A Polar Bear's Journey

It’s nothing short of a miracle that her tiny, defenceless cubs can survive in this forbidding environment

- By James Raffan

Nanu, a nine-year-old female polar bear, lives on the barrens of the Hudson Bay lowlands, south of Churchill, in the Canadian province of Manitoba.

Her den, which she dug into a creek bank, is nearly two metres in diameter and a half-metre higher than the exit tunnel. Air warmed by her body and breath rises into this upper space, bringing it to just below freezing. To save energy, she is able to lower her body temperature slightly from its normal 37°C.

A mother who has not eaten for three months is able to nourish her cubs in utero and still keep her metabolism quiet enough to conserve energy for three more months of fasting. That’s because, in addition to her own considerable insulation—two inches of thick downy underfur combined with a full mantle of long, hollow guard hairs—every polar bear has a layer of fat below the skin.

In December, in a state of suspended animation in her darkened den, Nanu 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, Siu and King, are immersed in the warm air trapped in the fur of Nanu’s belly. Mother’s rich milk, with 32 per cent fat, not only keeps these helpless newcomers alive but allows them to thrive when temperatures outside can be as far below zero as their mother’s body temperature is above.