Figure 4. Seasonal movements of a single adult female polar bear, September 2001 to September 2003.
The sea ice in Hudson Bay usually melts by mid-July and doesn’t begin reforming until November. During the four to five month ice-free season, polar bears from the Southern Hudson Bay subpopulations are forced onto land primarily in Ontario or onto islands within James Bay in Nunavut. During this time, bears do not actively forage for food, but only eat as the opportunity occurs. Cubs lose an average of 0.2 kg (0.5 lbs) per day and subadults (independent bears that have not yet reached reproductive age) and adult males lose an average of 0.9 kg (2.0 lbs) per day. When the sea ice reforms most bears return to hunt for seals. They remain on the ice until it melts again the following summer.
| Figure 5. Female polar bear den in a treed area in northern Ontario. Photography: D. Sutherland
|Figure 6. Female polar bear den on the edge of a peat palsa in northern Ontario. Photography: D. Sutherland|
Pregnant bears are the only exception to this annual pattern (Figure 4). These females are forced onto shore at the same time as other bears. In Ontario they move up to 120 km inland to find denning areas. They leave their denning areas between mid-February and mid-March. Data from satellite-collared bears and from summer airplane or helicopter surveys in Ontario indicate bears select treed areas, river banks, gravel ridges, and palsas (peat mounds that are raised due to the action of frost heaving) to construct their maternity dens (Figures 5 and 6).
Polar bears mate while on the sea ice in April or May, and cubs are born between late December and early January. Cubs are blind and weigh less than 1 kg (2 lbs) at birth. They first leave the den between mid-February and mid-March. The mother leads them out onto the sea ice and begins hunting for seals again. Pregnant bears survive their seven to eight month off-ice period by using stored body fat; they may lose more than 40 per cent of their body mass during this time. Most cubs are nursed until they are two years old; some are weaned after their first year. They are dependent on their mothers for their first two years. During this time they learn survival skills and hunting techniques.
Polar bears have a low reproductive potential. Female polar bears don’t begin reproducing until they are five or six years old. Once they become reproductively active they typically have litters of one to two cubs (litters of three cubs are rare) every three years. Polar bears can live up to 25 years, but there is evidence their ability to reproduce declines after age 20.