Science

Narwhal tusks, a stunning biological indicator

A study published in the journal Current Biology showed the dietary evolution of narwhals thanks to information contained in the tusks of ten narwhals in northwest Greenland, between 1962 and 2010. It is by measuring the level of mercury and certain isotopes of carbon and nitrogen present in this “horn” that researchers were able to follow the evolution of the environment and the diet of these cetaceans.

Mercury: between toxic substance and food indicator

Mercury is a volatile compound produced mainly by activities of human origin, such as metallurgical industries, mining or even energy production. It can also come from nature. Volcanic eruptions are a perfect example. Thus, this heavy metal – when it is not already present in the oceans – is first released into the atmosphere before some of it ends up in aquatic environments.

Mercury, in addition to being extremely polluting, is also a neurotoxic substance. According to Government of Canada site,methylmercury is the most common organic form of mercury in the environment. Certain biological processes such as bacterial activity in plants and sediments at the bottom of lakes, rivers and oceans can transform elemental mercury into methylmercury, the most toxic and bioaccumulative form. Methylmercury levels in animals increase down the food chain, from plankton to large fish, birds and mammals, including humans “. Thus, the quantity of mercury present in the tusks of narwhals makes it possible, among other things, to indicate that this sea unicorn is at the top of the oceanic food chain. Narwhal does not have the physiological capacity to remove mercury from its body, which, according to this study, would have

led to modify his eating behavior in order probably to decrease the accumulation of this compound in his body via changes in diet.

According to theWHO, as far as humans are concerned, mercury poses a threat to the development of the child in utero, it can also have toxic effects on the nervous, digestive and immune systems, and on the lungs, kidneys, skin and eyes.

READ also: Pregnant women more exposed to mercury and arsenic via seafood

Narwhal tusks: when the mouth is silent, the body speaks

According to a study that had been published in the journal Geophysical Research Letters, permafrost (frozen ground in the Arctic) would trap mercury which would then be released during its thaw. Likewise, melting ice is believed to play a significant role in the significant increase in mercury in the oceans in recent years. The poles are the regions of the world most affected by the increase in temperatures. In fact, between 1968 and 2010, ice cover in Baffin Bay declined by 11.4% per decade. These environmental changes could be detected in the tusks of narwhals, also serving as an indicator of dietary changes in these marine animals. According to Professor Rune Dietz of the Arctic Research Center at Aarhus University in Denmark and his team, “just like our hair or teeth, the narwhal’s tusk chronologically stores physiological information as it grows, thus preserving an invaluable record of ecological information throughout the lifespan of these animals.“.

Every year, the defense becomes covered with a new layer and becomes longer and thicker. © Rune Dietz

By studying the tusks of narwhals, the researchers observed a change in the prey consumed. Between 1962 and 1990, when sea ice cover was extensive and strong, narwhals consumed mainly prey living in sympagic (ice-covered) or benthic (deep water) environments, such as arctic cod. However, the more time passed, the more they turned to prey living in pelagic environments (open sea). Between 1990 and 2010, the consumption of borea-Atlantic hooked squid was preferred. Feeding changes in narwhals would be correlated with successive environmental changes, probably caused by the melting of the ice as well as by the resulting increase in the quantity of mercury.

READ ALSO: Melting permafrost threatens the planet

The narwhal, a cetacean that adapts

Rune Dietz and colleagues state that “for top predators associated with ice, such as narwhals, belugas and polar bears, the continued reduction of sea ice in the arctic has implications for their distribution, diet, ability to avoid predators , as well as on their health […]. Similar reports from soft tissue analysis of beluga whales, narwhals and ringed seals show that top arctic predators are consuming more subarctic and pelagic prey due to global warming and the disappearance of sea ice .

It is therefore anthropogenic (human) pressure with global warming (melting ice and increasing mercury in the marine environment) that would force the narwhal to adapt its feeding behavior. However, the diversification of the foraging of this species would suggest a strong capacity to adapt to the rapid climatic disruption present in the Arctic.

Thus, the ability of narwhal tusks to trace the chronology of their eating behaviors – induced by climate and toxic substances – is a promising advance in understanding the evolution of narwhals and the ocean ecosystem.

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