Free The Great White Bear by Kieran Mulvaney

those bursts are brief and, particularly if the seal escapes, followed by exhausted recuperation.
    When the ambient temperature is too high for comfort, polar bears respond by doing as little as possible, seeking out shade or shelter and lying still, expending no unnecessary energy and waiting for conditions to cool. Even the arrival of lower temperatures and the season's first ice and snow may offer only marginal respite.
    "Typically what you'll see them do is spread themselves on the ice, so their groin area can cool off," says JoAnne Simerson. "What I've seen them do, before there's snow and ice but the ponds are beginning to freeze, is actually punch through the thin layer of ice on the ponds and lie down and sit in the water."
    And, when sitting in water doesn't help, they can immerse themselves in it.
    Polar bears are excellent swimmers. They can—and frequently do—swim for hours at a time, although too prolonged an exertion can result in severe exhaustion and the need to recuperate for lengthy periods, and may be particularly dangerous for young cubs. The same long neck that enables a polar bear to plunge its head through ice holes and under water when hunting also allows it to keep its head above water for long stretches when swimming; and the fact that it is able to swim for long stretches owes a great deal to what are by far the largest feet in the bear world.
    At up to twelve inches in diameter—almost twice as big as a brown bear's feet—a polar bear's front paws are striking in their immensity; in the photograph at which I am currently looking, which I took on the shores of Hudson Bay a few weeks before writing these words, a resting bear is looking at the camera and at me, its muzzle lying on a paw that seems almost as large as its head.
    "They're like snowshoes," says Simerson; "it's all about weight distribution," a means of spreading the impact of a bear's bulk to make it easier for it to walk over ice. Indeed, when a bear is setting out uncertainly over ice that is particularly thin, it will often all but spread-eagle itself to distribute its weight as evenly as possible. But such large paws are also valuable for fatally swatting seals. And they make magnificent oars with which a bear can doggy-paddle through the water, while its rear legs curl up underneath its body and act like rudders.
    A polar bear's buoyancy is considerably aided by a thick layer of blubber, which not only helps keep a bear warm, but also enables it to store huge reserves of fat for lean periods and metabolize them when needed, a response to an environment of uncertain and uneven productivity. Polar bears frequently eat seemingly to excess when food is available and then slow down their metabolism substantially while drawing on fat reserves, a state known as "walking hibernation," when pickings are thin and food is hard to find.
    That layer of blubber can be as much as four and a half inches thick and is topped by skin that is, perhaps surprisingly, black. Equally surprisingly, a polar bear's fur is not actually white.
    It is in fact unpigmented, but because it reflects visible light it appears white to the human eye when it is clean and in sunlight. During sunrise or sunset, the fur may actually appear to take on the orange-yellow hues of the rising or setting sun; later in the season, in spring or late winter, before the annual molt that begins in April or May, bears may appear to be "off-white" or even yellowish.
    While sunlight is reflected off the pelt (contributing to its white appearance), the late Norwegian scientist Nils Øritsland found that some of it passes through and reaches the black skin, which naturally enough warms up.
    "Warm surfaces, of course, emit long-wave infrared radiation, or heat," notes Øritsland's colleague David Lavigne. (This is the principle behind night-vision optics.) "So the skin emits long-wave infrared and it turns out that, when it hits the hairs on the way out, they trap the