Video by bertiegregory / These tiny polar bear cubs wouldn’t let their mother sleep! Unlike other species of bear in North America, male polar bears do not den in wintertime. Female polar bears, however, den during the winter to give birth to cubs. This particular family belongs to a special population. Female polar bears that den near the Hudson Bay go through one of the longest fasting periods of any mammal. As the sea ice (their hunting ground) melts around July, they come onto land. In September/October they head further inland to build a den. In February/March, they break out of the den with their new cubs. They then head out onto the sea (which has frozen solid over the winter) to hunt seals. So for the eight months that they are on land, the mother must go without a good meal even while she is pregnant and suckling new cubs. As this fasting period is pushing the bears to their limit, any increase in Arctic temperature—and therefore an increase in the ice-free period—will have dr...