The narwhal’s surprising survival strategy: a million years of extremely low genetic diversity
University of Copenhagen researchers have mapped a West Greenlandic narwhal's genetic family history and made a surprising discovery: genetic variation in narwhals is very low compared against other mammals. The discovery calls into question the notion that high genetic diversity is needed for a species to survive. Instead of facing extinction, the narwhal population has been stable, until recently. How the narwhal has survived for as long as it has remains a mystery.
Why mess with a recipe for success? For narwhals, this appears to be the case. Researchers at the Natural History Museum of Denmark have DNA sequenced the genome of a narwhal from West Greenland, and opened a window to the past million years of its evolutionary history. The results suggest that genetic diversity among narwhals has been consistently low throughout the entire period. Nevertheless, the species has survived through to the present day.
"The narwhal has incredibly low genetic variation across its genome compared to other mammals, both in and beyond the Arctic. This is surprising, as high genetic diversity is normally associated with greater odds of long-term survival," explains Associate Professor Eline Lorenzen, who headed the study.
Roughly 170,000 narwhals currently inhabit Arctic waters surrounding Greenland, Canada, Svalbard and Russia. The genetic analyses demonstrate that their population has been stable for an exceptionally long period of time, and that their numbers have grown since the last ice age – although several areas are currently experiencing population declines due to hunting.
"The narwhal has survived massive historic changes to the extent of sea ice that defines Arctic ecosystems. Nevertheless, their stocks have increased, until recently, despite persistently low genetic diversity," explains Eline Lorenzen.
Narwhal’s enigmatic survival
Significant environmental change can affect a species in one of three ways: it can go extinct, move elsewhere or survive by adapting to the new circumstances. Should environmental change occur suddenly, adaptation will be based on existing genetic variation, as there will be no time for new variation to occur. Thus, greater genetic variation within a species provides more genetic material to select from, and thereby greater odds for species survival.
The case of the narwhal is an exception to this logic. The narwhal’s long presence in the Arctic, despite its extremely low genetic diversity, remains a mystery.
"We don't know why this is the case with the narwhal. But our study does question the notion that a species requires high genetic diversity to ensure its long-term survival, for example by increasing its resilience to climatic change," says Eline Lorenzen.
Researchers compared the narwhal genome against the genomes of four other endemic Arctic marine mammals: the bowhead whale, beluga whale, walrus and polar bear. All had far higher levels of genetic diversity than the narwhal, suggesting the pattern of low diversity is unique to this species.
The narwhal is one of only two species of toothed whale that are found in the Arctic year-round. It is known as the unicorn of the sea due to its elongated tusk that protrudes from the skull. Adult narwhals measure between 3.8-5 meters in length and weigh between 800-1600 kilos.