these environments because it is either locked up in snow or saline. Polar bears are able to produce water through the metabolism of fats found in seal blubber, and are therefore found primarily along the perimeter of the polar ice pack, rather than in the Polar Basin close to the North Pole where the density of seals is low. Annual ice contains areas of water that appear and disappear throughout the year as the weather changes. Seals migrate in response to these changes, and polar bears must follow their prey. In Hudson Bay, James Bay, and some other areas, the ice melts completely each summer (an event often referred to as "ice-floe breakup"), forcing polar bears to go onto land and wait through the months until the next freeze-up. In the Chukchi and Beaufort seas, most polar bears retreat each summer to the ice further north that remains frozen year-round, though a large portion of the population (15–40%) has been observed spending all summer on land every year in these areas since the 1980s. Physical characteristics The only other bear of comparable size to the polar bear is the Kodiak bear, which is a subspecies of brown bear. Adult male polar bears weigh and measure in total length. Around the Beaufort Sea, however, mature males reportedly average . Adult females are roughly half the size of males and normally weigh , measuring in length. Elsewhere, a slightly larger estimated average weight of was claimed for adult females. When pregnant, however, females can weigh as much as . The polar bear is among the most sexually dimorphic of mammals, surpassed only by the pinnipeds such as elephant seals. The largest polar bear on record, reportedly weighing , was a male shot at Kotzebue Sound in northwestern Alaska in 1960. This specimen, when mounted, stood tall on its hindlegs. The shoulder height of an adult polar bear is . While all bears are short-tailed, the polar bear's tail is relatively the shortest amongst living bears, ranging from in length. Compared with its closest relative, the brown bear, the polar bear has a more elongated body build and a longer skull and nose. As predicted by Allen's rule for a northerly animal, the legs are stocky and the ears and tail are small. However, the feet are very large to distribute load when walking on snow or thin ice and to provide propulsion