able to compare the new and old skulls,
0:17:51 > 0:17:53and they matched.
0:17:53 > 0:17:56Billy also helped to clarify the fractures on the bones.
0:17:56 > 0:18:00He was fed ox bones, this was one.
0:18:00 > 0:18:05And Buckland compared it with one that was found in the cave,
0:18:05 > 0:18:07and they closely match.
0:18:08 > 0:18:11This ability to crack massive bones
0:18:11 > 0:18:15explains why female hyenas look like males.
0:18:15 > 0:18:17It's also tied up intricately
0:18:17 > 0:18:20with the relationship they have with their cubs.
0:18:21 > 0:18:23Cubs are born underground
0:18:23 > 0:18:25and are fed on their mother's rich milk.
0:18:28 > 0:18:29At about three months of age,
0:18:29 > 0:18:31they emerge from the den
0:18:31 > 0:18:34and continue to suckle for almost another two years.
0:18:34 > 0:18:37Their mother helps feed the youngsters,
0:18:37 > 0:18:40as they can't yet crack and crush bones for themselves.
0:18:40 > 0:18:42Even at almost a year in age,
0:18:42 > 0:18:46when they're big enough to join the kill,
0:18:46 > 0:18:49their teeth and jaws are still not sufficiently developed
0:18:49 > 0:18:50to tackle big bones.
0:18:51 > 0:18:53The skull of a young hyena
0:18:53 > 0:18:55is quite different from that of an adult.
0:18:55 > 0:18:58It's got a flat top, narrow cheeks
0:18:58 > 0:19:02and relatively small teeth.
0:19:02 > 0:19:03An animal with a skull like this
0:19:03 > 0:19:07would not be able to crush and eat big bones.
0:19:07 > 0:19:09It takes almost three years
0:19:09 > 0:19:11for a young hyena's skull to grow to full size
0:19:11 > 0:19:13and reach mechanical maturity.
0:19:13 > 0:19:15And this is the result.
0:19:15 > 0:19:19This skull has a large, vaulted forehead
0:19:19 > 0:19:20that dissipates biting stress,
0:19:20 > 0:19:22carrying it away from the face.
0:19:22 > 0:19:25It's also got wide arches at its sides
0:19:25 > 0:19:27for the attachment of powerful jaw muscles,
0:19:27 > 0:19:30and robust premolars
0:19:30 > 0:19:33that have specialised crack-resistant enamel.
0:19:33 > 0:19:34Jaws like these
0:19:34 > 0:19:38can crack the dense bones of zebra and even giraffe.
0:19:41 > 0:19:45Developing this substantial specialised eating equipment
0:19:45 > 0:19:46takes time.
0:19:46 > 0:19:48So it may be several years
0:19:48 > 0:19:51before a young hyena can feed independently.
0:19:51 > 0:19:53This puts pressure on their mothers
0:19:53 > 0:19:56to become dominant and aggressive.
0:19:56 > 0:20:01They need to fight to get enough food for their cubs.
0:20:01 > 0:20:03The female's status in the clan's hierarchy
0:20:03 > 0:20:06will directly affect the survival of her young.
0:20:06 > 0:20:09The biggest, oldest, most established females
0:20:09 > 0:20:13are the most dominant and take a bigger share of the kill.
0:20:13 > 0:20:15So food and the need to fight for it
0:20:15 > 0:20:19has made females look and behave like aggressive males.
0:20:19 > 0:20:23But it has also had a strange side effect.
0:20:23 > 0:20:28Female hyenas have large amounts of the male hormone testosterone
0:20:28 > 0:20:32and, consequently, develop male-like reproductive organs.
0:20:32 > 0:20:34This can be a problem.
0:20:34 > 0:20:36Having a long, thin birth canal
0:20:36 > 0:20:38makes mating very difficult,
0:20:38 > 0:20:42and both mothers and cubs sometimes die during birth.
0:20:43 > 0:20:46The female's strange gender swap
0:20:46 > 0:20:49is one of the most unusual in the animal kingdom.
0:20:49 > 0:20:52And new science has now made sense of the old clues
0:20:52 > 0:20:54and solved this mystery.
0:20:55 > 0:20:58Hyenas are very intriguing animals.
0:20:58 > 0:21:02William Buckland's early observations of their bones
0:21:02 > 0:21:03in his hyena experiments
0:21:03 > 0:21:05started a study of these creatures
0:21:05 > 0:21:09that was to reveal their fascinating biology.
0:21:09 > 0:21:13Hyenas may have a frightening reputation,
0:21:13 > 0:21:16but their odd characteristics all have a reason.
0:21:16 > 0:21:19The story of their aggression and bizarre bodies
0:21:19 > 0:21:24is intimately tied up with their food and the survival of their cubs.
0:21:24 > 0:21:27They've evolved a perfectly formed bone-breaking jaw,
0:21:27 > 0:21:29but the time it takes to grow
0:21:29 > 0:21:32has resulted in one of the most unusual but dedicated mothers
0:21:32 > 0:21:34in the animal kingdom.
0:21:34 > 0:21:37So, to become the best parents,
0:21:37 > 0:21:40female hyenas have become more male,
0:21:40 > 0:21:43and male seahorses more motherly.