The Arctic and Antarctic are the coldest places on earth. The Arctic is a partly frozen ocean, hemmed in by large expanses of windswept tundra; the Antarctic is an ice- covered continent, surrounded by the world’s stormiest seas.
They are similar to each other — and unlike any other habitat — in that they have 24-hour daylight in summer and perpetual darkness in winter, but they are physically different in ways that have important effects on animal life. In the Arctic, many animals live on land; in the Antarctic, animal life is based almost entirely in the ocean.
ARCTIC AND TUNDRA
Covering about 12 million square km (4.6 million square miles) the Arctic Ocean is both the smallest and the shallowest ocean in the world. For several months in summer permanent daylight produces a constant supply of energy which is harnessed by vast quantities of planktonic algae. These form the first link in the Arctic Oceans food chain, which ultimately nourishes animals as large as whales and polar bears. Sea ice — or the lack of it — is a major factor in determining where large mammals live, especially during winter. When the surface area of the ice is at its greatest.
Polar bears and Arctic foxes can traverse the ice to find food, but seals and some other marine mammals must maintain breathing holes to survive.
Despite the icy conditions sea life is plentiful in the Arctic because cold water is rich in oxygen and the sea-bed sediment is rich in nutrients. On land, though intense winter cold means that trees cannot survive.
The result is tundra — an open often featureless landscape scraped smooth by glaciers during the last ice age. Today, Arctic glaciers are restricted mainly to mountains and to the ice-cap that covers Greenland but large areas of tundra remain permanently frozen underground.
This frozen zone – the permafrost layer — prevents spring melt-water from draining away creating waterlogged landscapes in a region where rainfall, or snow is paradoxically very low. In late spring and early summer, tundra plants grow, and flower, very rapidly. Geese and other migratory birds arrive to breed, and vast numbers of mosquitoes emerge from tundra pools. The migrants’ departure, when the short summer draws to a close. marks the end of another biological year.
Unlike the Arctic, mainland Antarctica is isolated from the rest of the world. It is covered with ice, up-to to 4,000m (13,200ft) thick which continues out to sea forming large ice shelves. On the Antarctic Peninsula — a finger of land pointing towards South America — summer temperatures rise to a few degrees above freezing point, but in the rest of the continent, average temperatures are below freezing all year round.
Algae and lichens grow on bare rocks in many parts of the Antarctic coastline, but the Antarctic Peninsula is the only part of the continent where terrestrial plants can survive. This is also the only place that has a significant range of terrestrial animals, although these are chiefly springtails, mites, and nematode worms-few of which are over 5mm (1/8 in) long.
The rest of Antarctica’s land—based animal life consists of species that feed in the sea and come ashore to breed, such as penguins, or those that scavenge food at these animals’ breeding grounds, such as skuas. With the exception of emperor penguins, vertebrates desert the ice at the end of summer to spend the winter at sea.
The Southern Ocean, which surrounds Antarctica, is one of the most biologically productive seas in the World. Although species numbers are relatively low, population sizes are often enormous because the non-stop summer daylight generates a vast food supply.
Krill — small crustaceans that form the diet of seals and whales are especially prolific: some of their swarms are estimated to weigh in excess of 10 million tonnes and are large enough to be seen by satellites in space. Although the Southern Ocean is always cold, it maintains a minimum temperature of about -18°C (28.8‘°F); below this, sea water freezes. As a result, the ocean is quite warm compared with Antarctica itself.
LIFE ON POLAR REGION
Although they live at opposite ends of the earth, the animals that inhabit the Arctic and Antarctic share many adaptations. Resilience to extreme cold is first and foremost among these, but almost as important is the ability to cope with a highly seasonal food supply.
For some animals, winter is a good time for catching food but, for most, hunger and cold make the long winter months a critical time of year. Such testing conditions mean that, in comparison with other parts of the world, the poles are inhabited by very few animal species. However, those that do thrive can be extraordinarily numerous.
COPING WITH COLD
Warm-blooded animals have to maintain a constant body temperature, which means that combating heat loss is a major priority in the polar environment, Cold-blooded animals can function with a fluctuating body temperature, but even they have limits — in sub-zero conditions, they can freeze solid.
Fish are particularly at risk, of freezing, for while their body fluids normally freeze at about -0.8°C (30.6° F) polar sea water is often slightly colder still. To help prevent freezing. the blood of many cold-blooded species contains proteins that lower its normal freezing point.
