Every year after mating female polar bears begin a long journey. For six months, the fertilized eggs are maintained in the female but prevented from growing (delayed implantation). Only in female polar bears with sufficient fat reserves will the eggs implant into the womb.

This is when they build their dens, which are usually constructed in valleys, on the side of hills, or in snow banks. Females use the same den multiple times, and scientists have dated den structures that go back over two centuries.

After building the den the female enters and allows the snowfall to cover them from sight. A couple months after entering she gives birth to cubs who weigh only ~500g (1.1 pounds). She will feed them over the next several months until it's time to leave the den in search of seals.

With rising temperatures and expanding oil and gas interests in their habitat this vulnerable time in the den is becoming more perilous. A study by Dr. Tom Smith of BYU found that the FLIR imaging used by oil and gas companies when surveying missed 55% of the time in identifying these dens.

Video by: daisygilardini