A new study from the University of Toronto uses polar bear feces to reveal how certain chemical contaminants can be trapped and accumulated in the body.
Polar bears tend to store certain contaminants in their bodies because they are at the top of the food chain, have high-fat diets, and have evolved to consume large amounts of fat.
“They’re like a trap for these chemicals,” he says Frank WaniaProfessor in the Faculty of Natural and Environmental Sciences at U of T Scarborough who was one of the authors of the study.
“Their intake of pollutants is very high, but their ability to eliminate them is very low.”
For the study published in the journal environmental science and technologyWania and PhD student Yuhao Chen developed a new method to study how certain chemicals called polychlorinated biphenyls (PCBs) accumulate in polar bears from contaminated food. They analyzed the food and droppings samples of polar bears from the Toronto Zoo to see how much PCB is trapped compared to how much is excreted.
Polar bears experience what is known as biomagnification, in which greater amounts of toxins accumulate further up the food chain. Since polar bears are at the top, they have ingested the highest amount of contaminants in their diet. (A polar bear has more pollutants than the seal it eats, the seal has more than one cod, the cod more than a small fish, etc.)
The researchers also found that PCBs tend to multiply at a higher rate in polar bears due to their high-fat diet and the ability of their digestive system to absorb these lipids.
While animals and humans are generally good at excreting most chemicals that shouldn’t be in the body, some contaminants have properties that make them harder to get rid of. Fat-soluble and persistent substances, including the pesticide DDT and certain types of PCBs, can accumulate in body tissues because they cannot be easily broken down or eliminated from the digestive system.
Sarra Gourlie, supervisor of nutrition at the Toronto Zoo, who provided the food and fecal samples for the study, says polar bears have evolved to ingest almost all of the fat they eat — mostly from seal blubber.
“They are able to extract about 97 percent of the fat from their food, so very little of it actually gets excreted,” she says.
Toronto Zoo’s polar bears are not fed wild seal blubber, which can be high in PCBs. Chen says polar bears living in the wild have many more pollutants than bears in zoos, which have a cleaner diet.
He says it’s important to monitor these pollutant levels because they can cause damage. Studies have linked high PCB levels in wild polar bears to lower testosterone levels, which can impair reproduction. It has also been linked to immune and endocrine disorders, which can lower survival rates.
“PCB levels have been found to exceed levels of concern to the point where they are expected to have negative impacts on polar bears living in the wild,” says Wania.
PCBs are a group of highly toxic chemicals that are banned worldwide but can linger in the environment for a long time. While the researchers only looked at PCBs in this study, they say the approach could also help monitor other chemicals trapped by biomagnification.
Non-invasive method for top predators and humans
The traditional method of studying biomagnification relies on analysis of tissue, which can only be done on dead animals or humans. As a result, there is virtually no research on biomagnification in humans or endangered apex predators. The researchers hope that the method they have developed can also be applied to other top predators in the wild, such as lions or tigers.
It could also be applied to humans. Chen says he’s currently studying the analysis of food and fecal samples from different people to see how pollutants can biogrow in the body and whether there are differences between individuals.
“We know these pollutants are present in humans, and we know how the body absorbs these pollutants,” he says.
“What we don’t know is how much of these contaminants are stored or excreted by our bodies when we eat a given amount of contaminated food.”