Polar Bear
Diet
Polar Bear swimming. Svalbard, Norway
© WWF-US/Eunice K. Park
The polar bear is the most carnivorous member of the bear family, feeding mainly on a diet of ice seals. They eat so much because their physiology is specialized to acquire large amounts of fat from marine mammals, the consequence being that it cannot derive sufficient caloric intake from terrestrial food.
Seals are a particularly energy-rich food source, especially for hungry mothers and their growing cubs. In fact polar bears can devour huge amounts of fat (from seals) when this prey is abundant. They are often described as lipophillic (fat loving).
The largest proportion of a polar bear's annual caloric intake occurs between late April and mid-July. Food availability during this period is critical for maintaining their proper body weight to survive the ice-free season, when prey is harder to catch for bears that remain on the sea ice and little or no food is available for bears that remain on shore.
For bears that come ashore in areas like the Hudson Bay in Canada, this food-free season can last three to four months or even longer now that climate change is beginning to take its toll and the ice pack forms later in the season.
What do they eat?
Polar bears feed mainly on ringed and bearded seals.
Depending on where they are located (e.g. Canadian, Norwegian, Russian arctic areas) they may also eat harp, hooded, and the occasional ribbon seal.
Polar bears are also scavengers, and will happily feed on the carcasses of beluga whales, grey whales, walruses, narwhals, and bowhead whales.
Polar bears can also occasionally and successfully hunt beluga whales and adult walrus.
When other food is unavailable, polar bears sometimes eat muskox, reindeer, small rodents, seabirds, shellfish, fish, eggs, kelp, berries, and human garbage (the garbage dump in Churchill, Canada, had to be closed and moved because of this activity).
How much do they eat?
- A healthy bear will try to eat on average around 2kg of fat per day.
- Its stomach can hold food equivalent to 15% to 20% of its own body weight.
- It can assimilate 84% of the protein and 97% of the fat it eats.
- A ringed seal weighing 55kg should therefore provide up to 8 days of energy for a polar bear (though it needs to eat more than this in order to store up reserves for the food-free season)
Depending on the age and the condition of the bear, it it will eat different parts of any kill.
Adult bears in good condition prefer the calorie-rich skin and blubber of seals, whereas young bears and adults in poor condition consume the protein-rich red meat as well.
Bears that have left their mother, but have not yet gained full adult size (and are often still learning how to hunt) often scavenge on the carcasses from other bears' kills It is also reported that these subadults may also be forced to accept a half-eaten carcass if they kill a seal but cannot defend it from a larger bear. This is why bears generally eat the fat first - it contains the most calories and is easier to digest.
Polar bears like to keep themselves clean, as it probably helps the insulating properties of their fur. After feeding, they will usually wash themselves with by taking a swim or rolling in the snow.
How do they find and catch their food?
Polar bears are estimated to spend well over 50% of their time in the hunt for food. And surprisingly, given their strength and agility, less than 2% of their hunts prove to be successful.
Bearded Seal
© Mike Spindler / U.S. Fish and Wildlife Service
It is reported that even in good hunting areas, a bear may catch only 1 seal every 4 or 5 days.
There are several ways in which polar bears attempt to catch their food...
"Still hunting"
The most common method of hunting by polar bears, it involves:
- The bear locating an active breathing hole using its keen sense of smell (ie a place where the seals are currently surfacing to take in air)
- The bear then crouches next to the hole, out of site, and remains very still
- When the bear detects (by smell) that a seal has surfaced, it uses startling speed to enter and drag the seal from the hole and out of the water
- It bites the seal's head (usually crushing the skull) or its upper body
This type of hunting can take less than one hour, but polar bears will wait much much longer if they need to. Polar bears also use this method along narrow open water leads.
Stalking
A bear may also hunt by stalking seals that are resting on the ice pack.
Should it come across the opportunity, it will approach the seal to around 100m and wait. If the seal does not flee, it will continue its advance slowly and in a crouch. If still not spotted by the time it is around 10 to 30m away, it will make a sudden dash to catch the seal, before it can slip back into the sea.
It may also approach a resting seal via the water, for example if the seal is by an ice edge as opposed to a narrow breathing hole. Again, when close enough, the bear will launch itself from the sea. One possible advantage to this method is it may cut off the seal's escape route. However the disadvantage is that the bear has to be very close before it can pounce.
Head of ringed seal above the water. Blomsterhalvøya, Spitsbergen (Svalbard)
© Sindre Kinnerød / WWF-Canon
Uncovering seal birth lairs
This method of hunting is only used in spring when the ringed seals give birth to their pups.
Ringed seals create caves under snow drifts that accumulate next to breathing holes in the ice.
Again, using sight and its keen sense of smell, a polar bear will locate a birth lair, carefully and quietly positioning itself next to it.
If a polar bear hears a seal in the lair, it will gradually rise up on its hind legs and crash down with its front paws to break through the lair's ice roof. This may take several attempts if the ice is very thick. In addition, the bear has to be quick if it is to break through before the seal pup escapes via the breathing hole.
Scavenging
As already mentioned, polar bears have a remarkable sense of smell and can detect a carcass from up to 32km away. This allows them to be effective scavengers, taking advantage of other bear's kills, or sick or wounded prey that has eventually died.
