Scientists uncover traces of climate history by cracking open narwhal tusks

A recently published study in Current Biology shows how tree-ring-like layers in narwhal tusk show growing concentrations of mercury as their environment deteriorates.

Study shows concentrations of mercury rising as sea ice deteriorates

A new study examines the layers of bone in a narwhal's tusk for clues to gradual shifts in their health, environment and behaviour over decades. (Paul Nicklen/WWF)

Want to see the impact of climate change? Crack open a narwhal tusk.

A team of researchers from Denmark, Canada, France and Greenland do exactly that in a new study, published March 10 in the journal Current Biology. The aim was to find out how the animals have been affected by climate change over the past 50 years.

Male narwhal's single, unicorn-like tusk is formed by dozens of layers of bone. Over the course of each year, a narwhal creates a new outer layer, preserving old bone layers beneath.

That means, like an ancient tree, cracking open a narwhal tusk reveals, in the authors' words, "an invaluable archive of ecological information" across the animal's 50-year lifespan, which scientists can use to deduce conclusions about their eating habits, migration and exposure to pollution.

This data makes it possible to "explore rarely captured fine-scale, individual-level responses to environmental change," the study says, like how a particular narwhal reacted to greater areas of open ice in a warming Arctic.

But more than individual behaviour, the tusks have a "unique capacity to characterize" the cumulative impacts of climate change on Arctic animals, building on more common point-in-time data that doesn't show how one bad year affects another.

A picture showing a detail of the layers present in a narwhal tusk. Every year, a new layer is added, preserving data about the animal's health and environment similar to the rings of a tree. (Rune Dietz/McGill News)

In the study, Aarhus University's Rune Dietz and McGill University's Jean-Pierre Desforges, together with a team of international researchers, analyzed the layers of 10 narwhal tusks gathered from northwestern Greenland between 1962 and 2010.

The results offer what the authors call a "rare insight" into the behaviour of these animals as they adapted to a rapidly changing climate.

The narwhal under study slowly changed their main source of food from species found under ice caps to open-ocean species like capelin and polar cod, adjusting their behaviour, migration and diet as Arctic ice receded.

As they did so, researchers found the levels of mercury contained in the tusks' layers dropping, as their reliance on halibut in the more polluted coastal waters of Greenland tailed off.

But worryingly, the tusks show that trend has recently been upended. While it's normal for animals to accumulate mercury over time as they consume the small concentrations found in their prey, Dietz and Desforges found mercury concentrations in the tusks increased rapidly between 2000 and 2010.

Like narwhal, polar bears are also accumulating mercury at a higher rate, though it's not totally understood why. (Getty Images)

The research isn't certain about why that is happening, but suggests it could be due to increased greenhouse gas emissions in southeast Asia, or a result of increased concentrations lower down the food chain, in narwhal's prey.

But whatever the cause, it's not unique to narwhal. Similar studies have found rising concentrations of mercury in polar bears and Arctic foxes occurring over roughly the same period.

Mercury, a potent neurotoxin, can be dangerous to animals when it accumulates at high levels.

Overall, the authors say, the study offers an "unprecedented insight" into narwhal's adaptability and overall health during a period of rapid change in their environment.

And considering the large number of narwhal tusks in museums and collections around the Arctic world, it's no surprise that scientists are excited they can be used in this way — as an ever-expanding timeline of the Arctic environment, written in bone.