Polar bears are among the most familiar faces of climate change. The iconic bears, which live and hunt on dwindling Arctic sea ice, became an officially threatened species in 2008 and continue to show declining numbers.
But the bears may be facing another threat, from pollutants that have been banned for decades. A study published in the January issue of Environmental Research linked elevated levels of PCBs to lower density in male bears' penile bones, which may disrupt the bears' reproductive abilities.
"Polar bears are, except for killer whales, probably the most polluted animals on our globe," Christian Sonne, a biologist at Aarhus University in Denmark and lead author on the research, told VICE News. "That's the magnitude we're talking."
PCBs, or polychlorinated biphenyls, were introduced in the early 20th century for industrial purposes, but were banned in the United States in the late 1970s due to environmental and human health concerns. Because they can be stored in body fat, they're found in higher concentrations in animals further up the food chain, and in animals like polar bears that have a high-fat diet.
Sonne's team gathered samples from the penile bones of 279 polar bears born between 1990 and 2000. The majority of the bones came from seven sub-populations of bears in Canada, while 34 came from a population in eastern Greenland. Hunters generally turn the bones over to local authorities as proof of a kill.
They then scanned the bones using a technique called dual X-ray osteodensitometry and compared the resulting bone densities to previous data on PCB levels in polar bears in Canada and Greenland. They found that higher PCB concentrations were correlated with lower bone densities, with the Greenland population suffering the worst levels of pollution.
"These pollution levels are above thresholds for adverse health effects on immune and reproductive systems," Sonne told VICE News. " It's a matter of their properties being endocrine disrupting and also cytotoxic [toxic to cells], and there are literally thousands of different pathways that can impact."
'We want to make sure that we don't approve chemicals that will become the DDTs and the PCBs of the future.'
Lower density in the penile bone could mean a higher potential for fractures and less success in mating. And the penile bone is not the only aspect of polar bear reproduction affected by contaminants like PCBs, which are in a class of chemicals known as endocrine disruptors. Other studies have linked such pollutants to smaller testes and ovaries, increased cub mortality, and psuedohermaphroditic bears — female bears with small penises.
"The density of the penile bone is a proxy of mating success," Shaye Wolf, Climate Science Director for the Center for Biological Diversity, told VICE News. "There's a whole slew of harm to the polar bears' reproductive health that has been linked to these contaminant loads. This study provides more evidence and more support that contamination from industrialized nations is harming Arctic wildlife."
Though there's not much to be done about chemicals that were banned decades ago, such evidence could affect how we test and approve new chemicals now, Wolf said. Pollutants produced in industrialized nations can travel to the Arctic via air and ocean currents, and it's unclear what kind of effect they'll have on wildlife.
"We want to make sure that we don't approve chemicals that will become the DDTs and the PCBs of the future," Wolf told VICE News.
Though PCBs are not a pollutant related to climate change, the challenges brought by a warming world can exacerbate the damage they cause. When polar bears can't find enough food, they burn more of their blubber, releasing PCBs into the bloodstream, where they become more toxic.
Polar bears in eastern Greenland have been shifting their diets to consume more sub-Arctic hooded and harp seals, which in general have a higher concentration of pollutants than the Arctic ringed seals they traditionally eat. A 2013 study found that this dietary shift resulted in slower declines in the levels of pollutants in the bears' fatty tissues.
The pollutants can also impair the polar bears' immune systems, making them more susceptible to new diseases and parasites, Sonne said. All of this means that the presence of PCBs might make life even tougher for polar bears struggling to adapt to a warmer Arctic.
"They really kind of work together," Sonne told VICE News. "I would call it a flank attack, just like a wall where you attack from both sides. They maximize each other."
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