Mercury in the narwhal’s tusk as a sign of climate change | Polarjournal
The tusks of narwhal bulls are imposing tools with their length of up to 3 meters. For a long time it was not clear what they were for. Today we know that they are not only a dominance feature but also a sensory organ in the darkness of the Arctic Ocean. The teeth are the upper left canine. (Photo: Michael Wenger)

For high Arctic marine life such as the narwhal, climate change is a major problem. This is because they, like all highly adapted animals and plants, hardly have a chance to readapt due to the speed with which the changes are coming. However, it is not only warming or food changes that bring change, but also more mercury from human production. Researchers have now found that the highly toxic element has accumulated heavily in the tusks of narwhals, creating an additional risk factor for the animals’ survival.

The international research team led by Runde Dietz and Jean-Pierre Desforges concluded that narwhal tusks form a kind of archive about food habits and also pollutant accumulations. By studying isotopes of nitrogen and mercury, they found that animal feeding habits have changed over the past sixty years. In addition, the amount of mercury in teeth has increased dramatically over the last 20 years. The research team attributes this on the one hand to more air pollutants from Southeast Asia, which are increasingly transported to the Arctic by wind and weather changes; on the other hand, the natural mercury cycle could also have been disrupted by climate changes and melting sea ice. Thus, the amount of mercury accumulated in narwhals, depending on their age. The older an animal was, the higher the amount. “Narwhals lack the physiological properties that help eliminate environmental contaminants,” explains Rune Dietz, a professor at Aarhus University in Denmark. “They can’t get rid of mercury by forming hair and feathers like polar bears, seals, or seabirds.”

The distinctive tusks of narwhals are found mainly in bulls. The animals have only one tusk, the upper left canine, which protrudes from an opening and, like all mammalian teeth, is living material. Blood and nerve paths pass through the tooth, and it thus receives all the nutrients. This also leads to pollutants and certain chemical substances, isotopes, being deposited in the dental material, in growth layers, similar to the rings in trees. However, in order to obtain sufficient sample material, 10 tusks from northwest Greenland, more precisely from Thule and Uummanaq, which had been killed by hunters in the course of time, were examined. The animals were of different ages and thus gave a good overview of the time from 1962 to 2010. Analyses of nitrogen and carbon isotopes showed that the older animals had fed primarily on sea ice-loving animals such as polar cod and Greenland halibut until about the mid-1990s. But as sea ice began to recede, more and more pelagic fish species like capelin were on the menu.

Highly elevated levels of mercury have been found not only in narwhals. The heavy metal has also been detected in many other animals, from fish to birds to polar bears and humans, in some cases far above the limits. (Photo: Michael Wenger)

At the same time, the mercury content increased more and more. Mercury is a highly toxic element that can cause severe damage to the nerves and brain even in small amounts. “Heavy metals like mercury and other contaminants accumulate at each link in the food chain,” explains Jean-Pierre Desforges, a post-doctoral researcher at McGill University. “The higher you are in the food chain, the more mercury you accumulate in your body throughout your life.” Elevated levels have been measured not only in narwhals, but in many other animal species as well, from fish to birds to polar bears, and even in humans. In some cases, it was shown that the limit values had been far exceeded in some cases. So it’s not just narwhals, but all the inhabitants of the Arctic who have to cope not only with habitat loss, but also with ever-increasing pollution.

Narwhals hunt for fish in groups. The animals perceive their environment with their tusks on that occasion and track down the fish. In a group they can hunt better and more efficiently. But their prey pattern has changed due to climate change and melting ice. Video: Michael Wenger

Dr Michael Wenger, PolarJournal

Link to the study Dietz et al. (2021) Current Biology 31 (1-8) Analysis of narwhal tusks reveals lifelong feeding ecology and mercury exposure; DOI:

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