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Narwhal tusks offer insights into Arctic's changing climate, shifting biodiversity

Narwhal tusks could provide a view into the earlier lives of the animals, offering insights into how they have survived as the climate changes around them, researchers say. Photo by Paul Nicklen/Current Biology
Narwhal tusks could provide a view into the earlier lives of the animals, offering insights into how they have survived as the climate changes around them, researchers say. Photo by Paul Nicklen/Current Biology

March 11 (UPI) -- The tree ring-like layers of the Narwhal tusks have offered scientists fresh insights into the shifting dynamics of Arctic ecosystems, revealing the influence of climate change on the region's sea ice and biodiversity.

The tusk of the narwhal is actually a canine tooth that extends from the upper jaw of male animals. Each year, as the tusk grows, the newest layer preserves aspects of the animal's physiology -- clues as to what the narwhal was eating and how it was responding to changes in the environment.

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Because narwhals can live upwards of 50 years, their tusks can help scientists reconstruct ecological shifts and climate change across several decades.

In a new study, published this week in the journal Current Biology, scientists analyzed the mercury levels, as well as the stable isotopes of carbon and nitrogen, trapped in the layers of several narwhal tusks.

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The analysis showed declines in sea ice coverage of the last several decades have altered the makeup of Arctic food webs.

"The higher you are in the food chain, the more mercury you accumulate into your body throughout your life," co-author Jean-Pierre Desforges said in a news release.

"Heavy metals and other environmental contaminants accumulate at each link in the food chain, so if you are at the top of the food chain, you end up consuming the greatest amount of mercury at each meal," said Desforges, a postdoctoral research fellow at McGill University in Canada.

Changes in sea ice coverage and ocean temperatures have altered the makeup of fish and marine mammals found in Arctic ecosystems, and thus, altered the composition the narwhal's diet.

"We have been able to trace this development in the narwhals' tusks. In each layer of the tusk, we measured the amount of mercury, just as we measured stable isotopes of carbon and nitrogen -- the so-called delta-13-C and delta-15-N," said lead study author Rune Dietz, professor at the Arctic Research Center at Aarhus University in Denmark.

The analysis showed that prior to 1990, the diet of narwhals living of the northwestern coast of Greenland consisted primarily of species closely linked with sea ice, including halibut and Arctic cod.

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After 1990, as sea ice coverage declined, narwhals became more and more dependent on open ocean species, including capelin and polar cod.

Additionally, around 2000, scientists found a marked increase in mercury levels in the narwhal tusks. The increase was unrelated to a shift in diet -- researchers estimate the cause was an increase in mercury pollution from coal combustion in Southeast Asia.

According to Dietz and his colleagues, the latest findings are concerning.

"The narwhal is the Arctic mammal most affected by climate change," he said. "At the same time, whales lack the physiological properties to eliminate environmental contaminants."

"They don't get rid of mercury by forming hair and feathers like polar bears, seals and seabirds, just as their enzyme system is less efficient at breaking down organic pollutants," Dietz said.

But while the latest paper confirmed the significant impacts of climate on Arctic food webs, the data analysis also showed the diets of narwhals are surprisingly adaptable.

The research also sets the stage for ongoing monitoring efforts.

"With our new discoveries, we now know that there is a bank of data in the narwhal tusks found in museums around the world," Dietz said.

"By analyzing them, we can hopefully get an insight into the narwhals' food strategy from different areas and periods many years back in time. This will provide us with a solid basis for evaluating how the species copes with the changed conditions that it now encounters in the Arctic," Dietz said.

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