Under the Microscope: Polar bears were in the news recently when the media widely disseminated news of the polar bear cub rejected by its mother in Germany's Nuremberg Zoo. Polar bears are among the world's largest land carnivores.
Unfortunately the polar bear is at high risk of extinction. Climatologists and zoologists predict that global-warming-induced melting of polar sea ice will reduce polar bear numbers by two thirds by the middle of the 21st century.
Male polar bears are up to 45 per cent larger than females and most adult males weigh 350-650 kg and are 2.5-3m in height. Adult females normally weight 150-250kg and are 2.0-2.5m in height. Females double their weight during pregnancy. Polar bear cubs weigh 600-700g at birth.
Polar bears are of the species Ursus maritimus and the species is distributed in 19 sub-populations across five countries - Canadian Arctic Archipelago, Greenland, Svalbard (Norway), Central Siberia and Franz-Josef Land (Russia), and Alaska. DNA studies point to significant interbreeding among the sub-populations.
Fossil and DNA evidence indicates that polar bears diverged from brown bears roughly 200,000 years ago. However, polar bears can breed with brown bears to produce fertile grizzly-polar bear hybrids indicating that they have only recently diverged and are not yet a truly distinct species.
But neither species can survive long in the other's environmental niche and they are usually classified as separate species.
Even though it spends time on land and ice, the polar bear is regarded as a marine mammal because of its intimate relationship with the sea. Seals are the mainstay of the polar bear diet. They use sea-ice as a platform for hunting seals and polar bear range is limited by the availability of sea ice. The most severe threat to polar bears is posed by global warming which is literally melting away their natural habitat.
A Canadian Wildlife Service study of polar bears in 1999 in the Hudson Bay showed that rising temperatures threaten polar bears with starvation. The local sub-population declined by 22 per cent because the sea-ice is melting earlier in the year, driving the bears to shore weeks before they catch enough food to see them through the scarce-food-period of late summer/early fall. Alternatively the melting sea ice may force bears to swim enormous distances putting them at risk of drowning.
Polar bears are enormously aggressive creatures. They are not used to people and are potentially very dangerous. The polar bear feeds mainly on seals, particularly ringed seals that make holes in the ice to breathe, but it will kill and eat a wide variety of animals - birds, rodents, shellfish, crabs, beluga whales, walrus calves, reindeer and other polar bears. When hungry enough they will eat plants. Of course, polar bears do not eat penguins because penguins live in Antarctica whereas polar bears live in the Arctic.
The physiology of the polar bear is specialised to digest the fat of marine mammals and they don't derive much nutrition from land animals. Therefore they survive mainly on live seals and walrus calves taken at the edge of sea-ice in winter and spring or on carcasses of dead whales or adult walruses. They live off their reserves of fat in late summer/fall when sea ice is minimal. Most animals can easily outpace polar bears on land or open water. The only regular predators of polar bears are humans.
Because the polar bear feeds largely on fish-eating carnivores it ingests large quantities of Vitamin A, which it stores in the liver. The extremely high concentration of Vitamin A makes the polar bear liver poisonous to humans.
The fur of the polar bear provides insulation and camouflage. Although the individual hairs are translucent, the fur in bulk appears white. There are stiff hairs on the pads of the bear's claws. This provides insulation and traction on ice. The bear's fur and thick blubber insulate it against the cold and bears overheat at temperatures above 10 degrees. The bear's 12cm layer of fat adds buoyancy to the animal in water in addition to insulating it from the cold.
Polar bears mate in April/May and the fertilised egg remains in a suspended state until August/September. The females eat huge amounts of food over these four months, doubling body weight in preparation for pregnancy. When food becomes scarce in autumn they dig a den in the snow and enter a state of hibernation. The cubs are born in December while the mother is still asleep. The cubs nurse while the mother remains dormant and then the entire family leaves the den in March. Cubs are weaned and separated from the mother by three years of age.
There is an interesting stuffed polar bear in the Natural History Museum, Dublin. The Irish explorer, Captain Leopold McClintock, shot the bear in 1852, used the blubber for fuel, ate the steaks, and kept the skin for display in the museum. You can see the bullet holes in the skin.
I will finish with a joke. A lawyer took his client ice-fishing. While they were sitting around the hole in the ice, a polar bear with a hungry look in his eye started charging at them from a half-mile off. The lawyer immediately pulled off his ice boots and started pulling on a pair of runners. The client said, "We're doomed - you can't outrun a polar bear on this ice." The lawyer calmly replied, "I don't have to outrun the polar bear - I only have to outrun you."
William Reville is associate professor of biochemistry and public awareness of science officer at UCC (http://understandingscience.ucc.ie)