Mama Bear

Arctic explorer James Raffan tells the story of a polar bear and her cubs-and their struggle to survive in a changing climate

Nanu, a nine-year-old female polar bear, lives on the Hudson Bay lowlands, south of Churchill, Man. Her den, which she dug out of a stream bank before the ground froze, is nearly two metres in diameter and half a metre higher than the narrow exit tunnel. Air heated by Nanu’s body and breath forms a bubble of warmth that rises into the space. While the temperatures outside can be in the –20s in January, the well-insulated cave can be brought to just below freezing with nothing more than her presence.

Though she hasn’t eaten for six months, she’s able to nourish her cubs in utero and still keep her metabolism quiet enough to conserve energy for at least three more months of fasting to come. That’s because, in addition to her own considerable insulation—five centimetres of thick, downy underfur combined with a full mantle of long, hollow guard hairs—every bear has a layer of fat just below the skin.

To save energy, Nanu is able to lower her body temperature slightly from its normal 37°C. In her state of suspended animation in the darkened confines of the den, she delivers two blind, deaf, toothless, downy-haired young who are less than one per cent of her size, with no body fat to speak of.

The cubs, Sivu and Kingu, are immersed in the fur of her belly, where the sound of their mother’s heart is as present and familiar as it was when they were in utero. Mother’s milk, with 32 per cent fat, keeps these tiny, helpless newcomers alive and powers their exponential growth.

By the time they are three weeks old, their fine hair is replaced by a dense undercoat and longer guard hairs. After 25 days, the soles of their little feet start developing hair as well, completing the insulation they need for moving around the den.