Bechshoft, T.; Derocher, A. E.; Richardson, E.; Lunn, N. J.; St. Louis, V. L. Hair Mercury Concentrations in Western Hudson Bay Polar Bear Family Groups. Environ. Sci. Technol. 2016. DOI: 10.1021/acs.est.6b00483
The Problem with Polar Bears:
The accumulation of toxic methylmercury is a serious threat to wildlife all over the world – especially top predators in polar regions, like polar bears. While humans know to be careful not to consume too much fish high in mercury, polar bears have no diet options besides marine mammals that contain high levels of the toxin. Young polar bears are often the most vulnerable to detrimental effects of pollutants, which include neurological impairment and hormonal disruption that prevents normal development and lowers their chances of successfully reproducing.
To learn more about levels of mercury in young polar bears, scientists from the University of Alberta and Environment and Climate Change Canada’s Wildlife Research Division acquired samples of hair from 33 young polar bears (2 years of age or less) and their mothers in Western Hudson Bay. They measured levels of total mercury in the hair to determine how polar bear cubs’ exposure to mercury compared to that of their mothers. This is one of the first studies to measure mercury in very young (< 1 year-old) polar bears, and to explore differences in mercury levels as cubs age.
While mercury is a naturally-occurring element, human activities, including combustion of coal, incineration of waste, and artisanal mining activities have released huge amounts of it from within the Earth, making it available to microorganisms that can convert it to its toxic, bioaccumulative form: methylmercury.
Studies have shown that methylmercury biomagnifies. This means that while concentrations in phytoplankton at the base of the Arctic foodweb are quite low, it builds up to higher concentrations in fish, and even higher concentrations in top predators who prey on fish or marine mammals. This means top predators like polar bears accumulate much higher levels of methylmercury than other creatures, and are at a disproportionate risk for adverse effects.
Biomagnification is especially worrisome in remote, sparsely-inhabited regions like West Hudson Bay, where many animals are already struggling due to other stressors, like habitat loss and prey scarcity due to climate change. It is crucial to understand how levels of toxic contaminants in mothers and their cubs are related, and what levels of methylmercury cubs are exposed to, so that we can better predict how bear populations will be affected by these pollutants and try to curb future exposures.
Methylmercury from far-away sources accumulates in Arctic environments, where it enters the foodweb that polar bears depend on. Before giving birth, mother bears transfer methylmercury along with blood to the young they are carrying. After giving birth, mothers continue to deliver mercury to their young along with their milk.
Stealing Hair Out From Under a Bear:
Scientists traveled to Western Hudson Bay, Canada and collected hair samples from tranquilized bears. Hair analysis was an ideal way to investigate contaminant exposure among polar bears, because it’s not very invasive and doesn’t affect the wellbeing of the bears. They managed to obtain samples from 24 different polar bear families with a total of 33 young polar bears.
The researchers measured total levels of mercury in the hair of mothers and different-aged young. Previous studies have shown that the majority of the mercury (~ 97%) found in hair samples is methylmercury, which is the main form the researchers were interested in, due to its toxicity. They also measured the age of the bears by counting layers of tooth growth, and estimated the weight of the bears based on their girth and body length, seeing as polar bears are a bit too heavy for your average scale.
What’s in the Hair of a Polar Bear?
The authors found mercury in all mothers and their young. In some mothers, levels were high enough to surpass previously estimated levels where neurological effects occur, meaning that some of these polar bears may be contaminated enough to be experiencing toxic effects.
The authors compared mercury levels in mothers and their young, considering factors like weight, age, and sex of each bear. They arrived at a couple of key findings, namely:
(1) Mercury levels increased with age. As shown in Figure 1, the researchers found that adult polar bears had the highest levels of mercury in their hair, while very young (one-year ore less) cubs had the lowest levels. The authors explained that this was, in part, probably due to food intake, as adults eat much more prey, exposing them to higher levels of mercury with ever meal.
(2) Mercury levels in the very young are strongly related to mothers’ mercury. Young cubs are still drinking their mothers’ milk, and have been receiving all of their nutrients from their mothers’ body. It follows, then, that their mercury levels would be closely related to those of their mother. As they grow and begin to eat on their own, mercury levels between mothers and their young become less strongly related.
(3) Harder-working bears have higher mercury levels. The researchers found that mothers with larger litters tended to have higher mercury levels (Figure 2). They also found that young male polar bears had higher mercury levels than their female counterparts. The researchers hypothesized that this might be explained by differences in energy expenditures: Mothers with larger litters have a bigger job, and so they expend more energy, and they have to eat more prey to catch up. Eating more prey means eating more mercury, leading to higher levels in these hard-working moms. Likewise, young males bears are likely more active, and growing faster, than females, so they are also likely to eat more prey, and more mercury along with it.
Arctic species are particularly vulnerable to toxic, bioaccumulative contaminants because of their tight-knit food webs and because they tend to eat a lot of prey that is already highly contaminated, like marine mammals and large fish that are near the top of the marine food web. We are well aware that polar bears are a struggling species due to multiple stressors, including habitat loss and prey scarcity due to climate change, as well as toxic effects of chemical contamination. However, there’s still a lot we don’t understand about how contaminants are affecting bears’ behavior, or what hand they might have in population declines.
As one of the few studies to investigate levels of mercury among young polar bears, this research helps us begin to understand how bear behavior affects levels of toxic chemicals, and also brings attention to the plight of these mighty polar predators.