Some insects can survive at -45°C (-49°F) without any ice forming in their bodies, Since mammals and birds cannot afford to let their internal temperature fall even slightly they need insulation to keep warm.
Fur and feathers are among the finest insulating materials that nature has devised, but many polar animals, such as whales, seals, and penguins, have additional insulation in the form of blubber — a layer of yellowish fat that is laid down under the skin. Blubber, which can be up to 30 cm (12 in) thick, is such an effective insulator that all these animals feel cold to the touch even when their internal body temperature is above 38°C (100.5 °F).
Blubber is particularly useful in the sea because water conducts heat away from the body 25 times faster than air. It also has another valuable function: because fat contains lots of energy, blubber can be used as a food reserve when supplies are low.
SURVIVING UNDER ICE
Whales and seals face problems during the long polar winter because sea ice restricts their access to air. They dive under the ice to feed, but they must then surface to breathe.
Some species avoid the problem by moving to lower latitudes. Those that remain behind survive either by maintaining breathing holes or by congregating in polynias- areas where the wind and currents keep the water ice-free. Seals start making breathing holes when the ice is thin, rasping away at it with their teeth.
As the depth of the ice increases with the progress of Winter, they continue to visit and work on their holes to ensure they remain clear. The Weddell seal, which lives further south than any other species, spends so much time keeping its breathing holes open that its teeth develop distinctive patterns of wear, and by late winter, its breathing holes can be 2m (6 1/2 ft) deep.
The seals have to find their holes in almost complete darkness because during the Antarctic winter the sun stays below the horizon for weeks. Whales rarely make breathing holes. instead they head for polynias, where they can come up for air in open water.
This less laborious strategy means they are not tied to one place but it does have its dangers: groups of whales can become ensnared in shrinking polynias, unable to reach the next stretch of open water. There are records of narwhals — the world’s most northerly whales — being trapped in their hundreds, making them easy targets tor hunters.
Near the poles, 24-hour daylight in summer creates the ideal conditions for rapid plant growth. This short-lived but profuse supply of food has a dramatic effect on tundra life, attracting vast numbers of migrants.
Geese come to crop the plants with their beaks, warders arrive to feed on worms and insects that live in swampy ground, while terns find the food both on the tundra and in water close to the shore.
This annual iflux of visitors is mirrored in the sea. Many of the world’s baleen whales head towards polar waters during summer to make use of the annual upsurge in planktonic life. However, unlike migratory birds, these huge mammals do not breed at high latitudes.
Instead, they put on weight and then return to warmer waters to give birth. During the breeding season. they often do not feed at all.
In the treeless Arctic tundra camouflage is one of the most effective ways booth of avoiding attack and of making an attack unseen. The summer and winter landscapes look so different that many tundra animals change their camouflage twice a year.
The Arctic fox is a classic example: its summer coat is usually brownish grey, but in early autumn it turns white: in spring, the process is reversed so that the fox blends with the rabidly thawing tundra, in some parts of the far north — particularly Western Alaska and northern Greenland — Arctic foxes develop a bluish winter coat instead.
Some researchers have suggested that this blue coat is an adaptation to coastal landscapes, where there ls less winter snow, but as these foxes have been widely introduced by fur farmers, the theory is difficult to prove.
The stoat, a member of the weasel family, changes coat colour in a similar way, as do ptarmigans and many tundra birds. Some, such as the snowy owl, keep their white plumage all year round, which suggests that good camouflage is most important in winter and less so in summer when food is easier to find.
Although the food supply in polar seas slowly falls in autumn, there is still a reasonable amount for animals to eat. On land, life is not so easy. The growth of tundra plants comes to a complete halt and, to make matters more difficult the plants them selves are often covered by deep snow.
For herbivores, this lack of accessibility is a major problem at a critical time of year. In the Arctic tundra, plant-eating animals reach plants in one of 2 different ways.
Reindeer (caribou) and musk oxen use their hooves to clear away the snow to reveal the lichens and dwarf willows underneath. Lemmings turn the snow to their advantage by burrowing in it. The snow protects them from predators and from the weather outside: no matter how cold or windy it is on the surface, the lemmings enjoy a benign micro-climate that allows them to feed all year.
In Antarctica, there are very few terrestrial plants, which means that almost no animals stay active in winter on food gathered from land. With so much ice, even food from the sea can be difficult to reach. Male emperor penguins guarding their eggs, do not even attempt to find it: huddling on the ice through the long night of winter, they go without food until spring.