‘Self-domesticating’ mice suggest some animals tamed themselves without human intervention

From the floppy ears of dogs to the curly tails of pigs, domesticated animals sport a different look from their wild cousins—a look that scientists chalked up to human intervention. Now, a new study of wild mice shows that they, too, can develop signs of domestication—white fur patches and short snouts—with hardly any human influence. The work suggests that the mice are able to tame themselves, and that other animals like dogs may have done the same before they were fully domesticated by humans.

Much of what we know about how animals change appearance during domestication comes from a famous experiment in Siberia in the 1950s. Researchers found that when they took wild foxes and let only the tamest breed, the foxes began to develop doglike features such as curly tails, smaller heads, and floppy ears. Nearly 100 years earlier, Charles Darwin dubbed this suite of traits “domestication syndrome.” But could these traits arise without any human intervention? An experimental accident suggests they can.

The accident began in 2002 when scientists studying mouse behavior and disease transmission trapped a dozen wild mice in a barn in Illnau, Switzerland. The animals were free to come and go and nest and mate as they pleased. Their new digs were also safe from predators—the mouse doorways were too small to allow domestic cats, owls, and martens to enter. The barn also contained plenty of free food and water, provided by the researchers every few weeks. The mice that didn’t mind the visits stuck around and eventually blossomed to a steady population of 250–430 animals. Some even began to run over the researchers’ shoes instead of scurrying away. That’s a sign that these animals had lost their fear of humans, even without the researchers deliberately breeding the most human-friendly mice, as scientists had done with the foxes.

Four years later, Anna Lindholm, an evolutionary biologist at the University of Zurich in Switzerland, began to notice white patches of fur on a few of the russet-colored mice. “It was really rare,” she says—in some mice, the white splotches made up of as few as eight hairs. From 2010 to 2016, the proportion of adult mice with white fur patches more than doubled, the team reports today in Royal Society Open Science .

Serendipitously, Lindholm had also been measuring the mice’s heads for another project. And, just like the Siberian foxes, the mice became smaller and their heads shrank—about 3.5% on average. That’s an “exciting” change that suggests self-domestication can occur as a result of natural selection, says Brian Hare, an evolutionary anthropologist at Duke University in Durham, North Carolina, who was not involved in the work.

This is the first study to show that self-taming can produce the same traits as human-directed domestication, says W. Tecumseh Fitch, an evolutionary and cognitive biologist at the University of Vienna. He and others have proposed that a group of cells involved in early development called the neural crest are responsible for the suite of traits associated with domestication syndrome. Those cells affect skin color, ear cartilage, facial bone formation, and the adrenal glands, which produce stress hormones and testosterone—all of which differ in domesticated and undomesticated animals. The new work, says Fitch, is consistent with his prediction—he would love to know the genes involved.

Hare agrees, and says the work may even have implications for how social species such as humans, dogs, and bonobos can solve problems—like how to share food and help each other out when stressed—that their wild relatives can’t.

The research, says Lindholm, provides a window into how domestication started for mice. Humans may have waited until these creatures showed some semblance of tameness on their own before helping the process along she says. “It’s simply from being near us that is likely to have caused these changes.”