The dangling ears of dogs, the curled tail of pigs … the long-tamed animals look different from their wild "cousins". Until now these physical characteristics had been entirely reduced to the intervention of man, who over the centuries has selected the most docile and mild specimens, leaving only the most suitable to live by his side reproduce.

But a new study on wild mice shows that the visible signs of domestication can also "stand alone", due to natural selection, without the man putting a hand in it. In the work published in the Royal Society Open Science, it tells of how a group of wild mice developed some typical features of domestication - such as patches of white fur and a shorter snout - only for having become accustomed to the proximity of man.

In practice, rodents would have started taming on their own: a process that could have affected even dogs and horses, before we started actively selecting them.

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Common sections. Much of the current knowledge on domestication and appearance of animals comes from a famous experiment carried out in Siberia in the 1950s. When the researchers tried to domesticate some wild foxes, making only the meekest mate, the new generation animals began to develop some traits that we now recognize in dogs: curly tail, dangling ears, shorter snout.

A century earlier Charles Darwin had grouped these and other characteristics (pendulous ears, patches of white fur, small size) under the label of domestication syndrome. But is it possible that these traits develop even without human intervention?

Image White patches on the hair: a trait associated with animal domestication. | Linda Heeb

Free but close. In 2002, a group of ethologists captured a dozen wild mice for scientific purposes, to study the transmission of diseases in rodents. For the animals a den was set up in a barn in Switzerland: a shelter where food was never lacking and from which they were free to go out whenever they wanted.

Many mice got used to frequent human visits and lost their fear. From 250 that they were, they reproduced until they became 430. Until, four years later, a biologist at the University of Zurich did not notice, on the back of some mice of this population, some unusual patch of white fur. From 2010 to 2016, the number of speckled specimens doubled. When the size of their head was measured for another project, it was realized that on average it had shrunk by 3.5%.

All by yourself. The mice had changed in a similar way to the Siberian foxes, without the man intervening to select them. Further studies will have to analyze the genetics behind this process, which could be guided by a group of cells involved in the early stages of embryonic development, those of the neural crest.

The research could open an observation window on the origin of some animal domestication processes. That could have started with some spontaneous changes in more docile animals and used to our presence.