General Description: Polar bears are large bears. They have strong legs with large, flattened feet with some webbing between their toes that help with walking on ice and swimming. The wide paws prevent sea ice from breaking by distributing weight while walking. The webbed feet results in making polar bears, unlike other bear species, considered to be “marine mammals” along with seals, sea lions, walruses, whales and dolphins. Taxonomically, however, they are still bears. They evolved 1-3 million years ago from the brown or grizzly bears, which still eke out a marginal life along the northern shore of the arctic oceans.
Unlike the massive polar bears that can grow huge on diets of abundant seals, their ancestor brown or grizzly bears in the arctic are small, have very lower reproductive rates and eagerly eat almost anything that exists in their environment.
Polar bears have evolved something else that is different from their ancestor brown or grizzly bears: most polar bears don’t den, however all brown or grizzly bears do. When grizzly bear food is covered in snow during the winter, this species must den because there is nothing to eat. In contrast, most polar bears have access to their food of seals all winter long, so there is no need for them to den. The exception to this is pregnant adult females. Pregnant female polar bears must den so that their tiny newborn cubs are born in a warm protected environment; the cubs would freeze to death in the frigid temperatures of the far north if they weren’t born in dens.
Sustained by their mother’s milk in warm dens during long winter months, they emerge in the spring large enough to survive with the help of their mother. Teaching her cubs to survive long enough to survive by themselves, however, will take the mother polar bear another two years and sometimes longer.
How have polar bears adapted to live in the cold?
Many of the polar bear's physical adaptations help maintain body heat and deal with their icy habitat:
- The outer layer of fur is hollow and reflects light giving the fur a white color. The white fur helps the bear remain camouflaged in icy and snowy habitat.
- The skin is black under the fur; this black is evident only on the nose.
- The footpads of a polar bears’ paws have a kind of “non-slip” surface allowing them to get traction on slippery ice.
- Polar bears have a thick layer of fat below the surface of the skin, which acts as insulation on the body to trap heat. This is especially important while swimming and during the frigid arctic winter.
- Large size reduces the amount of surface area exposed to the cold per unit of body mass (pounds of flesh) which generate heat.
- Polar bear dens can be 38 degrees F warmer than the outside temperature.
How big are polar bears?
Polar bears are the largest carnivorous land mammals on Earth. They are about 7-8 feet long measured from nose to the tip of their very short tail. Male polar bears are much larger than the females. A large male can weigh more than 1,700 pounds, while a large female is about half that size (up to 1,000 pounds). Bears can weigh about 50 percent more after a successful hunting season than they do at the start of the next; most of this additional weight is accumulated fat. A newborn polar bear weighs only about 1.5 pounds which is very small compared to the weight of its mother.
What do polar bears eat?
Unlike other bear species, polar bears are almost exclusively meat eaters (carnivorous). They mainly eat ringed seals, but also bearded seals. They also eat walruses, other species of seals and whale carcasses. Polar bears will search out bird eggs and other food sources but none of these are abundant enough to sustain the large body mass and dense populations of polar bears.
How do polar bears hunt seals?
Polar bears hunt seals by waiting for them to come to the surface of sea ice to breathe. When the seal nears the surface, the polar bear will bite or grab the seal and pull it onto land to feed.
Another vitally important food source in most areas are seal pups that are born and live in dens in the arctic ice. The polar bear identifies these dens by smell and other markers and pounces though the roof of the den to capture the young seals. In Hudson Bay, the availability of seal pups in the spring is increasingly limited by earlier melting of ice. In the Arctic, polar bears are at the top of the food chain; they eat everything and nothing (but native hunters) eats them.
Permanent, multi-year ice that doesn’t ever melt is more important to polar bears than the annual ice that melts and reforms every year; this multi-year ice is increasingly rare, but will likely persist for longer in the island archipelago of northwestern Canada than in Alaska or off the northern coast of Russia.
Where do polar bears live?
Most polar bears occur north of the Arctic Circle to the North Pole. There are some populations south of the Arctic Circle in Hudson Bay, Manitoba, Canada. Polar bears live in Alaska, Canada, Russia, Greenland and some northern islands owned by Norway like Svalbard.
How do polar bears communicate and associate with each other?
Polar bears tend to live solitary lives except when mating, when a female is raising her cubs forming a family group, or when many bears are attracted to a food source like a beached whale. Young polar bears spending the summer ashore on the Hudson Bay coast will frequently play with each other, most commonly with their siblings. When necessary or playing, polar bears communicate with each other with grunts, growls, roars or squeals.
Polar bears near Churchill on the coast of Hudson Bay are even known to play with chained sled dogs without killing them, which they could easily do.
What are the reproductive habits of polar bears?
