Vikings who conquered new lands unwittingly brought with them another sort of invader, a new DNA study says—mice.

Scientists studying the evolution of house mice already knew about a DNA pattern found only in mice in what's now Norway, a Viking homeland, and northern Britain, which Vikings colonized, said study leader Eleanor Jones, a population biologist at Uppsala University in Sweden.

The finding suggested to Jones and her team that the two populations, despite being separated by the sea, were related and that Vikings had possibly brought the mice to northern Britain.

The new study tested modern and ancient mouse remains from the sites of known Viking colonies and found the same telltale pattern, adding weight to the idea that the mice were brought by Viking colonizers.

Intimately Connected

Between the eighth and the mid-tenth centuries A.D., Vikings settled new colonies in several regions, including Scotland, Ireland, Iceland, the Faroe Islands, Newfoundland, and Greenland.

House mice likely stowed away in grain and hay stores on the big, deep-bellied boats that Vikings used to conquer these new areas, Jones said.

In general, as their name implies, house mice tend to prefer human company and may have even co-evolved to live among people. The first records of house mice living with people come from the Mideast's so-called Fertile Crescent and date to around 8,000 to 6,000 B.C.

With the new research, we can "we can follow [their] genetic story, which is tied very intimately to our own genetic history," she said.

Studying Mice and Men

For the study, Jones and colleagues first took DNA samples from wild house mice in nine sites in Iceland, one in Greenland, and four near the Viking archaeological site of L'Anse aux Meadows in Newfoundland. The team compared this DNA with ancient samples from mouse bones found at four archaeological sites in Iceland and a few in Greenland.

Researchers then zeroed in on a small fragment of mitochondrial DNA (mtDNA), which is passed down by mothers to their offspring. By comparing this fragment to mouse mtDNA from all those different sites, the scientists figured out which mice were related and which weren't. (Get a genetics overview.)

From this data, the team pieced together a family tree, which showed how house mice spread across Europe over the past few thousand years. That lineage matched with the path of the Vikings' expansion, she said.

House mice, Jones said, "carry the genetic signature of human history."

It's unknown whether the Vikings were aware of their hitchhikers, although there are records that the settlers brought cats on the ship, noted Jones, whose study was published March 19 in the journal BMC Evolutionary Biology.

"Once you get to Iceland and Greenland, there are no other small animal pests—they must be taking cats with them to deal with this mouse pest," she said.

Once on land, the hardy species would've had no problem establishing themselves in the new Viking communities, Jones added.

Viking-Mouse Study "Convincing"

The new study offers a "very convincing application of ancient DNA analysis" of mice to trace how people settle new territories, Fabienne Pigière, of the Royal Belgian Institute of Natural Sciences in Brussels, said by email.

The research also "opens new perspectives of research for the study of human-settlement history," said Pigière, who was not involved in the research.

For instance, the team found no evidence of house mice in Newfoundland during the Viking period, which suggests Vikings may have lived in the area for only a short time, she said. (See "American Indian Sailed to Europe With Vikings?")

Elizabeth Reitz, an zooarchaeologist at the University of Georgia in Athens, noted that smaller animals such as house mice are often found in archaelogical collections, but "many people ignore them in favor of emphasizing larger, domestic, animals—those used for food, traction, and raw materials."

"Yet small, incidental animals, such as house mice, can tell us a great deal about the human-built environment," Reitz said via email. "As this article shows, they can also provide us additional insights into the process of human colonization and trade."

The study is also a reminder of how people can alter new environments, study leader Jones added.