Narwhal tusks tell a haunting tale


Narwhal tusks tell a haunting tale

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Researchers have long debated what the 10-foot-long tooth protruding from a narwhal’s head actually is. by. Perhaps it has something to do with sexual selection, and males with longer horns attract more females. Or maybe things feel the salinity. Or maybe a narwhal uses its tusk to drive its prey to the bottom of the ocean.

Whatever the purpose, scientists know for sure: the Arctic region, which narwhals call home, is heating up twice as fast as the rest of the planet, and by analyzing these fangs, researchers can gain surprisingly detailed information about how animals behave. with catastrophic change. It doesn’t look good.

Writing in March in the journal Current Biology, the scientists described what they found in 10 tusks collected from animals in northwestern Greenland. Because a tusk grows continuously over the many decades of a narwhal’s life, the researchers were able to read the large teeth like rings on a tree. They found that between 1962 and 2000, the mercury in the fangs increased by an average of 0.3 percent per year, but between 2000 and 2010 it increased by 1.9 percent per year. This is consistent with the increase in mercury discovered in the bodies of other top predators in various regions of the Arctic, possibly due to air pollution blowing in from the south.

Scientists are also finding evidence in the tusks that narwhals’ diet is changing, from consuming species associated with sea ice to eating more species from the open ocean. This corresponds to a dramatic decrease in Arctic sea ice since 1990.

“Instead of doing 40 years of work to get 40 years of data, in one year of work you can get narwhal tusks and go back 50 years in time,” says McGill University wildlife toxicologist Jean-Pierre Desforges , one of the main authors of the paper. “So that’s the really remarkable thing.”

A cross section of a narwhal tusk, showing the layers of material.
Enlarge / A cross section of a narwhal tusk, showing the layers of material.

Jean-Pierre Desforges

Mercury is a powerful neurotoxin that bioaccumulates in species as they ingest it throughout their lives. When an organism at the bottom of the food chain consumes mercury, it accumulates in its tissues. So something bigger eats that animal and its mercury, and so on in the food chain.

Some top predators, like the polar bear, bioaccumulate a lot of mercury, but they can also expel it – bears abduct it in their thick fur. Not so lucky for the smooth-skinned narwhal. “For an animal that lives a long time, these whales can live more than 50 years, they are accumulating mercury year after year,” says Desforges. “This is why they reach really high levels and, of course, that is what worries us. If these levels are high enough, they could have a negative effect on the species. “That could include reproductive or cognitive effects, since mercury is a neurotoxin.

The other worrying sign the researchers found on the tusks hinted at the whales’ changing food sources. They looked for stable isotopes of carbon and nitrogen, residues from the narwhals’ diet that remain on their tusks. Carbon reveals information about the prey’s habitat, for example whether it lived in the open sea or closer to land. Nitrogen tells you its trophic level or where it was in the food chain. “Together, they give you an idea of ​​the general feeding ecology of the species,” says Desforges.

As with mercury, Desforges was able to map how this diet changed over time. Before 1990, whales had been feeding on “friendly” prey associated with the frozen habitat: the arctic cod and the halibut. Later, their diet began to shift towards more “pelagic” or open ocean prey, such as the capelin, a member of the scent family. “We’re not looking at the actual stomach contents of the prey or anything like that,” says Desforges. “But essentially we are arguing that this temporal pattern matches extremely well with what we know about the extent of sea ice in the Arctic, which after 1990 begins to fall quite dramatically.”

As the sea ice has thinned, narwhals have changed their diets.  At the same time, levels of mercury (Hg) have been increasing.
Enlarge / As the sea ice has thinned, narwhals have changed their diets. At the same time, levels of mercury (Hg) have been increasing.

Jean-Pierre Desforges

There could be a thing or two going on. As the sea ice retreats in the Arctic, the ecosystems below it may reorganize, leading to population declines between cod and Arctic halibut. In that case, the narwhals would have to resort to hunting species from the open ocean to compensate for their dietary deficit. On the other hand, those cod and halibut populations are not necessarily declining, but simply moving north. Or it could be that as the Arctic waters warm, there are more capelin around and the narwhals are unwilling to pass up a hearty meal.

But if a fish is a fish, why would it matter what narwhals eat, as long as they get enough food? It turns out that not all fish are the same. “Arctic species are more nutritious and energetic,” says Desforges. To survive the cold, fish need to accumulate fat, which means more calories for predators that feed on them, such as narwhals. “If they’re moving prey to less arctic species, that could have an effect on their energy level consumption,” adds Desforges. “Whether that’s true remains to be seen, but it’s certainly the big question we need to start asking ourselves.”

This reorganization of the diet, which may or may not be a problem for the narwhal, could collide with increased levels of mercury, which is it so a problem for any animal. These two threats may be more problematic in combination than alone. “That’s the tricky part,” says Desforges. “We basically have data that suggests things are changing, but we really don’t have an idea of ​​how that is affecting whales here.”

The power of this fang analysis technique is that it can theoretically allow scientists to look further back in time than in the 1960s. Taking a tissue sample from a live narwhal only gives you data on how the individual is doing. at that moment. But natural history museums around the world have narwhal tusks in their collections dating back more than 100 years.

“Museum collections offer a great opportunity to observe these changes in an even deeper time,” says Moe Flannery, senior manager of bird and mammal collections at the California Academy of Sciences, who was not involved in this work. “The museum specimens contain this hidden information that is not easily accessible, but is accessible to researchers studying changes over time. “

Searching ahead However, over time, it’s hard to say what a rapidly changing Arctic will hold for the narwhal and what signs of climate change we might find on its tusks in the future.

This story originally appeared on wired.com.

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