When the cubs were born, the researchers hand-fed them. They also attempted to touch or pet the foxes when they were two to two-and-a-half months old, for strictly measured periods at a time.
The aggressive and fear avoidance responses were eliminated from the experimental population
If the cubs continued to show aggressive or evasive responses, even after significant human contact, they were discarded from the population – meaning they were made into fur coats. In each selection, less than 10% of tame individuals were used as parents of the next generation.
"As a result of such rigorous selection, the offspring exhibiting the aggressive and fear avoidance responses were eliminated from the experimental population in just two to three generations of selection," Trut wrote
in a study published in 2009.
The foxes at the fox-farm were never trained to become tame. They lived in cages and had minimal contact with humans. Belyaev's aim was to create a genetically-distinct population, so he simply selected for particular behavioural traits.
"Belyaev had one main goal at the beginning of experiment: to reproduce the process of historical domestication at the experiment, during a short time," says Trut. "This goal didn't change. But during the experiment the understanding of evolutionary process changed."
By the fourth generation, the scientists started to see dramatic changes.
The cubs were beginning to behave more like dogs. They wagged their tails and "eagerly" sought contact with humans. They whined, whimpered and licked researchers just like puppies would.
The foxes could 'read' human cues and respond correctly to gestures or glances
The process was surprisingly quick. "By intense selective breeding, we have compressed into a few decades an ancient process that originally unfolded over thousands of years,"
wrote Trut in 1999.
These foxes were called the "elite of domestication", and as the generations passed the proportion of these elite cubs grew. By 2005-2006, almost all the foxes were playful, friendly and behaving like domestic dogs. The foxes could "read" human cues and respond correctly to gestures or glances. The vocalisations they made were different to wild foxes.
"The proudest moment for us was creating a unique population of genetically tame foxes, the only the one in the world," says Trut.
It was not just the foxes' personalities that were changing
The key point is that the experiment offers a hint as to the stages by which domestication takes place.
"Before, we knew that dogs and wolves were descended from the same ancestor, but we didn't know how," says Hare. "What came first? The fox experiment showed that just by selecting for friendliness, all these other changes, including an increase in social skills, happened by accident."
In fact, Belyaev and Trut soon found that it was not just the foxes' personalities that were changing. Their bodies were too.
"The main surprise was that, together with changing of behaviour, many new morphological traits in tame foxes start to appear from the first steps of selection," said Trut.
The domesticated foxes had floppier, drooping ears, which are found in other domestic animals such as dogs, cats, pigs, horses and goats. Curlier tails – also found in dogs and pigs – were also recorded.
All these changes were brought on by selecting for one trait: tameability
What's more, "in only a few generations, the friendly foxes were showing changes in coat colour," says Hare.
The process seems to be ongoing. "At the more advanced steps of selection, changes in the parameters of the skeletal system began to arise," Trut wrote. "They included shortened legs, tail, snout, upper jaw, and widened skull."
The foxes started looking more delicate and, put simply, "cute".
Their reproductive habits also changed. The domesticated foxes became sexually mature about a month earlier than non-domesticated foxes. Their mating season was longer and they could breed out of season. On average, their litters had one more cub.
All these changes were brought on by selecting for one trait: tameability. This gives us a big clue to how domestication works.
The physical traits Belyaev and Trut found, like the floppy ears, were those you would expect in a juvenile. But the domestic foxes carried them through into adulthood, suggesting the selection process had slowed down aspects of their development.
This might have something to do with chemicals in their bodies.
Selection has even affected the neurochemistry of our foxes' brains
Belyaev reasoned that selecting for tameability changed the mix of hormones and neurotransmitters the foxes' bodies made. He
believed behavioural responses were "regulated by a fine balance between neurotransmitters and hormones at the level of the whole organism".
For example, the drooping ears of the domesticated foxes might be a result of slowing down the adrenal glands. This could arrest the cells before the ear has time to stand to attention.
"Selection has even affected the neurochemistry of our foxes' brains," wrote Trut. One example she described was a drop in the "hormone-producing activity of the foxes' adrenal glands."
Domestic foxes also had higher levels of serotonin than farm-bred foxes. That is intriguing, because serotonin is "thought to be the leading mediator inhibiting animals' aggressive behaviour." Serotonin, like other neurotransmitters, is critically involved in shaping an animal's development from its earliest stages.
The project continues to this day. As of August 2016, there are 270 tame vixens and 70 tame males on the farm. However, it has run into financial problems.
"The current situation is not catastrophic, but not stable at the same time," writes Kharlamova. "The main reason of instability is of course the expense of this experiment."In the 1990s, the institute supported itself by selling fox pelts. At the end of the 1990s, they started to sell the foxes as house pets. At present, a Florida-based company called the Lester Kalmanson Agency Inc
imports foxes for those who want to keep them as pets. Each fox costs $8,900, because of the delivery costs.
With the foxes now tame, the researchers are trying to identify the genes that change under selection for tameness. "The main current goals are focused on molecular-genetics mechanisms of domestic behaviour," says Trut.