As a species, polar bears are under many threats to their survival and the stress may be taking a toll on individual bears’ growth. Comparing bear skulls from the early part of the 20^th century (1892-1939) to skulls from the latter half (1961-2002) shows changes in size and shape.

The skulls from the later period were between 2 percent and 9 percent smaller, according to a team from the Department of Arctic Environment, Aarhus University in Denmark.

The bear’s environment in the two periods differed in sea ice extent and pollution levels. In the latter half of the century, industrial pollutants built up in the Arctic and the extent of sea ice decreased. Pollutants included compounds containing carbon and halogens (fluorine, chlorine, iodine, bromine). Although some of the compounds have been phased out of use in homes and industry, many are still used. The pollutants also contribute to reduced bone mineral density in the bears, leaving them vulnerable to injury and osteoporosis.

Christian Sonne, a veterinary scientist on the team, said, “Polar bears are one of the most polluted mammals on the globe.”

The loss of sea ice means bears have to use more energy to hunt, decreasing the energy available for growth. In addition, being hunted may be related to a depleted gene pool and inbreeding.

The researcher reported their findings in the August issue of the Journal of Zoology. Their findings were also the subject of a BBC News story in the August 25 issue, ‘Stress’ is shrinking polar bears .

The Polar Bear Specialist Group of the International Agreement for the Conservation of Polar Bears held its 15^th meeting in Denmark this summer and in its resolutions called for international study of the effects of global warming and pollution on the species.