Sometimes injury is to blame, but many very thin bears have suffered nothing more than a run of bad luck. The main problem is the unpredictable nature of the food supply. If the ice is too thick, the seals cannot make breathing holes so they go elsewhere. Satellite tracking shows that bears travel huge distances to track the seals, covering at least 30 kilometres a day for several weeks. As soon as the ice breaks up in the spring, seals can breathe anywhere so the bears cannot catch them.
Summer is a lean time for polar bears.
Without a permanent food source, males are unable to carve out territories for themselves and their families as most bears do. Instead of a stable family life, with each mature male defending his own patch and protecting his females and cubs from intruders, polar bears go their separate ways after mating. Most of the families that an itinerant father later encounters do not share his genes, and he sees the cubs as just another source of food. The danger of infanticide turns breeding females into refugees. Pregnant polar bears living around Hudson Bay and the islands off the Canadian coast leave the hunting grounds during November.