Between climate change, human conflict and habitat loss, polar bears have more than enough to worry about. But it turns out these vulnerable conservation icons also face a more unlikely threat: industrial chemicals that were banned almost 40 years ago. A new paper in the journal Proceedings of the Royal Society B finds that these and other contaminants still find their ways into the bodies of polar bears, where they can wreak havoc on their reproductive systems and fertility today.
The main chemical culprits in question are PCBs, or polychlorinated biphenyls, manmade chemicals that got their start in an Alabama chemical plant in the 1920s, and were later manufactured around the world. In 1979, PCBs were banned over fears that they posed a threat to human and environmental health. Yet research in recent years reveals how these contaminants persist in the environment for decades, and may have traveled all the way to the testes and sperm cells of polar bears.
It goes without saying that fertility is crucial to any species’ survival. But it’s especially crucial for polar bears, because they have one of the slowest reproductive rates of any mammal. If a female unknowingly mates with an infertile male one year, she’ll be out of luck for that breeding season—one of only about five she has in her lifetime. In other words, any threat to polar bear fertility holds the potential to thwart population growth in a big way.
“The biggest danger of such a secret infertility is that the males are actually taking up the chance for a female to conceive,” says Viola Pavlova, an ecologist with the Academy of Sciences in the Czech Republic and lead author on the paper. “So it is very important for the females to have enough males available to breed with.”
PCBs could impact fertility in polar bears and other top Arctic predators in a variety of ways. As hormone disruptors, they can interfere with normal hormone function in the body in both animals and in humans. In humans, researchers have found a correlation between high PCB exposure and changes in semen motility and quantity in humans, amongst other effects including liver damage and skin diseases. Worse, they are stable, meaning they can persist in the environment for decades.
The compounds—of which there are 209 different types—were first produced commercially by the Swann Chemical Company in Anniston, Alabama. Chemical giant Monsanto later bought that company and became the primary producer of the product in the U.S., while other companies around the world began producing it as well. By 1979, PCB production was banned completely in the U.S.—but by then, roughly 150 million pounds had already been dispersed in the environment. That’s the equivalent weight of roughly 8,000 school buses.
Many of those particles rode air and ocean currents up to the Arctic either dissolved in water or attached to silt, and traveled into the gills of fish or the mouths of bottom-dwelling marine creatures feeding in silty areas, researchers have found over the years. The compounds accumulate in animals’ fat cells, and can stay there for a lifetime.
When a top predator like a polar bear eats a fatty seal that has eaten a contaminated fish, the polar bear will ingest the highest dose of PCBs than any other animal in the food chain. Animals at the top of the food web therefore suffer the brunt of the contamination, and Arctic animals suffer especially because they rely on a fatty, energy-rich diet to survive in their harsh environment. (This process, known as biomagnification, is the same reason why larger, predatory fish like tuna have a higher risk of being contaminated with mercury.)
Pavolva’s team focused on populations of polar bears in East Greenland and Svalbard from the mid-1990s, which had shown signs of slowed population growth and were known to have had elevated levels of PCBs. To assess the role that PCBs may have had, her team first analyzed data on PCB impact on rodent fertility. Then, they used a computer model to extrapolate how a similar impact could thwart polar bear populations based on their mating patterns.
(While directly studying effects of PCBs on polar bear testes and fertility would have been ideal, this was not a possibility for the study, Pavlova says. “It’s a vulnerable species, it’s protected,” she explains. “It would be very difficult to do anything like that in captivity and getting that from wild animals is fairly impossible.”)
This is the first time anyone has studied how PCBs may be impacting male polar bear fertility. Previous research has focused on female fertility, but males tend to contain higher concentrations of PCBs than females do, likely due to in part to breast milk consumption, Pavlova says. “The cub gets contaminated, but on the other hand the female gets rid of some of its burden (when she breast feeds),” she says. “The males can’t get rid of the contamination that easily.”
The researchers found that PCBs could indeed have impacted male fertility by making infertile males unviable mates. The mechanism by which the males could become infertile from contamination is unclear; it could include a range of hormone system effects, or direct impacts on the quality of the sperm itself, says John Meeker, a researcher at the University of Michigan who studies the impact of environmental contaminants on human health. Other studies of hormone disrupting chemicals suggest weakening of the penile bone in polar bears, along with other bones, suggesting that it has other health consequences that reduce the overall fitness in both males and females.
Meeker, who was not involved in this study, points out that pinpointing the pathways to health impacts is complicated by the large number of different PCBs. “Even within a specific class they might have different mechanisms,” says Meeker. “It could be wide ranging.”
Concentrations of the compound have decreased in the environment over time, but lingering sources can still reach animals and humans today, says Meeker. Of particular concern for humans are places like toxic waste sites containing contaminated materials, but also old buildings—including schools built in the 1950s to the 1970s that could have sources of PCBs stuck in outdated materials including caulking and fluorescent light ballasts.
But the most common avenue of human exposure is through food consumption, particularly fatty fish, Meeker says. Whereas humans can generally choose whether or not to eat seafood, top marine predators including seals and dolphins cannot. Inuit communities whose traditional diet includes these top predators have also shown evidence of elevated exposure to the chemicals.
Of all predators, killer whales show the highest levels of PCBs, higher even than those of polar bears, says study author Christian Sonne, a biologist at Aarhus University in Denmark. “Their liver can’t metabolize the same way as polar bears and they can’t excrete the same way,” explains Sonne, pointing out that killer whales don’t make fur, which is an important excretion route of the compounds for polar bears.
Now, as climate change reroutes ocean currents and sea ice patterns, Arctic food webs and accumulation patterns of PCBs could shift as well. Increased starvation could also allow PCBs to accumulate in animals’ blood, having unknown health consequences, Sonne says.
The effects of PCBs on polar bears are just one of many ways that so-called hormone disrupting chemicals may impact wildlife and humans—and a reminder that new chemicals produced today take a toll in other unforeseen ways. Even as older chemicals have been removed from products over time, new chemicals have taken their place in a sort of conveyor belt of change, says Robert Letcher, an environmental chemist at Carelton University in Canada.
Letcher adds that not all of these chemicals have proven to have negative effects on animals, but “the ones that persist in the top predators are the ones that are worrisome because they have survived the gauntlet of processes that can break them down.” Studies like Pavlova’s, he says, are important in better understanding the effect this could have on polar bears and other species in the long run. “If its fertility and ability to reproduce has been somehow impacted in a negative way, then you are putting into question the ability of this species to carry on,” says Letcher.
And in today’s world, he adds, there is no safe place for a species to run, swim or fly to. “There is no pristine environment,” says Letcher. “There is no utopic population of marine mammal that has escaped. It’s universal.”