Narwhal Tusks Show Climate Change and Pollution Reshape the Arctic

There’s no question that we’re messing up the oceans big time. But shocking new research shows the world’s addiction to fossil fuels is changing the bodies of big ocean predators like narwhals. A study published this month in Current Biology uses narwhal tusks to examine how the animals store mercury over time, finding that levels of the pollutant have shot up in the whales’ bodies over the past two decades.

Narwhals are definitely one of the funnier looking critters in the sea due to their tusks, which spiral out, unicorn-like, from the front of their heads. While the tusks may resemble horns, they’re actually just overgrown front teeth. (There’s even records of some male narwhals sporting two horns.)

These tusks also contain valuable information on the life of each narwhal. Because they’re connected to the bloodstream of the whale, each new growth layer contains valuable information, telling a story about what the animal ate that year and any exposure it may have had to pollutants. Since narwhals can live up to 50 years, tusks can cover a lot of ground and be a pretty valuable historic record. The best analogy, said study author Jean-Pierre Desforges, a postdoctoral fellow at McGill University, is that of a tree ring.

“Each of the individual layers in a tree gives you a lot of information about the condition of the tree in that year of growth,” he said. “It’s the exact same way with a narwhal tusk. We can count up [the layers] and get a number on how old the animal is, and we can link each individual layer to a date in time, broadly speaking, to a year. If the animal is 50 years old, we can count 50 layers in a tusk, and date it back all the way to 1960.”

This specificity is important when analyzing the exposure to pollutants of a top-level predator like a narwhal, especially when it comes to something like mercury. Once it gets in, mercury stays in the food chain, entering the bloodstream of each successive predator that eats a smaller animal tainted with mercury. For predators like narwhals, which traditionally dine on larger fish like halibut and cod, that means they can accumulate a lot of mercury in their bloodstream.

In the study, researchers looked at tusks ranging in length from just under 5 feet (1.5 meters) to 8 feet (2.5 centimeters) from 10 animals living off the coast of northwest Greenland. Some tusks had layers dating back to 1962, enabling the researchers to cover a nearly 50-year span. They found that small changes in the mercury in the tusks happened from 1990 to 2000, correlating with a change in the whales’ diet—but that the levels really started to skyrocket after the new millennia.

Human fossil fuel use is driving a lot of changes to the narwhals’ environments that could affect their tusks. First, there’s how their physical environment is changing. Until the mid-1990s, most of narwhals’ hunting grounds were covered with ice, but Arctic sea ice took a sharp turn for the worse a few decades ago.

The narwhals’ diet shifted as the sea ice did, moving from the larger fish that lived near sea ice in their traditional diet to smaller fish that live on the open ocean, like species of smelt. These new fish are of a step down on the food chain than the bigger species, which should suggest that the narwhals’ tusks would contain less mercury.

Narwhal tusk records match “really well with sea ice records,” Desforges said. “The fact that our dietary patterns [found in the tusks] matched that sea ice pattern was telling to us that there’s probably some sort of climate change affect.”

But then there’s mercury emissions themselves. Human activity—including the use of coal-fired power—emits thousands of tons of mercury into the atmosphere each year, which makes its way into the oceans and enters the aquatic food chain. And while the retirement of coal-fired power plants in Europe and North America have helped lower some of those emissions, the continued use of coal in China and India is still cause for concern. Some evidence suggests that increasing ocean temperatures can drive up mercury levels in fish—even as we’re decreasing the amount of mercury we’re spewing into the air.

It’s this last point that Desforges said is the most concerning: The level of mercury in the narwhals’ tusks shot up even as their new diet should have made mercury levels stay on the lower end. That may suggest larger charges are happening with how mercury is moving around the planet’s oceans as well as human influence in the ocean.

“In our study, we show that mercury changes over time, due to dietary factors, but after the year 2000, the mercury pattern shifts away from a strong association with diet and it goes more toward the human impact angle,” he said. “We’re seeing changes in mercury that are disassociated with diet, meaning that humans are having an impact on mercury [in the ocean], especially in recent decades.”

And this rapid biological response to human influence has serious implications for species up and down the food chain, Desforges said. That’s true even for top predators who may think they have nothing to worry about (cough, cough, humans).

“We’re adding to a growing body of literature in Arctic research that climate change is not only affecting the physical environment, but that impacts translate all the way up the food web to top predators that live for a long time,” Desforges said. “These are slow-adapting species, low reproductive rates—but they are responding quickly to this rapid change in the environment.”