"The frozen seas are worlds unto themselves" "Beneath their ceiling of ice they have an eerie, almost magical stillness, cut off from the storms that rage above" "In the winter, the feeble slanting rays of the sun bring little warmth and the temperature seldom rises above minus fifty degrees Centigrade" "For much of the year, it is dark and cripplingly cold" "Yet, there is life here - at both ends of the earth... the Arctic and the Antarctic" "For most animals, whether they live in or out of water, the winters, when much of the sea is frozen, bring the greatest challenge" "The Northern hemisphere" "It's February, and as the Earth tilts on its axis, the sun's rays creep slowly northwards and the Arctic emerges from its harsh winter" "The Arctic is a frozen ocean surrounded by continents" " and when the surface of the sea between the continents freezes from shore to shore, land predators walk out onto it - to hunt" "It's early March and the sea is still covered with ice" "But there are patches of open water - polynyas - that never freeze over" "Here, where tidal currents are squeezed between islands, the water movement is so strong that ice cannot form" "Walruses spend the winter in polynyas" "Here they have permanent access to the air, but they can also retreat to the sea to shelter to hunt" "Other sea mammals overwinter in the polynyas as well" " in this one, a young bowhead whale" "Here the current is really fast and the shifting ice is dangerous" "This whale became trapped when ice encircled it last autumn" "There is no food here, but a whale must breathe and the only place that it can do so, for miles around, is in this tiny hole" "It's living entirely on its reserves of fat, but now they are dangerously low" "It will be some months yet, before it can escape" "Elsewhere, other whales have also been trapped" "These are belugas" "Their tiny hole in the ice has been kept open, not by currents but by the belugas' continuous movements as they rise to breathe" "Open water is now some twenty miles away" "It will be two months yet before the ice melts" "The belugas are extremely thin - and most of them are horribly scarred" "But their wounds were not inflicted by the ice" "A whale would be a huge prize for any meat-eating hunter and these belugas, trapped by the ice, are within reach of polar bears" "Well aware of the danger, the belugas stay submerged as long as they can" "But they can only hold their breath for about twenty minutes" "Catching a four-metre long whale that weighs one ton is no easy task" " even if that whale is weakened by starvation" "But a beluga is well worth waiting for" "Day by day, as the hole gets bigger, it becomes increasingly difficult for the bear to land a whale" "Keeping its fur in good condition and free from salt is important for warmth" " and the bear uses snow like blotting paper" "These belugas have been attacked by many bears over the last six months" " and some have been caught" "It may have taken a long time and a lot of patience, but a catch, when it's made, brings abundant rewards of energy rich blubber" "Gulls rely on bear-kills at this time of the year" " and the colour of blood staining the ice attracts them from a long way away" "The remaining belugas still have a long wait before they are released from their prison and the threat of slaughter" "Arctic foxes also rely on the polar bears to hunt on their behalf" "They're the jackals of the north and scavenge from bear kills whenever they can" "In winter and early spring, they are wholly dependent on bears" "Only in the summer, when the sea ice melts, will they regularly catch prey for themselves" "They are not strong enough to tackle adult seals, but can certainly take new-born pups or birds" "This canny individual is going to bury its prize" " it may need it during the uncertain times ahead" "The presence of bears affects the behaviour of almost all the animals here - big and small" "The bears tend to avoid the fringe of fragmented ice bordering open water" " where travelling can be very laborious" "But that very fact makes this area - the pack ice" " a particularly good place for seals to pup" "Harp seals breed here" "Their pups are born with white coats which camouflage them very effectively against the snow" "Harp seals have a very short nursing period" " a necessity on this unstable pack ice" "But suckling is intensive" "The pup feeds for just twelve days on milk that is 45 percent fat" "Then it is abandoned and has to fend for itself" "A male hooded seal" "This impressive nasal display is used to warn away other males and to win a mate Hooded seals also breed on pack ice" "Their pups suckle for only four days" " the shortest nursing period known for any mammal" " and all because the threat of polar bears caused them to breed on the unstable ice" "Another Arctic seal opts for the solid ice further north" "Because it's easy for bears to hunt here, these ringed seals must be particularly vigilant and have to hide their pups" "Ringed seals are comparatively small, so they can give birth to their pups in little caves, or lairs, under the snow" "Here, a pup is out of sight and protected from bad weather" "In late March and into April, female bears emerge from winter dens with their new cubs" "The mother has not eaten for at least five months and she's hungry - very hungry" "If she doesn't succeed in catching a regular supply of seals, her milk will fail and her cub will die" "Bears have an extraordinarily sensitive sense of smell and can detect seal pups hidden in the snow from two kilometres away" "But a female ringed seal uses several lairs and a bear will almost certainly have to break into a number before it finds one that is occupied" "This is a crucial time for the cub" "By watching its mother hunt and by copying her actions, it's beginning to acquire the rudiments of its own hunting skills" "Play is also important for developing muscles and