Longevity & Causes of Death

Polar Bears

Longevity & Causes of Death

Longevity

  1. Polar bears can live 20 to 30 years, but only a small percentage of polar bears live past 15 to 18 years.
  2. The oldest known polar bear in the Arctic lived 32 years. The oldest known polar bear in a zoological park lived 45 years.

Aging Studies

  1. Each year, as a polar bear grows, a thin layer of cementum is added to 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.

Predators

  1. Adult polar bears have no natural predators except other polar bears.
  2. Cubs less than one year old sometimes are prey to wolves and other carnivores.
  3. Newborn cubs may be cannibalized by malnourished mothers or adult male polar bears.

Intraspecific Mortality

  1. On rare occasions, males kill other males while competing for mates. Males also periodically kill females protecting cubs. Their motives aren't clear, but in some cases part of the carcasses have been eaten, so extreme hunger could be an explaination.

Human Impact

  1. Hunting.
    • Polar bears have been hunted for thousands of years.
      • 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.
      • Commercial hunting of polar bears for hides began as early as the 1500s and flourished by the 1700s.
      • 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.
    • For the last several decades, hunting has been the greatest known cause of polar bear mortality.
      • 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.
      • Hunting is government-regulated in Canada, Greenland, and the United States. Hunting is currently banned in Norway and parts of Russia.
  2. Environmental threats.
    • Climate change impacts in the Arctic, including increasing temperatures, shifts in the distribution of multi-year ice, and changes in precipitation patterns, threaten polar bear populations in a number of ways.
      • Increasing temperatures are associated with a decrease in sea ice (both the amount and length of time sea ice forms).
      • Since polar bears use sea ice as a platform to prey mainly on ringed and bearded seals, changes in sea ice can lead to less access to prey and an increase in malnourished or starving bears.
      • Because of their strong association with the ice, ringed seal populations may also plummet due to loss of sea ice and result in even further reduction in polar bear food supplies.
      • In addition, with less sea ice, female polar bears may have a more difficult time reaching traditional denning areas.
    • Oil spills from drilling platforms or tankers potentially threaten polar bears.
      • A polar bear's fur loses its insulating properties when covered with oil.
      • Oil spills could diminish or contaminate polar bear food sources.
      • Scientists are trying to determine whether noise from industrial subsistence or recreational activities could disturb polar bears and deter them from important habitats.
    • The presence of toxic chemicals in polar bears may have long-term effects on their health and longevity.
      • Toxic chemicals from worldwide industrial activities are carried to the Arctic by air currents, rivers, and oceans.
      • Arctic animals in higher food chain levels concentrate greater amounts of toxic chemicals in their tissues than those below them. Because polar bears are top predators in the arctic, they are exposed to especially high levels of toxic chemicals.
      • Human-made toxic chemicals such as heavy metals, polychlorinated biphenyls (PCBs), dichlorodiphenyltrichloroethane (DDT), and organochlorines are present in the Arctic. Arctic seals have particularly high levels of organochlorines concentrated in their blubber. Since polar bears mainly feed on seals, organochlorines become even more concentrated in the fat layer of polar bears. This potentially leads to a higher than normal mortality rate of polar bear cubs who acquire these toxins from their mother's milk.
      • Scientists continue to monitor the levels of toxic chemicals in polar bears to determine their long-term effects.
    • Radionuclides, from nuclear waste dumping in the Russian Arctic, may have detrimental effects on polar bears, and the arctic ecosystem as a whole.

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.
  3. Some polar bear populations have shown increasing evidence of food deprivation including an increase in the number of underweight or starving bears, smaller bears, and fewer cubs that don't survive for as long. This takes place in areas that are experiencing shorter seasons with access to sea ice, reducing the bears' ability to hunt seals.

Disease and Parasitism

  1. 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 roundworm 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.