Already imperiled by melting ice and a brew of toxic chemicals, polar bears throughout the Arctic, particularly in remote dens near the North Pole, face an additional threat as flame retardants originating largely in the United States are building up in their bodies, according to an international team of wildlife scientists.
The flame retardants are one of the newest additions to hundreds of industrial compounds and pesticides carried to the Arctic by northbound winds and ocean currents. Accumulating in the fatty tissues of animals, many chemicals grow more concentrated as larger creatures eat smaller ones, turning the Arctic's top predators and native people into some of the most contaminated living organisms on Earth.
In urban areas, particularly in North America, researchers already have shown that levels of flame retardants called polybrominated diphenyls, or PBDEs, are growing at a rapid pace in people and wildlife. Although they have been found in much lower concentrations in the Arctic, scientists say their toxic legacy will persist there for years because they are slow to break down, particularly in cold climates.
In polar bears, the effects are unknown. But in tests on laboratory animals, PBDEs disrupted thyroid and sex hormones and damaged developing brains, impairing motor skills and mental abilities, including memory and learning.
Scientists say that other industrial chemicals with properties similar to PBDEs are already weakening the bears' immune systems, altering their bone structure, skewing their sex hormones and perhaps even causing small numbers of hermaphroditic bears.
What remains uncertain, however, is whether those physiological changes are killing bears or reducing their populations. Some experts suspect that many cubs, which are contaminated by their mother's milk, are not surviving.
An even more immediate threat to the world's 20,000 to 25,000 polar bears is global climate change, which is melting their hunting grounds. Bears in Canada's western Hudson Bay -- the most well-researched population -- declined from 1,100 in 1995 to fewer than 950 in 2004. In Alaska, wildlife biologists for the first time have documented that polar bears are drowning. Scientists predict that some populations could become extinct by the end of the century as more sea ice melts.
Derek Muir of Canada's National Water Research Institute, who led the new research, said the geographical patterns in contamination suggest that the East Coast of North America and northwestern Europe are the primary sources of the flame retardants.
