The biological purpose of the male narwhal’s tusk is one of the mysteries of science. There are several theories: it might attract females, act as a stabiliser for sonar navigation or perhaps be used to slap and stun prey. Whichever is true, an international team of researchers has found its own use for the mysterious horn: as a record of the changing status of the Arctic environment.
As the narwhal’s tusk grows, it preserves information about the animal’s feeding behaviour and the contaminants to which it has been exposed. Studying this record is a similar principle to tree-ring science. By measuring isotopes of carbon and nitrogen, as well as concentrations of mercury embedded in the layers of ten narwhal tusks, the team was able to unpick individual and population responses to the changing Arctic environment. ‘The big advantage is that from a tusk, you can get data up to 50 years back in time, which would be a lifetime’s work for a scientist,’ says study author and Arctic scientist Rune Dietz from Aarhus University in Denmark.
Mercury contamination is a significant problem in the Arctic. Since the 1850s and the onset of industrialisation, mercury levels have been consistently high in the Arctic environment. Today, concentrations are 20 times higher than the background or naturally occurring level. The study’s findings reveal that from around the year 2000, the amount of mercury to which narwhals have been exposed has risen significantly. This corroborates similar studies that have observed rising mercury levels in beluga whale teeth, polar bear hair and seabird feathers.
It’s a problem for the health of larger animals, which consume the mercury that has passed through the food chain, increasing in quantity as it does so. ‘Mercury can accumulate in the brain, where it disrupts normal neurological function. The brain controls almost all physiological systems, like the immune and endocrine systems,’ says Jean-Pierre Desforges, a study author from McGill University in Canada. ‘This potent neurotoxin is freely circulating in the environment. Because they don’t grow hair, narwhals and other whales can’t remove it from their tissues as well as polar bears, seals or humans.’
There are probably two reasons for the increased concentrations, says Desforges. ‘First, that humans are releasing more mercury into the atmosphere. Second, that climate change and reduced sea-ice is changing mercury cycling.’ As sea-ice is lost, mercury becomes more available for consumption and therefore accumulates in marine mammals.
As much as 94 per cent of the mercury found in Arctic animals today is anthropogenic, with 65 per cent originating in Southeast Asia and China. The 2017 Minamata Convention, which has 127 signatories, aims to reduce global mercury emissions. It set targets of reducing mercury use in manufacturing by 50 per cent and in energy by 33 per cent. However, many nations are still burning large quantities of coal, which releases mercury as a byproduct, and gold mining is another big contaminator. Climate change is also increasing mercury run-off from ancient permafrost deposits – a phenomenon exacerbated by the more intense and frequent wildfires of recent decades.
Aside from mercury, the researchers were also able to explore the way in which changes in sea-ice coverage have influenced the narwhal’s feeding behaviour. To do this, they looked at carbon and nitrogen isotopes, which can indicate the types of prey that predators are consuming. ‘The dietary data revealed two major time periods, which coincided almost perfectly with changes in sea-ice coverage,’ says Desforges. ‘Before the 1990s, when sea ice was plentiful in the Arctic, narwhals were feeding in an ice-associated food web dominated by Arctic cod and Greenland halibut. After the 1990s, we’ve seen a rapid decline in sea ice and this was associated with a shift in the carbon and nitrogen isotope values suggestive of an increase in open-ocean prey, such as polar cod and sub-Arctic capelin.
‘Climate change is happening and the biological community is responding,’ adds Desforges. ‘We can add ours to the list of hundreds of studies showing that this is observable and documentable.’