Narwhals' (Monodon monoceros) giant horn is like nothing else in the ocean, and its function is still debated. It turns out their success is a mystery of its own, since they have the low genetic diversity we normally associate with imminent extinction. Besides raising interesting questions about the narwhal's history, the discovery has implications for conserving other similarly genetically homogenous species.
In a harsh time for marine mammals, narwhals are an exception. A recent increase in their numbers to 170,000 has caused the IUCN to bump them from “near threatened” to “least concern” among endangered species. However, comparisons of the genetics of widely separated narwhal populations have suggested they lack diversity, without which species are usually considered unable to cope with changes such as newly emerging diseases.
Dr Eline Lorenzen of the University of Copenhagen and colleagues assembled the genome of a narwhal from West Greenland, allowing a much more in-depth analysis than had been conducted in comparisons of limited sections of narwhal DNA. They found the lack of diversity dates back millions of years, rather than reflecting recent inbreeding. Their results are published in the open access journal iScience.
"There's this notion that in order to survive and be resilient to changes, you need to have high genetic diversity, 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," Lorenzen said in a statement.
Lorenzen wondered if the low diversity is a feature of Arctic marine mammals, but found the bowhead whale, the ice-deprived walrus, and the recently weaponized beluga whale have more normal population genetics. Nevertheless, she is keen to expand the number of animals tested in this way to see if narwhals have company.
The most likely explanation, Lorenzen thinks, is that narwhals experienced slow declines caused by climatic changes, given them time to adapt and preserve the most valuable genetics, followed by a relatively rapid recovery 30,000-40,000 years ago.
With so many animals and plants in danger of extinction, wildlife bodies and governments are making painful choices about priorities. Many biologists argue some species are beyond saving, and resources should be redirected elsewhere. Very low genetic diversity is one of the measures frequently proposed for assessing this. However, if narwhals can thrive for so long, through so many changes to the planet, perhaps other rarer creatures can recover despite a limited gene pool.
Narwhals are restricted to the Arctic, and might be expected to be suffering from the loss of sea ice. Although that has not happened yet, and Atlantic populations survived centuries of hunting for their tusks, which helped preserve the myth of the unicorn, concerns remain they may be affected if polar warming crosses an unknown tipping point.
Scientists are pretty confident they have unraveled the mystery of narwhals' tusks. At the very least it's not an evolutionary disadvantage or it wouldn't have survived.