improving co-ordination" "As the days go by, the sun rises higher and remains above the horizon even at night" "The female bear continues to hunt until her cub is too tired and can't keep up" "She has smelt something" "The pup escapes through a hole in its lair that leads to the sea below" "Only one in twenty hunts is successful" "But this mother must find a seal pup soon if her cub is not to starve to death" "As spring turns into summer, the sun's heat begins to melt the sea ice" "Now the ocean is accessible and the Arctic's summer visitors return" "Migrating birds arrive from the south to nest and feed on the seafood that's now within their reach" "Brunnich's guillemots are the northern equivalent of penguins" "But they have retained the power of flight, for they need to reach cliff ledges where their nests will be safe from predatory bears and foxes" "Nonetheless, they are as at home in the water as in the air" "They dive down to a depth of fifty metres or more to catch fish" "In June, the ice begins to fracture" "The cracks - or leads - form useful corridors of open water for air-breathing animals" "Belugas, migrating to their feeding grounds, use these leads to penetrate the ice-covered seas and reach areas where their preferred food," "Arctic cod, has spent the winter" "The males regularly dive to about five hundred metres to find fish" "The females and young, which have smaller lungs, only go to about 350" "In late June and July, narwhals arrive" "The females, who usually lack tusks, come first with their new calves" "The males follow a little later" "They also move up the leads in search of fresh feeding grounds" "Bowheads." "Up to 18 metres long and weighing a hundred tonnes, these are the only large whales that stay in the Arctic all year round" "They are not after fish They are seeking smaller prey" "Despite having the largest mouths in the animal kingdom, the size of a small garage, they eat tiny crustaceans" " copepods - straining them from the water with the four-metre long strips of baleen that hang from their upper jaws" "In the summer they store enough energy to last them through the following winter, when food will be less abundant" "As the ice melts away, the polar bears are forced to head for land" "They are excellent swimmers and can cover a hundred miles of continuous open water if need be" "Off east Greenland, there is little ice left by August, so walruses haul out to rest on land" "At this time of the year they are moulting, getting rid of their old scarred and parasite-ridden skin" "A bathe in the cold water brings some relief from the itching" "But even there, the odd scratch is irresistible" "They make daily excursions out to deeper water" "Down at 20 metres, they root around in the sediment, using their sensitive bristles to search out soft-shelled clams" "Once they find a clam, they suck its flesh from the shell with their powerful, muscular mouths" "Walruses can feed for about five minutes at this depth before they have to return to the surface to breathe" "Elsewhere in the Arctic, belugas are gathering in their thousands" "They congregate in just a few large estuaries" "Belugas of all ages and sizes come here" "There are even young calves" "Some are so young - born only a week or so ago - that they need help" "They swim on their mothers' backs so that they can breathe more easily" "As the tide moves up the estuary, the belugas follow, swimming into surprisingly shallow water" "Like walruses, they also need to moult" "A combination of warm fresh water and vigorous rubbing against the gravel does the trick" "They remain here for days, or even weeks, so it's likely that socialising is also important to them" "After moulting, they head back out to sea to feed" "It's now autumn and the sea begins to freeze over, once again" "Thin sheets of ice form at the surface and pile up, layer upon layer, gradually creating an impenetrable barrier" "By late November, the Arctic Ocean is sealed once again by ice" "The lights of the aurora play in the winter sky" "At the other end of the planet - in the Antarctic" " there is the southern aurora" "Antarctica is now emerging from winter" "This is the coldest, windiest place in the world" "Temperatures are still hovering at a numbing minus 50 and the returning sun has virtually no warmth" "Very few animals can survive such extreme conditions" "But emperor penguins can" "Standing on the frozen sea, they endure the full force of the Antarctic storms" "Only by huddling together can they can survive the appalling winter months" "They take it in turns to bear the brunt of the gales" "They can only live here at all because Antarctica is surrounded by the Great Southern Ocean and no land predators have been able to reach it," "so unlike Arctic animals, they're not threatened by polar bears" "The sea is still frozen" "But one seal, nonetheless, manages to stay here, even throughout the winter  the Weddell seal" "Underwater, it's protected from the storms above" "But it must have access to the air all year - in order to breathe" "And they keep their breathing holes open - with their teeth" "Only by continually scraping away at the ice can they maintain access to the air" "But that means that their teeth get badly worn down" "Then they can no longer hunt and eat effectively" "Weddell seals die young" "The continent of Antarctica is so isolated and so high - almost five thousand metres in places" " that it's considerably colder than the Arctic" "Ice slides slowly down from its centre towards its rim in immense glaciers" "During winter, the continent effectively doubles in size as the sea freezes over" "Ice forms around its shores and extends outwards for hundreds of miles around the entire land mass" "Under the sea-ice live small shrimp-like creatures - krill" "They have been here all winter" "During these dark months, they feed by scraping algae from the ice" "Remarkably, they also