Narwhal tusks may indicate effects of climate change, say scientists

The iconic and mysterious narwhal tusk that once inspired tales of mythical creatures may also tell stories of their own, scientists have learned.

A study in Current Biology, published last month, found that male narwhal tusks are able to provide a lifetime of chronological information on how much mercury the animals are exposed to and also possibly what they are eating.

“The idea is to use the narwhal tusk kind of like tree rings,” said Jean-Pierre Desforges, a co-author of the paper and a postdoctoral fellow studying marine mammal toxicology at McGill University.

To carry out the study, Desforges and other researchers cut open and analyzed the tusks of 10 male narwhals, provided by Greenland hunters.

What they found in the tusks, which grow continuously throughout narwhals’ lives, is similar to what has been found in other predators of the Arctic: the amount of mercury — a neurotoxin — in the environment, and the tusks, has accumulated rapidly over the last 20 years.

The authors believe this sharp rise likely reflects an increase in global emissions and also possibly a shift from an ice-based food web to one more dominated by open water, the result of climate changes.

Unlike some other Arctic predators, such as polar bears, narwhals aren’t able to expel this mercury through fur or feathers.

Desforges says that if those levels get high enough they could lead to reproductive or cognitive problems that could cause generational issues.

He also acknowledges that there is one other predator above narwhals in the Arctic food chain — humans.

While this was not a focus of the study, Desforges says that studies looking at beluga have shown that the levels of mercury in the teeth are strongly linked with levels of mercury in the animal tissue.

“It is a pretty good reflection of the mercury burden in the whole body,” he said.

Mercury tends to accumulate mostly in parts of the animal like the liver, kidney and brain, not blubber, said Desforges.

In order to answer whether an animal is safe to eat, he said it comes down to knowing what the mercury levels are for a specific animal and balancing out the pros and cons.

“It’s always that risk and benefit, right? [By not eating narwhal] you’re avoiding mercury exposure, which is great, but you’re also avoiding a lot of other good healthy foods,” he said.

Similar to trees, narwhal tusks, such as those seen here, grow a new layer every year. By cutting them open, a team of scientists was able to determine the amount of mercury consumed by the animals over the course of their lifetimes. (Photo courtesy of Jean-Pierre Desforges)

Narwhal tusk expert Martin Nweeia, a dental researcher with Case Western Reserve University and Harvard University who was not involved with the paper, said that although the study was well-written and the mercury analysis looked good, the conclusions about climate-induced dietary changes do have some problems.

By including only tusked male narwhals in their study sample, information on diet trends is incomplete, Nweeia said.

“You’d have to throw in everything, you can’t kind of skew your data,” he said.

For him, this would involve sampling both tusked and non-tusked females and males, a process that requires a fatty acid analysis of tissue samples.

The need for such analysis is also flagged in the study as an area to follow up on, as is looking at actual stomach contents of the narwhals.

Nweeia agrees this is needed and says the best way forward on it would be to work with Inuit and let traditional knowledge guide that work.

“I’d be curious what hunters think because they’re cutting open stomachs all the time,” he said.

“They know exactly what that diet is.”

While Nweeia says that climate change could be a factor in changing the diet of narwhals, he won’t be convinced until these gaps in knowledge are filled.

The study also highlights the ability of the tusks to fill in other gaps of knowledge.

“What’s cool about this is from one year of sampling you get a narwhal tusk and you can go back up to 50 years or more, depending on how old the animal is,” Desforges said.

Given how narwhal tusks have been collected for centuries and many have wound up in museums around the world, Desforges hopes that one day those some of those specimens can be analyzed to expand the mercury exposure timeline beyond the last 60 years.