Just like a tree trunk, the tusk of the narwhal acquires a new growth ring every year. Analyses of those rings have now revealed some interesting facts about the animals' diet, and about changes in their environment.
For the study, an international team of scientists examined 10 of the tusks. These were obtained from Inuit hunters in northwest Greenland, who had already killed the animals as a source of subsistence. It should be noted that the narwhal tusk is, in fact, an elongated canine tooth, found only on males of the species.
The appendages measured 150 to 248 cm (59 to 98 in) in length, and came from individuals living between the years 1962 and 2010. All of the tusks got cut in half lengthwise, so the researchers could see and examine the growth rings within. Each ring represented one year of the whale's life.
More specifically, the scientists measured levels of mercury – and of stable isotopes of carbon and nitrogen – within each layer. This data provided an indication of how high the whales' ingested prey items sat on the food chain, plus it offered clues as to whether those creatures lived near the sea ice or in the open ocean.
It was determined that up until 1990, a period in which the ice cover was "extensive but varying," the narwhals fed mainly on sea-ice-adjacent prey such as halibut and Arctic cod. As the ice cover declined between 1990 and 2000, the whales switched to eating chiefly open ocean prey, such as capelin and polar cod. Because those fish sat lower on the food chain, the mercury levels in the tusk layers for those years was correspondingly lower.
That said, even though the narwhals' diet didn't change after 2000, the tusk mercury levels did proceed to rise significantly. Due to the fact that a similar rise was observed in the bodies of other Arctic animals, it is believed that the change came about due to the presence of mercury in increased coal combustion emissions from southeast Asia.
"With our new discoveries, we now know that there is a bank of data in the narwhal tusks found in museums around the world," says the lead scientist, Prof. Rune Dietz of Denmark's Aarhus University. "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."
A paper on the research – which also involves scientists from McGill University (Canada), the Greenland Institute of Natural Resources, the Center for Permafrost (Denmark) and the University of Waterloo (Canada) – was recently published in the journal Current Biology.
Source: Aarhus University