Polar bears breed in the late spring as the temperatures begin to rise in the Arctic. Like other bear species, however, they don’t really become pregnant at the time of breeding as the tiny embryo (or blastocyst) will not implant in the female’s uterus until fall when true gestation starts. This is called delayed implantation and allows a female bear to physiologically assess her condition prior to starting gestation and the process of birthing, nursing and carrying for her offspring for the next three years. The period of actual gestation following implantation to the birth of cubs is only about 60 days.
In the Hudson Bay population, where the reproductive biology of polar bears has been most extensively studied, it appears that a polar bear female carrying a blastocyst must achieve a body weight of at least 490 pounds to have the blastocyst implant and start gestation. If this threshold is not achieved, the blastocyst will reabsorb, the female will continue to hunt seals all winter, attempting to be fatter a year later and able to carry off a successful pregnancy.
In the beginning of the winter, a pregnant female will dig a den in a snow bank and begin the process of gestation that leads to the birth of her cubs. Depending on the area, pregnant females may enter dens anytime between early October and December. Time of exit from dens occurs between late February and April. Most females dig their dens in a snow bank on land, but some also den on the floating sea ice. In Hudson Bay, females may dig a den in the earth instead, but they use areas where the snow will build up and provide insulation. In the middle of winter in some of the coldest places on Earth, female polar bears give birth to cubs. Litter size is most commonly two cubs but sometimes litters can be one, three or, very rarely, four cubs.
Female polar bears in the Hudson Bay area spend remarkable periods of time fasting, the longest known of any mammal species. This fasting period before denning and in dens averages about 180-186 days. In Hudson Bay, pregnant females can successfully fast for as long as 240 days. The long period of fasting makes this species especially vulnerable to environmental changes like a warming climate that reduces the amount of time they have available to build up the fat reserves they need to survive both fasting and to bring off a successful pregnancy.
When the cubs are born they are completely dependent on their mother. They stay in the den nursing on her rich milk until spring, when they emerge and start exploring the world as their mother heads out to the ice to catch the seals she needs to replenish the weight she’s lost during her period of fasting. Over the next two years the cubs will learn from their mother how to catch seals themselves and to develop the other skills needed to survive and grow to adult size. Typically, cubs will stay with their mother until they are two-and-a-half years old but in some cases they will stay for a year more or a year less. If the mother is able to replenish her fat reserves sufficiently she can produce a litter of cubs that survive until weaning every three years. When food declines in abundance, there is a longer period between successive successful litters and litter sizes are smaller.
The conditions developing in Hudson Bay are such that females will no longer be able to give birth to and successfully raise a little of cubs. When this happens, the adult bears will survive until they die of old age and the population will be doomed because of lack of young bears. Scientists are fearful that this pattern is also starting to happen in the more northern polar bear populations as the amount of arctic ice continues to shrink.
Rising temperatures in the world’s oceans are causing sea ice to disappear for longer and longer periods during the late summer, leaving polar bears insufficient time to hunt. This is a worldwide problem, and the Endangered Species Act has listed polar bears as threatened everywhere in the world they occur. Polar bears can only survive in areas where the oceans freeze, allowing them to hunt seals living under, on, or in the frozen polar ice cap.
Orbiting satellites have been able to track the seasonal extent of sea ice since 1979, and the trends are very disturbing for the future of the polar bear. The minimum extent of sea ice occurs in mid-September and new records lows for this minimum are now regular events. The most recent minimum low was set in 2012, surpassing the previous minimum of 2007. The trend is for the last summer sea ice in the arctic to be farther and farther from shore, making it necessary for polar bears to swim increasingly long distances from shore to reach the ice. Worse, the last remaining sea ice is over deep and unproductive waters that yield less prey.
In the southernmost populations around Canada’s Hudson Bay, the sea ice does not persist throughout the summer. We can expect to see a similar trend when the sea ice disappears in the arctic. In Hudson Bay, polar bears now spend summer months on shore when the ice has melted and there is no ice platform from which to hunt seals. As a result, they must fast for months. Remarkably, Hudson Bay polar bears have been able to catch enough seals during the winter to tide them over during this period of on-shore fasting. But this situation is changing fast because of climate change. Now the ice is melting earlier and forming up later, leaving an ever-shorter period in which to hunt. Hudson Bay bears are now much skinnier, have fewer cubs, the cubs they do have more frequently don’t survive to adulthood, and the interval between successful litters is growing. There is also more cannibalism of cubs by male bears.
The patterns seen in Hudson Bay are beginning to occur now in more northern populations. This pattern is especially well documented on the north Coast of Alaska but appears to be the case worldwide. The increasing amount of open water between shore and the sea creates long swims for polar bears. During this long swim, cubs and adult bears have died. If they do reach the remaining ice it is over unproductive deep water where there are few seals to hunt.
Additionally development is increasing in ocean floor exploratioin and offshore oil extraction in the open waters that were previously sealed by frozen ice. This brings people, disturbance and potentially ruinous oil spills to the previously pristine arctic polar bear habitat.