Which creature is the largest terrestrial predator on the planet, has the most adorable cubs, and makes its own fresh water?
Polar bears, also known as Ursus maritimus, or “sea bears,” live in the Arctic Circle and on nearby land masses and sea ice in the U.S. (Alaska), Canada, Russia, Greenland, and Norway.
These mammals are hypercarnivorous bears, which means that over 70% of their diet is meat and fat. They hunt ringed and bearded seals for their food primarily, spending most of their time hunting off the edge of sea ice or waiting by the holes in the ice where the seals come up for air. Depending on the time of year, they catch between 1-20% of the seals they hunt, so each success is vital to their survival.
When no sea ice is present, they live off of their fat reserves, and they can resort to eating whale carcasses, walruses, bird eggs, birds, fish, and to a small extent, berries, roots and kelp. Adult females typically weigh between 400 – 700 pounds, while adult males can weigh twice as much or more, between 500 – 1,700 pounds. They can stand up to 11 feet tall (a bit more than 3.3 meters) and their lifespan is approximately 25 years.
Polar bears, the only type of bears that swim in the sea, have many characteristics that are adapted for cold temperatures, swimming, and hunting. These include two thick layers of fur (an outer layer and an under layer) and a thick layer of fat that provide insulation, an oily coat that sheds the water off after swimming, 42 razor sharp teeth, a keen sense of smell, and large furry, webbed feet (the size of dinner plates) with bumpy footpads and short, sharp, stocky claws that allow them to swim, distribute their weight on the ice, and grip the ice as they pull up from the water’s edge.
A polar bear’s hair is actually clear, yet it contains whitish keratin in its outer layer and a hollow core that scatters and reflects light, often giving it a whitish hue. Their skin underneath is black which absorbs the heat from the sun to help keep them warm.
Polar bears can swim up to 6 miles per hour and up to 3-4 minutes underwater when they are hunting for seals near the shore or on ice floes. They can also swim incredibly long distances. The longest recorded swim was over 426 miles in nine days straight by a female polar bear, a trip equivalent to the distance from Boston to Washington, DC. Some think these long swims may be increasing due to the melting sea ice and their need for food.
Since polar bears’ access to fresh water is limited to the snow, their body can actually metabolize the fat of the seal blubber they consume into water… In other words, polar bears make their own fresh water! Additionally, their incredible sense of smell enables them to detect a carcass or a live seal up to 3 miles away!
Those Adorable Cubs!
Mating season begins in April when the polar bears gather on the sea ice during peak seal hunting season. Pregnant females proceed to eat twice their body weight and then dig out a maternity den for themselves, typically in snowdrifts a few kilometers inland from the coast. Some dens are made underground in the permafrost or on the sea ice. Expectant mothers then go into a dormant state in their dens for several months.
Each litter consists of 1-3 cubs weighing less than 2 pounds each, and they are born blind with a light down fur. The mother nurses her cubs in the den until they are about 20-30 pounds. The family then ventures outside each day for about 2 weeks while the mother breaks her 5 month fast, grazing on vegetation while the cubs get stronger by walking and playing. They then start their long journey to the sea ice so she can hunt for seals while continuing to nurse her cubs for another 2 years.
Their Role in The Ecosystem
Polar bears are known as apex predators because they are at the top of the food chain in their region. They are also considered a “keystone species,” playing a critical role in the Arctic ecosystem — their presence affects the types and numbers of many other species in the region. The World Wildlife Fund considers polar bears to be an important indicator of the health of the Arctic marine ecosystem.
Ringed seals in the Arctic appear to regulate the population of polar bears and vice versa, and this seems to account for the differences between the seals in the Arctic and the Antarctic. Seals in the Arctic use more breathing holes, are more restless on the ice, and the fur of the baby seals is usually white, seeming to work as a camouflage from predators. Baby seals in the Antarctic are born with dark fur, where there is no threat from polar bears.
Other species in the ecosystem are also impacted by the presence of polar bears. For example, Arctic foxes and glaucous seagulls rely on the bears’ remaining seal carcasses as a source of food.
Centuries ago, we humans had reverence for our place in nature. We understood the importance of limiting our consumption, using every part of our hunt, and being sure not to deplete the natural resources we depend on for our survival. Indigenous people of the Arctic still live this way. They hunt polar bears for their meat and fat for food, fat for fuel to light their homes, dried gallbladder and heart for medicine, fur for clothing and footwear, tendons for sewing thread, and large canine teeth as sacred talismans and symbols of protection.
A Vulnerable Species
As time has gone on, there have been increasing threats to the polar bear population including the fur trade, toxicity from environmental contaminants, habitat disruptions from fossil fuel exploration and development, and now the melting of sea ice due to planetary warming. Various measures have been implemented to curb these impacts including the International Agreement on the Conservation of Polar Bears in 1973, identifying polar bears as a ‘vulnerable species’ by the International Union for the Conservation of Nature, and more.
Today, many leading polar bear biologists are extremely concerned about the impacts of climate change on polar bears. One trend that biologists and hunters are seeing is an increase in hybridized bears, also known as ‘grolar bears’ or ‘pizzly bears!’ Biologists think that as polar bears migrate further south in search of food, and grizzly bears migrate further north for new opportunities of food, the incidence of mating between these biological sister species will continue to grow. Despite this adaptation, the U.S. Geological Survey is projecting that two-thirds of polar bears will be gone by 2050 and some are forecasting that polar bears will become extinct by 2100.
While I was doing the research for this article, I experienced a period of grief that went quite deep. It was painful and challenging, and fortunately, it eased up after I expressed this during our weekly Bio4Climate meeting. I continue to learn about the power and promise of the restoration of biodiversity and the magnificent creatures with whom we share the Earth.
Let’s follow our bearings together as we recognize International Polar Bear Day today, Sunday February 27th.
By Louise Mitchell