The Domesticated Silver Fox
Humans are dependent on domesticated animals for many things. Domesticated animals have been a source of food, fur, fertilizer, security, and companionship for all of recorded history, and archaeologists have found evidence of domestication dating back tens of thousands of years.
The animals we have domesticated for our farms and homes are very different from their wild brethren, both physically and genetically. However, the actual process of domestication - how our ancestors tamed the wild species thousands of years ago - has remained largely a mystery.
In 1959, a Russian geneticist named Dmitri Belyaev embarked on an experiment that would fill in some of the pieces of this puzzle.
In the 1950s, studying genetics in the Soviet Union was a dangerous occupation. The Lamarckian inheritance doctrine of Trofim Lysenko, an agricultural researcher with no scientific background but considerable political power, had been declared by the Soviet government to be the "only correct theory" of inheritance. Scientists who opposed the Lysenko "theory" -- that acquired characteristics could be passed on to offspring -- were considered bourgeois and fascist, and many geneticists were sent to labor camps or even executed.
Through the 1950s, Dmitri Belyaev managed to stay under the radar at the Central Research Laboratory of Fur Breeding in Moscow by studying genetics in secret, using "animal behavior" studies as a cover. The cold climate of Siberia proved much more hospitable for Belyaev's research, and he moved to Novosibirsk to help establish the Siberian Department of the Soviet Academy of Sciences. By the end of the decade, Lysenkoism had begun to decline, and Belyaev was able to conduct research openly once again.
Belyaev sought to understand the genetic effects of domestication on animals, noting some interesting similarities between different domesticated animal species. Ears frequently became floppy, fur frequently became lighter in color, and tails became curled or sometimes shortened. The general trend of these changes appeared to be one of pedomorphosis, or the retaining of juvenile traits into adulthood. The mechanism appeared to be genetic.
To test this hypothesis, Belyaev designed a novel experiment. Beginning with a population of 130 silver foxes (Vulpes vulpes) from a commercial fur farm, he and his team selectively bred only those that responded to the human researchers without fear or aggression. Those that snarled or bit were not permitted to mate. Those that allowed themselves to be handled and actively sought human contact were classified as Group I. Only the members of Group I were allowed to breed.
The experiment continued for more than 50 years. Over that time, the behavior and physical appearance of the foxes changed drastically.
Foxes That Fetch
Physical changes to the Group I population had become apparent by the tenth generation. Some foxes had patches of white fur on their faces abd bodies. Others had floppy ears and waggable tails similar to dogs. After 20 generations, some foxes had shorter legs and tails, and some had changes to their skull size resulting in over and under-bite.
As the experiment progressed, the percentage of foxes put into the "elite" group - the ones most amenable to human contact and thus permitted to mate - increased substantially. Ten generations in, about 18 percent of the foxes were docile enough to breed. After 20 generations, more than one third were in the docile category. After 40 years of breeding, more than 3/4 of offsprring were adequately docile.
Aside from the physical differences, the domesticated group of foxes had different development patters as well. Domesticated silver foxes responded to sights and sounds one to two days earlier than non-domesticated ones.
Domesticated foxes also took much longer to develop a fear response - nine weeks as opposed to six weeks for their counterparts. Belyaev's team found this to be connected with the production of corticosteroids - hormones that regulate responses to stress. Not only did the domesticated foxes have lower overall corticosteroid levels, but they began producing the hormone much later in their development.
On the other hand, domesticated foxes had higher levels of serotonin, a neurotransmitter that produces feelings of happiness and well-being. These neurological changes have been linked to genetic changes in the genes that regulate hemoglobin production, which plays a significant role producing the neurochemicals for mood and stress response.
This change allowed the foxes to become more socialized to the human experimenters, with many actively seeking human attention and affection. In the later years of the experiment, many of the foxes were sold as exotic pets. In video clips, these foxes can be seen playing with their owners and acting extremely dog-like, fetching balls, wagging their tails, and affectionately licking their owners' hands.
One Piece in the Domestication Puzzle
Perhaps the most surprising result of the silver fox domestication experiment is not the result itself, but the relatively short time period needed to achieve it. Docility had begun to emerge in the first decades of the experiment and had come to dominate the population within four decades.
While there is still considerable mystery about how humans domesticated animals early in our history, the silver fox experiment has provided some extremely useful insight into the genetic and chemical changes that turned the wolf into Man's Best Friend.
Sources and Further Information
- American Scientist - Early Canid Domestication: The Farm-Fox Experiment
Foxes bred for tamability in a 40-year experiment exhibit remarkable transformations that suggest an interplay between behavioral genetics and development
- Animal evolution during domestication: the domesticated fox as a model
We review the evolution of domestic animals, emphasizing the effect of the earliest steps of domestication on its course.
- Selection for tameness modulates the expression of heme related genes in silver foxes
Julia Lindberg et al. Behavioral and Brain Functions 2007, 3:18.
- Study of the Molecular Basis of Tame and Aggressive Behavior in the Silver Fox Model
International Collaborative Study between The James A. Baker Institute for Animal Health, Cornell University, USA,The Institute of Cytology and Genetics, Russian Academy of Sciences, Russia, and Department of Biology, University of Utah, USA