Weather extremes: How to keep warm in winter

As sub-zero temperatures grip Britain, the challenge for most people is keeping out the cold. Jonathan Brown and Michael McCarthy talk to the animals to find out how it's done

Togetherness

Look outside this morning: here comes the real freeze, with snow across half the country and icy air across the rest. But before you give way to that involuntary shiver, remember there are lots of natural ways in which warm-blooded animals can defy low temperatures in winter. Our fellow creatures manage very well without two pairs of socks, thermal underwear or a glass of whisky and one way of keeping warm is to huddle.

We are warm-blooded creatures and our bodies produce heat: when we get together this is naturally multiplied, sometimes uncomfortably so (especially on the Tube). Humans tend not to huddle together for psycho-social reasons: most of us like to keep our private space inviolate. But for some creatures it is necessary.

When birds roost in large flocks, they are not only providing a common defence against predators, but also sharing their warmth on cold nights, especially the smaller species. One bird, the emperor penguin, is perhaps the most remarkable example of how togetherness can be used to provide insulation. Penguins breed exclusively in the cold currents of the southern ocean. They are accustomed to low temperatures; and the emperor, which actually breeds on the icy Antarctic land mass, is the best insulated of all. Under its skin is a thick layer of blubber. Then there is a layer of downy feathers that trap air and finally a layer of waterproof feathers that keep out the icy sea water. This means it has no problem at all with temperatures down to, say, -10C. But when it goes inland to breed, the emperor penguin encounters the lowest temperatures regularly endured by any living thing, with blizzards in the Antarctic winter taking the thermometer down to as much as 70 below. The birds cope by forming a crowd. Those on the outside get coldest quickest, so they tramp round and round in a circle to keep warm, until it is their turn to move to the centre of the huddle and share the warmth, while other birds take their places and begin the circular shuffle themselves.

Fur

Temperatures in the Arctic plunge to -35C in winter. But that doesn't worry the polar bear, Ursus maritimus, which spends the most productive part of its life cycle adrift on giant ice floes.

The most northerly species of bear can handle such extreme conditions because of its fur. The top layer of guard hairs is made of long, oily, hollow, quill-like follicles that help the 600kg mammal stay afloat during its regular dips into the freezing sea. Creamy-white to blend in with the frozen surroundings, the outer coat is also water repellent, allowing the bear to shake itself dry, in the same way that a dog does after swimming. It also prevents the inner layer of fur from getting wet. This dense, soft undercoat acts like a warm fleece. Air pockets between the hairs trap heat and aid buoyancy. Another of the polar bear's tricks is to allow itself to be covered in snow when it is hunting or asleep, dramatically reducing the wind chill it experiences.

Its greatest enemy - apart from the humans that hunt it for meat and fur - is warmth. During the icy season, the polar bear's main food source, the ringed seal, is readily at hand, forced to surface at ice holes where the largest land carnivore spends many hours lying motionless in wait. It is during the summer months - when the ice recedes - that the polar bear becomes vulnerable. Because of its dependence on cold, it is particularly at risk from global warming. Climate change is already being observed in the Arctic and as the icy season becomes shorter, the polar bear is faced with longer periods without food. Its body condition deteriorates and reproduction rates drop. Already, the polar bear has virtually disappeared from the most southern parts of its range.

Blubber

Mammals that spend most or all of their time in the water, such as whales, seals or dolphins, have developed a highly specialised form of fat to insulate them. For those that must dive to capture their prey, fur is an inadequate protector, as pockets of air trapped by the thick hairs are squeezed out under extreme water pressure. And deep down in the waters of the Arctic or Antarctic oceans the temperature can drop to a truly inhospitable -40C.

Blubber is a thick layer of fat that encases the body and can make up half of total body weight. It contains narrow blood vessels and, in addition to maintaining temperature, it provides a vital energy store for the many sea animals that travel large distances to breed and feed. Blubber also helps streamline the bodies of fast-swimming creatures, helping to lower the amount of energy they need to propel themselves through the water as well as making them more buoyant. The colder the waters the animal inhabits, the thicker the layer of blubber. The fat-layer is acquired from the mother's milk. A baby grey whale, for example, may consume up to 30 gallons a day of milk which has the consistency of soft margarine.

It was the hunt for blubber that drove much of the whaling trade for centuries and resulted in the calamitous destruction of these giants of the seas. Whalers would render down cetacean blubber in giant cauldrons to make oil. Inuit peoples have long valued blubber as an energy-rich food source. Seals or pinnipeds share their time between the water and the land, so most of the 33 species have a thick layer of fur as well as blubber to give them an additional layer of warmth when ashore.

