A. Longevity.
1.Polar bears can live 20 to 30 years, but only a small proportion of polar bears live past 15 to 18 years (Stirling, 1988).
2.The oldest known polar bear in the Arctic lived 32 years. The oldest known polar bear in a zoological park lived 41 years (Stirling, 1988).
B. Aging studies.
Each year as a polar bear grows, a thin layer of cementum is added to the outside of each tooth. Age can be estimated by examining a thin slice of tooth and counting the layers. To estimate the age of a live polar bear, researchers can extract one small, vestigial premolar tooth.
1.Adult polar bears have no natural predators. Males occasionally kill other males competing for mates. Males periodically kill females protecting cubs.
2.Cubs less than one year old sometimes are prey to adult male polar bears and other carnivores, such as wolves.
3.Newborn cubs may be cannibalized by malnourished mothers.
D. Human interaction.
1.Hunting.
a. Polar bears have been hunted for thousands of years.
(1)Evidence of human polar bear hunts have been found in 2,500- to 3,000-year-old ruins. Arctic peoples have traditionally hunted polar bears for food, clothing, bedding, and religious purposes.
(2)Commercial hunting of polar bears for hides began as early as the 1500s and flourished by the 1700s.
(3)Kills increased substantially in the 1950s and 1960s when hunters began using snowmobiles, boats, and airplanes to hunt polar bears. Public concern about these hunting methods led to an international agreement in 1973 banning the use of aircraft or large motorized boats for polar bear hunts. b. Hunting is the greatest single cause of polar bear mortality.
(1)Today, polar bears are hunted by native Arctic populations primarily for food, clothing, handicrafts, and sale of skins. Polar bears are also killed in defense of people or property.
(2)Hunting is government-regulated in Canada, Greenland, and the United States. Hunting is currently banned in Norway and Russia.
2. Environmental threats.
a. Oil spills from drilling platforms or tankers potentially threaten polar bears.
(1)A polar bear's fur loses its insulating properties when covered with oil.
(2)Oil spills could diminish or contaminate polar bear food sources.
b. The presence of toxic chemicals in polar bears may have long-term effects on their health and longevity.
(1)Toxic chemicals from worldwide industrial activities are carried to the Arctic by air, rivers, and oceans.
(2)Arctic animals in higher food chain levels concentrate greater amounts of toxic chemicals in their tissues than those below them. Polar bears, at the top of the food chain, develop the highest concentrations of all.
(3)Human-made toxic chemicals such as polychlorinated biphenyls (PCBs), dichlorodiphenyltrichloroethane (DDT), and chlordanes are present in the Arctic. These chemicals have been found in significantly high levels in the tissues of polar bears.
(4)Scientists continue to monitor the levels of toxic chemicals in polar bears to determine their long-term effects. c.Radionuclides, from nuclear waste dumping in the Russian Arctic, may have detrimental effects on polar bears, and the Arctic ecosystem as a whole.
E. Starvation.
1.Starvation is the greatest threat to subadult polar bears. Subadults are inexperienced hunters, and often are chased from kills by larger adults.
2.Older, weaker bears also are susceptible to starvation.
F. Disease and parasitism.
As in any animal population, a variety of diseases and parasites can be responsible for
polar bear illnesses. Polar bears are especially susceptible to the parasitic worm
Trichinella, which they contract by feeding on infected seals. Trichinella larvae encyst
in various parts of the polar bear's body, usually muscle tissue. If enough larvae encyst
in one area, such as the heart, the tissue becomes severely damaged. Death may result.
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