Unknown Facts About Polar Bears
Written by: Dr. Owen Slater
Polar Bears: More Than Just the Largest Land Carnivore
Polar bears are often described as the largest land carnivore on Earth. However, as my colleague, University of Alberta polar bear researcher Dr. Andrew Derocher states, all three terms are technically inaccurate.
Because polar bears spend all or the majority of their time on sea ice and rely on living in this marine environment to survive, they are classified as marine mammals. Due to this classification, they are also not the largest, with transient killer whales taking the top spot on the marine mammal list.
Of species that spend a portion of their time on land, but the majority in a marine environment (semi-aquatic marine mammals), the elephant seal would still beat out the polar bear for the title. As for the carnivore piece, this term refers to eating muscle meat, which is known to not be sufficient to sustain polar bears.
Polar bears must eat large amounts of seal blubber, and they do this when fat seal pups are plentiful in the spring and early summer months on the Hudson Bay. With every mouthful of blubber they consume, they are able to convert almost 100% of this ingested fat into their own stored energy, which is essential for their survival and reproduction. As such, they are considered primarily lipivores rather than carnivores.
Why Polar Bears Thrive on a Fat-Filled Diet
With such a high cholesterol and triglyceride diet, if polar bears were like people they would quickly develop cardiovascular disease due to plaque formation leading to clogged arteries and eventually heart attacks.
Instead, a fat bear is considered a healthy bear, and they are able to avoid the consequences of such high levels in their blood through genetic adaptations that prevent harmful levels of fatty acids from sticking to the walls of arteries. Instead, the fats are transported through the blood and stored as energy dense adipose tissue, primarily just below the skin over the backend or rump.
A fat Churchill polar bear can be composed of approximately 50% fat and their weight can double in size when they come off the sea ice in the summer.Why Polar Bear Livers Are Toxic to Humans
How Polar Bears Survive Long Fasting Periods in Churchill
The thick layer of fat that polar bears accumulate over the hunting season allows them to survive when food is unavailable. For polar bears that call Churchill and surrounding area home, this means they are essentially fasting when all the sea ice melts from the bay in July or August and until enough ice returns in November or December. With climate change, this ice-free period in the Hudson Bay has increased in length by about 3 weeks since the 1980s. During this fasting period, bears lose about 1kg or 2.2 lbs of fat per day. To conserve energy, most adult bears spread out along the coast and spend their days resting close to shore after digging day beds to stay cool during warm summer days. Some bears will forage for food such as berries, eggs, animal carcasses or attempt to hunt prey, but the amount of energy they gain from these less energy dense foods that are much lower in fat than seal blubber is thought to be completely offset by the energy required to obtain it.
Even after months of fasting, the large male bear on the right still had considerable fat reserves in late October compared to the slimmer bear on the left. Image credit: Owen Slater
Why Polar Bear Livers Are Toxic to Humans
Polar bear livers are toxic to people due to the extremely high concentration of vitamin A, with European explorers unfortunately learning this firsthand during their early Arctic expeditions. Vitamin A is a fat-soluble vitamin, so excessive amounts are trapped in fat and not excreted in urine like water-soluble vitamins. Instead, it bioaccumulates through the food chain starting with plants and algae, through to fish, then seals and finally with polar bears at the top of the chain. Specialized cells in the liver store the vitamin A and polar bears livers have such high levels (upwards of 30,000 IU/gram) that if a person were to consume a portion of the liver they would experience severe vomiting, headaches, skin sloughing and potentially death. Record-Breaking Polar Bear Hunting Tactics Underwater
Record-Breaking Polar Bear Hunting Tactics Underwater
It is common knowledge that polar bears hunt by waiting at seal breathing holes, but they use a variety of hunting tactics, including submerging themselves underwater to ambush seals hauled out on ice floes. This is known as aquatic stalking and one polar bear was documented to set a record when he held his breath for over 3 minutes and travelled about 45-50 meters underwater in an attempt to catch a bearded seal near the island of Svalbard, Norway. This event was video recorded by Rinie van Meurs, who was guiding at the time. He subsequently reached out to Dr. Ian Stirling, who was a polar bear biologist with the Canadian Wildlife Services and they wrote a paper describing the record-breaking feat. A link to the paper is provided here:Polar Bear Footprints: Scent Trails That Lead to Mates
Polar Bear Footprints: Scent Trails That Lead to Mates
Scented footprints can lead to mating opportunities. Imagine trying to find a mate when home ranges of Western Hudson Bay polar bears are upwards of 350,000 km2 and constantly changing from day to day due to shifting pack ice. To do this polar bears rely on their sense of smell and pheromones left behind in footprints produced by specialized glands in their feet. The snow and ice can retaining these pheromones for days.
A polar bear smelling a footprint to gain important information about the bear that left it behind. Image credit: Owen Slater
Bears coming across tracks will take a quick sniff and perform a flehmen behaviour, which involves opening the mouth and slight lifting of the upper lip to allow the pheromones to enter the vomeronasal organ through small openings just behind the incisor teeth on the upper jaw. This scent organ can determine the chemical signature of the footprint including what sex the bear is and whether a female is in estrous in the spring. If she is, the adult male will follow the track, sometimes for days before eventually catching up with the female, at which point he is likely to have to fight with other large males who also followed her scented trail.