shrink in size and revert to their juvenile form to save energy" "As the temperature rises in spring, the ice begins to melt and little air bubbles trapped within it are released" "Microscopic algae grow around these bubbles and the krill now graze on them, gathering them up with their beating legs" "As the sun's rays grow stronger and penetrate deeper into the water, floating algae begin to flourish and krill leave the dwindling ice and gather in swarms as they harvest this new crop" "Far to the north, beyond the blanket of sea ice, chinstrap penguins have been overwintering in the open ocean" "The occasional iceberg gives them the chance of a rest" " if they can get onto it" "But at this time of year, where they really want to be is on land" "It's just getting there that's tricky" "It's spring and the penguins are returning to breed" "Their need to get ashore is now urgent and imperative" "Doing so is all a matter of timing - and picking the right wave" "But their journey has only just begun" "Most of them will have to walk many miles before they can find a nest site" "This is Zavodovski Island, which has the largest penguin colony in the world" "About two million chinstraps breed here" " and they come to this island for a good reason" "It's an active volcano" "The heat from the crater and the fumeroles keeps the slopes free from the ice and snow, allowing these chinstraps to start breeding earlier than those further south" "But then again, living on an active volcano is not without its risks" "Unlike the emperors, these penguins are able to lay their eggs on the bare ground" "Little wonder so many of them choose to brave the mountainous waves in order to get here" "Further south, near the continent, the blanket of sea ice is beginning to break up" "Icebergs are gigantic fragments of ice that have broken away into the sea from the front of glaciers" "During the winter they were frozen into the sea ice, but now they are set adrift once more" "As the bergs break up, they form brash ice" "It litters the backwaters of the Antarctic Peninsula" "Minke whales make their way into these placid waters in summer" "This is the most abundant whale in the Southern Ocean" "Minkes are one of the smallest of all the baleen whales" "And like all others, they come here to feed" "The majestic humpback whales are also summer visitors" "They have travelled thousands of miles from their winter breeding grounds in the tropics to gather the food that becomes available here in summer" "In just four months, they accumulate enough fat to provide them with energy for the whole of the rest of the year" "All these animals have come here in search of one thing - the krill" "Krill is the mainstay of the Antarctic food web" "It occurs in phenomenal quantity" " billions of individuals in a single swarm, and swarms can stretch for miles" "Fur seals also collect this rich superabundant food" "Krill swarms are very patchy, but once found, feeding is easy" "Humpbacks engulf hundreds of thousands of them in a single gargantuan mouthful" "When the going is good, the whales feed continuously, each eating up to two tonnes of krill in twenty-four hours" "Further south, near the continent, the sea ice is still sound" "Here, where the ice remains for most of the summer, emperor penguins make their home" "These have been feeding out at sea and are now ready to make their way back to the colony to feed their chicks" "Instead of heading straight for the ice edge, the penguins hesitate some distance from it" "They're nervous" "They dive down and investigate the ice edge" "And for good reason Leopard seals patrol this border" "Leopard seals are the Antarctic's equivalent of polar bears" "They are the top predators" "But they hunt most successfully in the water, so by and large, the animals they prey on are safer out on the ice" "They have a lazy grace that belies their ferocious nature" "Confident that the coast is clear, the emperor penguins head for the ice" "But they certainly don't linger" "Now they have a long walk back to the colony" "Emperor penguin colonies are some way back from the ice edge" "In winter they may be as much as a hundred miles from it, but as the summer progresses and the ice melts, the edge comes ever closer to the colony" "So by the time the chicks are fledged and ready to take their first swim, the water is close by" "This colony is in the lee of a headland and that prevents the ice from being broken up by ocean currents" "The returning adults are so full of food that they can barely walk" "But with no predator to threaten them now, they can take their time" "Somehow, in this melee of sixty thousand or so penguins, a parent has to find its chick" "It returns to the place where it last left its chick in the hope that it might still be close by" "But chicks tend to wander, so the adult has to call to it" "The chick responds - and they slowly home in on one another" "The plaintive entreaties of the chick stimulates the adult to regurgitate a mouthful of fish" "With the return of one parent, the other is free to go to sea to feed for itself" "Aware of the leopard seal's presence, the penguins press together at the ice edge, unwilling to be the first to risk diving in" "Occasionally, the seal comes out onto the ice and attempts to grab one" "But its most successful strategy by far is to lie in wait" "It hides behind a corner of ice" "The emperors gain confidence and make a dash for it" "The first wave of penguins escape" "Once in open water, they will be safe" "But the seal is alerted by the noise, and through the mass of bubbles, it makes its attack" "Almost invariably, it makes a kill" "Encouraged by the absence of the seal, the remaining penguins make a break for the open sea" "In time, their chicks will fledge" "And when the Antarctic autumn is near its end, these adults will walk across the newly formed ice, to endure yet another winter on the frozen sea"