Why should domesticated species share these characteristics? Pig farmers would not have cared whether animals had curly tails. Early cattle breeders had nothing to gain from producing cows with black- and-white spotted hides. And yet the domestication syndrome is very real.

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What’s going here?

One way to answer this is to turn to an unusual, six-decade-long experiment involving the domestication of silver foxes in Siberia. Recently, my Russian colleague Lyudmila Trut and I have told the story of that study in our book, “How to Tame a Fox (and Build a Dog).” As Lyudmila, who has been leading this work every day since 1959, and I explain, the experiment was launched in part to understand the domestication syndrome.

That label, of course, was not in use in the 1950s, when the Russian geneticist Dmitry Belyaev conceived of and started the project to figure out how domestication happens. He hypothesized that the process was driven by our distant ancestors’ selection of animals according to one critical trait — that they were less aggressive and fearful toward humans than was typical for their species. This tameness, Belyaev proposed, would have been crucial to breeding animals for other things we wanted, such as meat or a ride. It just would not do to be trampled by our food source, maimed by our protectors or kicked by our vehicles.

Belyaev knew about the traits that make up the domestication syndrome, because he had spent a great deal of time working with domesticated animals. He surmised that somehow or another, all the components — the floppy ears, mottled coats, smaller skulls and more — were genetically correlated with tameness. And so when we select based on tameness, these other traits just come along for the ride in a kind of genetic hitchhiking.

He and Lyudmila began to test this idea in 1959. Every year they assessed hundreds of foxes and selected only those with the most “prosocial” interactions with humans — the ones that licked people’s hands, wagged their tails and whined sadly when interactions with humans were over. These were the foxes chosen to parent the next generation.

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They would then assess whether subsequent generations became tamer over time, and equally important, whether traits associated with the domestication syndrome began popping up. They did, and quickly — remarkably quickly, given the thousands of years it took for our ancestors to domesticate dogs, cows and other creatures. Within the first decade of the fox domestication experiment, the animals were not only markedly tamer, offering up their stomachs for belly rubs, but some of them had curly tails and mottled fur.

Lyudmila remembers one fox in particular from this time. In 1969, the 10th generation of foxes was born, and among them was a pup that she named Mechta, the Russian word for dream. In wild foxes, a pup’s ears are floppy until it is about two weeks old, at which point its ears take on the ramrod-straight look we tend to picture when we think of foxes. When Mechta was three weeks old, her ears had not yet straightened. They still hadn’t at four weeks, nor at five. Mechta looked exactly like a dog pup.

Lyudmila desperately wanted to show Belyaev, but he was so busy that spring that he couldn’t come to the experimental farm outside the city of Novosibirsk until Mechta was three months old. To Lyudmila’s surprise and delight, however, Mechta’s ears remained as floppy as ever when he showed up. When Belyaev saw the pup, he exclaimed, “And what kind of wonder is this?!”

By this time in the study, the domesticated foxes also had dramatically reduced stress hormone levels, indicating they were more comfortable around humans than their wild brethren, and the females had slightly longer breeding seasons. In the decades to follow, the frequency of these characteristics increased, and the foxes also developed juvenile facial features, smaller skulls and increased levels of neurotransmitters such as the “happiness chemical,” serotonin.

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Belyaev was right: Select animals based on tameness and only tameness, and many of the traits that make up the domestication syndrome come along for the ride.

That answers the question at one level, but why does selection for tameness also lead to all of the other characteristics that make up the domestication syndrome? A promising new idea proposed by a trio of scientists may help answer that question. In a paper published in 2014 in the journal Genetics, these researchers suggested that the key may be changes in the development of neural crest cells, a type of stem cells. When vertebrates are still tiny embryos, neural crest cells migrate to different parts of the body including the brain, skin, jaws, teeth, larynx, ears and cartilage.

The researchers think that choosing animals based on tameness also selects for a reduced number of neural crest cells, and that the floppy ears, curly tails and other traits associated with domestication result directly from these “mild neural crest cell deficits.” Changes to neural crest cell development, the argument goes, may be the mechanism that allows the domestication syndrome to hitch a ride with tameness. Time will tell whether this hypothesis holds up, but if it is correct, it provides the missing link in how domesticating based on tameness also produces the domestication syndrome.

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There are exceptions to the domestication syndrome, as well as many mysteries surrounding it. German shepherds, for example, have erect ears. It’s possible that selective breeding for rather aggressive behavior may have tinkered with components of the domestication syndrome.

And at this point, you may be wondering about America’s other favorite pets: cats. Why they possess some traits of the syndrome but still have pointy ears, we don’t know. One speculative possibility is rooted in the idea that cats’ bond to humans, and their reliance on us, is weaker than in dogs, and that even during the domestication process, cats probably had to fend for themselves more than dogs. Natural selection, according to this idea, may have strongly favored pointed ears for the benefits they provide during hunting.

Lee Alan Dugatkin is an evolutionary biologist and historian of science at the University of Louisville. He and Lyudmila Trut are the authors of the recent book “How to Tame a Fox (and Build a Dog).”