A new University of Toronto study uses polar bear faeces to reveal how certain chemical pollutants get trapped – and build up – inside the body.
Polar bears tend to store some pollutants in their bodies because they are at the top of the food chain, have a very fatty diet and have evolved to absorb large amounts of fat.
“It’s a trap for these chemicals,” says Frank Wanya, a professor in the Department of Physical and Environmental Sciences at T University Scarborough, who was one of the study’s authors.
“Their intake of pollutants is very high, but their ability to expel it is very low.”
For the study published in the journal Environmental science and technology, Wanya and Ph.D. Student Yuhao Chen has developed a new method for studying how certain chemicals, called polychlorinated biphenyls (PCBs), build up inside polar bears from contaminated food. They analyzed the diet and stool samples of polar bears from the Toronto Zoo for the amount of PCBs trapped compared to the amount excreted.
Polar bears experience what is known as biomagnification, where higher levels of toxins accumulate higher up the food chain. Since polar bears are at the top, they have consumed the highest level of pollutants in their diet. (Polar bears have higher levels of contaminants than seals they eat, seals contain more cod, more cod than small fish, etc.)
The researchers also found that PCBs tended to biomagnify at a higher rate in polar bears due to their high-fat diet and the ability of their digestive system to absorb those fats.
While animals and humans are generally good at flushing out most chemicals that shouldn’t be in the body, some pollutants are difficult to get rid of due to their properties. 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 excreted in the digestive tract.
Polar bears have evolved to absorb nearly all of the fat they eat — mostly seal blubber, says Sarah Gurley, a superintendent of nutritional sciences at the Toronto Zoo, who provided the food and fecal samples for the study.
“They are able to extract about 97% of the fat from their diet, so very little of it is actually excreted,” she says.
Polar bears at the Toronto Zoo do not feed on wild seal blubber, which can contain high levels of PCBs. Polar bears living in the wild have much higher levels of pollutants than those in the zoo, which has a cleaner diet, Chen says.
He says it’s important to monitor levels of these pollutants because of the damage they can cause. Studies have linked high levels of PCBs in wild polar bears to lower levels of testosterone, which may affect reproduction. It has also been linked to disorders of the immune and endocrine systems, which may reduce survival rates.
“Polychlorinated biphenyls have been found to exceed alarming levels to the point where they would be expected to have a negative impact on polar bears living in the wild,” Wanya says.
PCBs are a group of highly toxic chemicals that have been banned globally, but can persist for a long time in the environment. While the researchers only looked at PCBs in this study, they say this approach may also help monitor other trapped chemicals through biomagnification.
Non-invasive method for predators and humans
The traditional method for studying biomagnification is based on tissue analysis, which can only be performed on dead animals or humans. As a result, there is virtually no research on biological amplification in humans or major endangered predators. The researchers hope that the method they have developed can be used on other predators in the wild such as lions or tigers.
It can also be used on humans. Chen says he’s currently looking at analyzing food and stool samples from different people to see how pollutants can biomagnify within the body and whether there are differences between individuals.
“We know that 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 pollutants are stored or expelled by our bodies when we eat a certain amount of contaminated food.”
High-protein diet may harm polar bears
Yuhao Chen et al., Investigation of the thermodynamics of biomagnification in zoo-dwelling polar bears by equilibrium sampling of food and stool samples, Environmental science and technology (2022). DOI: 10.1021 / acs.est.2c00310
Presented by the University of Toronto
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