In 1959, geneticist Dmitry Belyayev began catching silver foxes in Siberia. Keeping them in captivity, he selected the tamest ones and allowed them to breed.
He then identified the friendliest cubs among the offspring, letting them reproduce, and so on until eventually, he had a population of foxes with no fear of people.
But, to everyone’s surprise, the tame animals had developed other unexpected features. White patches had appeared on their fur, their ears drooped and their snouts were shorter.
Belyayev had discovered what became known as ‘domestication syndrome’; when human-friendly varieties of a species are artificially selected, the genes promoting tameness give rise to other traits as well.
A related experiment was carried out recently in Switzerland; the results have just appeared in Royal Society Open Science. The study began 16 years ago, when Madeleine Geiger and colleagues at the University of Zurich trapped 12 house-mice in local farm buildings. The captives were released into a barn where food was provided for them.
This was no animal prison, however; the mice were free to exit the barn through holes too small for cats martens or owls to gain entry. Mice from outside visited the barn occasionally but didn’t establish colonies within it.
The inmates lived in conditions which may have resembled those encountered by their ancestors when they began exploiting human settlements in the Middle East for the first time, 15,000 years ago.
The mice in the experiment were allowed to reproduce freely. Unlike Belyayev’s foxes, they were not selected for tameness or any other attribute. They lived normal lives, apart from regular encounters with the researchers studying them. All nest-boxes and hiding places in the barn were inspected at roughly ten-day intervals.
Adults were handled during these checks and again when their pups were 13 days old. Every few weeks or so, the mice and their pups were captured for examination and newly mature adults fitted with transponders.
In an average lifespan of 28 weeks, a mouse would have been handled two or three times and captured an additional three to four times.
The mice bred successfully, generating a population which stabilised at 250 to 430 individuals, a population density of 3.5 to 6/m2. This was well below the peak density of 10/m2 rodent communities are capable of attaining under normal conditions.
“In the course of a decade,” according to the report, “this mouse population exhibited significantly increased occurrence of white patches of fur and decreased head length.”
The proportion of mice with white patches doubled between 2007 and 2016. Head lengths also decreased significantly.
The researchers think that individuals with a strong antipathy towards people, left the barn, while more laid-back ones tended to stay on. Some with an innately friendly disposition towards humans, though born outside the barn, had moved in.
Thus, a semi-artificial form of selection-for-tameness was operating in the experiment. It worked to exclude mice with strong ‘fight or flight’ tendencies.
Domesticated animals and birds gradually cease to resemble their wild forebears; a Friesian cow, for example, is a very different beast from its wild auroch ancestor. Dogs have diverged from wolves.
When house-mice reached Western Europe about 3,000 years ago, they had already been changed genetically through the ‘domestication syndrome’. Some mouse populations today depend entirely on people for survival.
Mere exposure to human presence, it seems, changes wild populations. Are the mammals and birds, which frequent our parks and gardens today, different from their wilder ancestors?