How Norwegian lemmings rapidly evolved into a new species

Thirty‑five millennia may sound like a long time, yet for a mammal, it is practically yesterday. A new study uses whole‑genome data to show that the Norwegian lemming, Lemmus lemmus, split from its Siberian cousin only about 35,000 years ago.

“The Norwegian lemming is a key ecological species in the Fennoscandian tundra. Among other things, it serves as primary food for many predator species, including some threatened ones such as the Arctic fox,” said David Díez del Molino, a researcher at the Centre for Palaeogenetics, Stockholm University.

Lemmings shape Arctic life

Ecologists have tracked lemming booms and busts for decades because those cycles ripple through the tundra food web, shaping the breeding success of Arctic foxes, owls, and even geese.

When rodent numbers crash, fox litters shrink, snowy owls skip nesting, and grasses rebound after seasons of heavy grazing.

Unlike most small mammals, Norwegian lemmings overwinter aboveground by tunneling under snow, a habit that keeps them active when many rodents lie low. Their winter wanderings aerate soil, mix plant nutrients, and feed predators during the hungriest months.

During peak years, the species can reach densities of hundreds per acre. Yet its loud squeaks and bold charge‑and‑bite defense make predators think twice, granting the animal an outsized ecological voice relative to its three‑ounce frame.

Lemmings became a separate species

Researchers compared nine modern and two ancient lemming genomes and found a clean split from the Western Siberian lemming with no evidence of crossbreeding, even in areas where the two meet today.

That result stands in sharp contrast to polar bears and brown bears, whose DNA still shows ancient admixture from repeated hybridization events.

“Our findings indicate that the Norwegian lemming is among the most recently evolved mammals, diverging from its sister taxon, the Western Siberian lemming, at a remarkably shallow time depth,” said Edana Lord, a postdoctoral researcher at the Centre for Palaeogenetics.

The team’s high‑resolution sequencing also resolved relationships within the genus Lemmus, confirming that Eastern Siberian lemmings belong to a separate species and helping clean up a century of contested rodent taxonomy.

A species born in an ice age

The divergence date places the split just before the Last Glacial Maximum, when ice sheets peaked between roughly 26,500 and 19,000 years ago.

Glacial barriers likely trapped an ancestral lemming population in a Scandinavian refuge, cutting gene flow with Siberian relatives and setting the stage for rapid speciation.

Once the ice retreated, the newly minted Norwegian lineage expanded westward, but climatic swings had already written separation into the animals’ DNA. Modern population genetics shows no trace of later contact, suggesting that geography and behavior together kept the young species distinct.

Because the entire living population coalesces to a common ancestor only about 6,400 years ago, every Norwegian lemming alive today shares surprisingly shallow roots, a reminder that dramatic evolutionary change can happen fast.

Color blocks crossbreeding

Hybridization often lingers for thousands of years after a split, yet these rodents seem to have slammed the door shut.

One reason may be their vivid black‑and‑yellow coat, a pattern that functions as an aposematic warning and an instant badge of identity, reducing mating mistakes with the drab Siberian form.

Aggressive ritual boxing, loud chirps, and high‑speed chases further isolate the species by favoring individuals that court their own kind and bully outsiders away from snow burrows.

Genes that power survival

Sequencing uncovered unique changes in the LYST pigment gene that help produce the lemming’s splashy jacket, a mutation class also linked to altered melanosome function in laboratory mice.

The same region shows signs of a recent selective sweep, hinting at strong predator‑driven pressure for conspicuous colors.

Other mutations appear in BCO1, APOB, and RELCH – genes involved in fat transport and storage; in mice, knocking out BCO1 triggers fatty‑liver disease and shifts cholesterol pathways. Extra‑efficient lipid metabolism may keep lemmings fueled through subzero winters when seeds are scarce.

The researchers also found clusters of rapidly evolving olfactory receptor genes, suggesting that a refined sense of smell guides the rodents through deep snow tunnels to mates, food, and lurking stoats.

Lessons for Arctic mammals

Climate upheaval can spin off new species in a geological blink, and the lemming’s youth underscores how quickly isolation plus selection can rewrite a mammal’s genetic blueprint.

Similar patterns play out in Arctic whales, foxes, and hares as shrinking ice opens and closes migration corridors.

“This work represents a big step in our understanding of lemming speciation and evolution,” said Love Dalén, a professor in Evolutionary Genomics at Stockholm University.

Professor Dalén noted that tracing ancient DNA in more Pleistocene specimens could reveal exactly when each adaptive mutation took hold.

The study is published in Proceedings of the National Academy of Sciences.

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