Getting away from it all

Another way of dealing with the cold is to get away from it, and the creatures best able to do this, of course, are the most mobile ones: birds. Bird migration, one of the wonders of the natural world, takes small, warm-blooded creatures hundreds or even thousands of miles away from winter weather towards warmer climates.

Sometimes you hardly notice: chiffchaffs, whose returning two-note song is one of the first sounds of the spring, slip away unseen towards Morocco. But swallows are very visible as they collect in large parties on telephone wires every autumn before setting out for southern Africa, where they will find congenial temperatures, and even more important, a continuing source of insect food.

Migration also works in reverse: some birds come from the cold north to spend their winters in warmer Britain. These include two Scandinavian thrushes, the redwing and the fieldfare. Go into a park in Oslo in the summertime and you will see redwings hopping around, with their attractive cream-coloured eyestripe and red flanks. But at this time of the year, they've left icy Norway behind, and are eating the berries on British bushes. Fieldfares have grey heads and lower backs: you can see flocks of them now in the countryside. Other animals besides birds migrate: grey whales swim from polar seas to mate in the tropics, and large hoofed mammals, such as reindeer in the Arctic and wildebeest in Africa, travel long distances to find fresh grazing. Even butterflies do it: the monarch, a large orange and black North American species, makes a journey every autumn in spectacular groups, millions strong, from the USA to Mexico.

Hunkering down

Hibernation is a successful winter survival strategy for many warm-blooded mammals in cooler climates, from hedgehogs and dormice to bears and bats; and it also works well for cold-blooded creatures such as amphibians (newts) reptiles (tortoises) and insects (butterflies).

Hibernation tends to involve a cosy den, and a very deep sleep. Unfortunately, humans are not properly equipped for it as the state is often more complicated than a simple snooze: in some cases it involves a radical slowing-down of heartbeat, breathing, metabolism and other processes, to conserve energy that would otherwise be lost in the struggle to find food and keep warm. Body temperature falls in hibernation to only a few degrees above the temperature outside (in a hamster, it can drop to 0C), but chemical reactions inside the body prevent the animal from freezing to death. Such fuel as is needed during the long sleep is provided by body fat, built up by constant feeding during the autumn.

Smaller animals such as bats and dormice tend to go into the deepest sleep; inside a dormouse's nest, its heart may beat only once every few minutes. Bigger beasts, such as American black bears, sleep more lightly: their temperature drops, but their heartbeat hardly slows. In warmer spells, bears can wake easily, and some females give birth during the hibernating months. The Reverend Gilbert White, whose 1789 classic, The Natural History of Selborne, about the wildlife of his Hampshire village, was the first serious piece of birdwatching, was convinced that swallows hibernated, although he did not know where. But swallows play a different game...

The human experience

The human body has a number of ways of countering the cold. The average naked person will start to feel chilly when the surrounding temperature drops below 25C. As the temperature decreases, blood vessels dilate, blood pressure rises and in an echo of our more hairy days, the tiny muscles attached to hair follicles contract causing goose bumps - a fruitless attempt to trap warm air close to the skin. Shivering sets in, caused by the contraction of larger muscles, and can increase the internal production of heat five-fold. In order to preserve warmth in vital organs, the blood supply is cut to non-essential extremities such as hands, feet and nose.

Ice swimmers and those who inhabit the chilliest climates find they become conditioned to the cold through prolonged exposure and by eating energy-rich diets. Scientists find the number of people killed by similar drops in temperature is fewer in northerly towns and cities than it is in the warmer south. Adult mortality is 15 per cent higher on a winter's day than in the summer.

Modern man owes his ability to handle the weather to his much more distant cousins. The first evidence for the controlled use of fire dates to 1.5 million years ago with the discovery of charred bones at Swartkrans in South Africa. The technique spread as man migrated, reaching Asia and Europe anything up to a million years later. Fire revolutionised the world of early humans. It provided warmth and allowed man to journey farther afield, exploiting colder regions in search of new hunting grounds. It warded off wild animals and gave psychological reassurance to frontier communities and a focus for social gatherings, storytelling and tradition. Cooking meat on the fire dramatically unlocked the calorific energy of food and helped combat disease. Fire provided a means of communication through smoke signals, a method of clearing ground for agriculture and unleashed a revolution in metallurgy. But it was the Romans who devised the first form of central heating. Today highly efficient gas central heating, insulated walls and roofs, draught-proof, double-glazed windows and doors have turned homes of the better-off into a snug sanctuary.