Tamer over time
Domestication syndrome changes animals’ physical, hormonal traits
We humans have been domesticating animals for at least 10,000 years, and we’ve done so for many reasons. Pigs, cows and sheep give us food. Horses, yaks and water buffaloes provide transportation. Dogs and cats offer companionship and, sometimes, protection.
Something strange has happened along the way: Many of our domesticates, especially the mammals, share an unusual combination of characteristics. They tend to have curly tails, floppy ears, mottled coats and childlike faces featuring rounded snouts. Their skulls are generally smaller than their wild ancestors’, their hormone profiles are markedly different and their reproductive seasons are longer. These traits are clumped into what has come to be known as the domestication syndrome.
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.
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, 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 Trut began to test this idea in 1959. Every year, they assessed hundreds of foxes and selected only those with the most “pro-social” interactions with humans. These were the foxes chosen to parent the next generation.
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 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.
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.
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.
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.