Mice May Self-Domesticate With Little Human Intervention

self domesticating mice

Story at-a-glance -

  • Domestication may occur not only as the result of purposeful breeding by humans but also in a much more passive way
  • Just by hanging around humans, mice were able to “self-domesticate” basically on their own, raising questions about whether dogs may have also begun to self-domesticate prior to becoming pets to humans
  • Domesticated animals tend to have floppier ears, white patches on their fur, curlier tails, and smaller heads and snouts, a phenomenon known as “domestication syndrome”
  • In wild mice living in a barn with regular exposure to humans (and food and water provided), the percentage of adult mice with white patches of fur increased more than twofold from 2010 to 2016 — a major sign of domestication syndrome

By Dr. Karen Shaw Becker

In the 1800s, Charles Darwin noticed that domesticated animals have specific features that set them apart from their wild counterparts. In addition to being more tame, they also tend to have floppier ears, white patches on their fur, curlier tails, and smaller heads and snouts.

This phenomenon, known as “domestication syndrome,” was long thought to be the result of human intervention leading to domesticated animals. In fact, in 1959 the “Russian farm-fox experiment” took place, in which Russian researchers attempted to create domesticated foxes.

By specifically selecting only the friendliest foxes to breed, the animals began to show marked changes within 10 generations, including not only wagging their tails at people but also physical changes indicative of domestication: more folded ears, curlier tails, smaller heads and changes in fur color.1

It turns out, however, that domestication may occur not only as the result of purposeful breeding by humans but also in a much more passive way. Just by hanging around humans, mice were able to “self-domesticate” basically on their own, raising questions about whether dogs may have also begun to self-domesticate prior to becoming pets to humans.

Fifteen-Year Study Shows Mice May Self-Domesticate

In 2002, researchers began looking into behaviors and disease transmission among mice, trapping about a dozen to live in a barn in Switzerland as part of the study.2 The barn allowed the mice to come and go as they pleased, but also protected them with small doors intended to keep predators like cats, foxes and owls out.

The researchers provided regular food and water for the mice, giving them plenty of incentives to stay and little reason to want to leave. Years later, anywhere from 250 to 430 mice call the barn home at any given time. As part of the study, the researchers estimate that during an average mouse lifespan, mice are handled two to three times and captured another three to four times.

“Owing to regular experimental handling of mice and monitoring of nests over 14 years, we hypothesize that these wild mice have been habituated and unintentionally selected for tameness for approximately 20 generations,” the researchers explained. Indeed, some of the mice began to display signs that they were becoming tame, like running over the researchers’ shoes3 — a rather brazen move for a mouse.

Then, in 2006, the researchers noticed white patches appearing on some of the mice. Initially some of the spots were tiny and they were rarely found, but this changed significantly as the years went by.

The percentage of adult mice with white patches of fur actually increased more than twofold from 2.5 percent in 2010 to 5.4 percent in 2016, the study found — a major sign of the so-called domestication syndrome. Looking into the phenomenon further, the researchers also revealed that the mice had gotten smaller, with both head length and body weight decreasing significantly over the same period.

If tameness is heritable, the trait will continue to be passed on to future generations, increasing the mice’s tolerance of humans over time. In addition to the physical changes, domestication and tameness are associated with a downregulation of the fear/stress system, which suggests that domestication syndrome may alter the mice’s typical fight-or-flight response to stress.4

Neural Crest Cells May Explain Domestication Syndrome

Why domesticated animals would present with such a hodge-podge of traits, from curly tails and white patches to tameness and snout size, has puzzled researchers for years, but a growing number of studies suggest embryonic stem cells called the neural crest may play a key role.

Writing in the journal Genetics, researchers suggested that domestication syndrome may result from deficits in neural crest cells during development. “Most of the modified traits, both morphological and physiological, can be readily explained as direct consequences of such deficiencies, while other traits are explicable as indirect consequences,” they say.5

Neural crest cells play a role in many tissue types in the body, including pigment cells and those in the skull, jaws, ears, teeth and the adrenal glands, which would affect the fight-or-flight response.

"When humans bred these animals for tameness, they may have inadvertently selected those with mild neural crest deficits, resulting in smaller or slow-maturing adrenal glands," study author Adam Wilkins, Ph.D., of the Humboldt University of Berlin said in a news release. "So, these animals were less fearful."6

For the featured study, it’s possible that shyer mice naturally left the barn to avoid human contact while mice that were attracted to (or at least not bothered by) humans stayed behind.

What Does This Mean for the Domestication of Dogs?

There’s much to be discovered about the domestication of dogs, but the mice study suggests that dogs may have begun to self-domesticate prior to becoming fully dependent on humans. It’s thought that modern dogs have one common ancestor — the Eurasian grey wolf.

At some point in the past, roughly 30,000 or 40,000 years ago (the exact timeframe is a subject of great debate), a subspecies of wolf likely began to live near humans, perhaps as the animals searched for food at human settlements. Wolves that lived closer to humans likely became gradually more tame, even without direct human involvement, which may have primed them for future domestication.

For their part, mice have been living commensally with humans, meaning they derive benefit from humans while humans are unaffected, for as long as 15,000 years. This is about 3,000 years earlier than the start of crop agriculture — the time it was long believed mice took up residence with humans.

Instead, it turns out that mice made homes alongside their human neighbors during the hunter-gatherer era, as soon as humans started to put down some more permanent roots. They’ve likely been on the path to domestication ever since.