A new study from the University of Toronto at Scarborough using polar bear poo reveals why certain chemical contaminants can get trapped and build up inside the body.
Polar bears tend to store certain contaminants in their bodies because they are at the top of the food chain, eat very fatty diets, and have evolved to absorb large amounts of fat.
“They’re like a trap for these chemicals,” says Frank Wania, a professor in the department of physical and environmental sciences at U of T Scarborough who was one of the study’s authors.
“Their intake of contaminants is very high, but their ability to expel them is very low.”
For the study published in the journal Environmental science and technology
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Wania and PhD student Yuhao Chen developed a new method to study how certain chemicals, called polychlorinated biphenyls (PCBs), accumulate inside polar bears from contaminated food. They analyzed the diet and fecal samples of polar bears from the Toronto Zoo to see how many PCBs are trapped versus how much is excreted.
Polar bears undergo what is called biomagnification, where greater levels of toxins accumulate higher up the food chain. Since polar bears are at the top, they consumed the highest level of contaminants in their diet. (A polar bear has higher levels of contaminants than the seal it eats, the seal has more than a cod, the cod has more than a small fish, etc.)
The researchers found that PCBs tend to biomagnify at a higher rate in polar bears due to their high fat diet (fat is a type of lipid) and the ability of their digestive system to absorb these lipids.
While animals and humans are generally good at expelling most chemicals that shouldn’t be in the body, some contaminants are more difficult to eliminate due to their properties. Those that are fat soluble and persistent, including the pesticide DDT and some types of PCBs, can accumulate in body tissues because they cannot be broken down or easily excreted through the digestive system.
Sarra Gourlie, a nutritional science supervisor at the Toronto Zoo who provided the food and fecal samples for the study, says polar bears have evolved to absorb almost all the fat they eat, mostly seal fat. .
“They are able to extract about 97% of fat from their diet, so very little of it is excreted,” she says.
Polar bears at the Toronto Zoo are not fed wild seal blubber, which can contain high levels of PCBs. Chen says polar bears living in the wild have much higher levels of contaminants than those in the zoo, which have a cleaner diet.
He says it’s important to monitor these levels of contaminants because of the damage they can cause. Studies have linked high levels of PCBs in wild polar bears to lower levels of testosterone, which can impact reproduction. It has also been linked to disruptions in the immune and endocrine systems, which can reduce survival rates.
“The levels of PCBs have been found to exceed levels of concern to the point where they are expected to negatively impact polar bears living in the wild,” adds Wania.
PCBs are a group of highly toxic chemicals that have been banned globally, but can persist 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 large predators and humans
The traditional way of studying biomagnification relies on tissue analysis, which can only be performed 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 be used on other large predators in nature, such as lions or tigers.
It could also be used on humans. Chen says he is currently looking to analyze food and fecal samples from different people to see how contaminants can bio-magnify inside the body and if there are differences between individuals.
“We know these contaminants are in humans and we know how the body absorbs these contaminants,” he says.
“What we don’t know is how much of these contaminants are stored or expelled by our bodies when we eat a certain amount of contaminated food.”
Reference: Chen Y, Lei YD, Wensvoort J, Gourlie S, Wania F. Probing the thermodynamics of biomagnification in zoo-housed polar bears by equilibrium sampling of food and fecal samples. Environ Sci Technology. 2022. doi: 10.1021/acs.est.2c00310
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