What Can Narwhal Tusks Tell Us About Climate Change? Quite A Lot, Scientists Say

Digitally created narwhals. (Photo by: David Fleetham/VW PICS/Universal Images Group via Getty ... [+] Images)

Narwhals have been dubbed the “unicorns of the sea” because of the striking spiral “horns” that protrude from their heads. In the medieval times, narwhal tusks were prized because they were believed to have magical powers.

There’s no evidence that narwhals have magic horns, but their tusks are playing an important role in helping scientists understand how climate change is impacting the arctic. 

Much like the trunk of a tree, narwhal tusks, which are actually enlarged teeth, grow in layers as the animals age. When the tusks are cut open, it is possible to study changes in biochemical markers that occurred throughout the animals’ lives. 

Scientists analyzed changes in mercury exposure in ten narwhal tusks purchased from Northwest Greenland, several from animals over 40 years old, for a new study published in Current Biology. The tusks were purchased from Inuit subsistence hunters.

The researchers found that narwhals' exposure to mercury increased steadily over the past four decades, although there was a temporary leveling out in the 1990s. There was a dramatic uptick in mercury exposure around 2000. The researchers concluded that sea ice decline as a result of climate change has played a role in how much mercury narwhals ingest by altering narwhal diets and increasing the bioavailability of mercury in the ocean.

During the 1990s, sea ice began to melt rapidly. Isotope signatures on the horns indicate that the fish narwhals were preying on shifted during this time period, according to Jean-Pierre Desforges, a researcher at McGill studying marine mammal toxicology and author on the new paper. Narwhals began eating more open ocean fish, rather than just fish that live in icy regions.

Narwhal (Monodon Monoceros). (Photo By Encyclopaedia Britannica/UIG Via Getty Images)

The open ocean fish narwhals eat are at a lower trophic level, or lower in the food web, compared to their previous diet of ice-associated and benthic fish. Toxins like mercury build-up in higher trophic animals as they absorb them from prey. Narwhals were eating fish with less mercury when they began including the open ocean fish in their diets.

Despite the changes in diet, mercury levels rose again around 2000. Desforges said human emissions from activities like fossil fuel combustion may have caused an increase in mercury in the arctic. 

The arctic tundra has long been referred to as a “pristine habitat,” but it hasn’t actually managed to escape humanity’s pollutants. In fact, the arctic acts as a “sink” for various industrial chemicals carried in by the wind and water currents, according to Desforges. Mercury emitted across the globe ends up in the arctic.

Warming temperatures allow bacteria in the water to turn inorganic mercury to methylmercury, which is easier for animals to absorb.

Desforges described the increase in both pollution and the bioavailability of mercury as a “double threat” in mercury exposure for top predators like narwhals.

Narwhals aren’t the only top predator in the arctic impacted by increases in mercury.

Lorrie Rea, a professor at the Water and Environmental Research Center at the University of Alaska Fairbanks, is examining mercury levels over time using Stellar sea lion whiskers. Her work has found that the proportion of sea lion pups the researchers define as at high risk of toxicological impact from mercury is increasing by 3.7% per year. 

There are documented neurological impacts in young harbor seals with high mercury concentrations in their hair, and changes in immune function and oxidative stress response in sea lions with mercury build-up.

Rea said she is excited to see that the narwhal tusk study is published.

“It is a novel tissue to look at for mercury, and it provides a great opportunity to understand the story for individual animals over time,” she said. “It was also interesting to see that this group in another part of the Arctic are also seeing this recent dramatic increase in mercury levels like we are seeing in the Aleutian Islands of Alaska in Steller sea lions.”