A lack of genetic diversity among animal populations is often seen as a sign of impending doom; without genetic variation, a species cannot adapt to changing conditions and will eventually go extinct. But narwhals are complicating this theory, the elusive, oddly-toothed whales are faring quite well in the wild. Last year, the IUCN changed the animals’ conservation status from “near threatened” to “least concern,” but as Sam Wong reports for New Scientist, a new study has shown that diversity in the narwhal gene pool is remarkably low.
For the new report, published in the journal iScience, a team of Danish researchers sequenced the DNA of a narwhal from West Greenland. By determining how closely one individual’s parents were related, scientists can reconstruct genetic lineages of ancestral populations, Wong explains. And the researchers found that genetic diversity among narwhals is low, as previous studies have also suggested.
In other species, reduced genetic variation has been attributed to inbreeding, which can happen when a population dwindles over time, or to more acute population bottlenecks, which are events, like an environmental crisis, that dramatically reduces population size and leads to the loss of gene variants within the population. But the narwhal genome lacked signs of inbreeding, according to the study authors. Their population sizes are relatively robust; a recent estimate placed their numbers at around 170,000, the researchers explain.
Wondering whether the narwhals’ low genetic diversity might have something to do with their Arctic habitat—the animals spend their entire lives in the frigid waters off Canada, Greenland, Norway and Russia—the study authors also looked at genetic data from four other Arctic mammal species: the beluga, the bowhead whale, the walrus and the polar bear. None displayed the same lack of diversity as the narwhal—not even the beluga, the narwhal’s closest relative.
So what, exactly, is going on with the so-called “unicorns of the sea”? A population boom tens of thousands of years ago might be key to understanding the species puzzling lack of diversity, the study authors suggest. Through scientific modelling, the team was able to determine that narwhals began to experience a slow but constant population decline around two million years ago; by 600,000 years ago, only around 5,000 individuals were left. Narwhal numbers began to pick up around 100,000 years ago, roughly coinciding with the onset of the last glacial period, which in turn suggests that the population uptick was caused by “an environmental driver, possibly linked to an increase in Arctic sea ice,” the researchers write. Then, between 30,000 and 40,000 years ago, narwhal numbers started rising rapidly—and, according to the researchers, “genetic diversity may not have had time to increase accordingly.”
Narwhals may have been able to fare well despite their loss of genetic diversity because the population decline that began millions of years ago happened slowly, giving the animals time to “evolve different mechanisms to cope with their limited genome,” says Michael Vincent Westbury, lead study author and a postdoctoral researcher at the Natural History Museum of Denmark. But today, narwhals are vulnerable; unlike other whales, they do not migrate beyond Arctic waters, and whether the animals will continue to thrive as climate change causes rapid changes to their habitat remains uncertain.
“Our study can't comment on whether narwhals will be able to adapt, or if they have the plasticity to be resilient in these rapid changes,” says study co-author Eline Lorenzen, a molecular ecologist and curator at the Natural History Museum of Denmark.
But the new paper does suggest that there is reason to take a more nuanced view of how reduced genetic diversity impacts the future of a species. “There’s this notion that in order to survive and be resilient to changes, you need to have high genetic diversity," Lorenzen says. “But then you have this species that for the past million years has had low genetic diversity and it's still around—and is actually relatively abundant.”