Photo credit: the USFWS (the US Fish and WIldlife Services)

This photo, taken by the US Fish and Wildlife Service of Alaska, shows an abandoned den.

As food availability decreases many animals adopt fasting strategies to maximize their stored body fat. Similarly animals will also lower their metabolic rate during times of intense reproductive activities. The genus Ursus - made up of the Ursus americanus, the black bear, the Ursus arctos, the grizzly bear, and the Ursus maritimus, the polar bear - is known for it’s unique overwintering strategies. As overwintering strategies in polar bears are studied, the similarities found by physiologists strengthen phylogenetic relationships between the Ursus genus.
Denning in black bears and grizzly bears is very similar (8). On average the duration is five months. During overwintering bears rely on fat stores, avoiding the net catabolism, of protein and net production of urine and maintain a constant lean body mass. In both black bears and grizzly bears this adaptation is believed to be limited to the denning period (7).

Denning is a common strategy employed by polar bears during winter months. Polar bears like many mammals mate with multiple partners throughout their lives. As a result child-rearing responsibilities fall to the mother. Female polar bears care for their young for approximately two and a half years and mate every three years (1). In winter months adult female bears, move inland to find a suitable den. Polar bear dens can be up 40 degrees warmer than external temperatures (11).

Studies have found the early disturbance of these dens in the spring can result in the premature abandonment of dens and decreased cub survival. This decreased survival can be attributed to several factors: the inability to acclimate to severe weather temperatures, the incomplete development of motor skills, and a decrease in body size and surface to volume ratio (2).

Photo credits: Daniel J. Cox @ NaturalExposures.com