Many researchers have noted that beyond tameness, dogs appear to retain certain traits associated with juvenile wolves, especially behavioral traits such as whining, barking, and submissiveness. Russian geneticist Dmitry Belyaev focused on tamability as a guiding characteristic. His idea was not only that early humans would have selected the tamest animals to live with them, but also that selecting for a single trait could give rise to an entire set of changes in form, physiology, and behavior. Belyaev thus launched an experiment that would last longer than his life, seeking to test whether selecting for tameness would indeed produce a set of domesticated traits similar to those seen in dogs (Trut, 1999).Belyaev chose the silver fox for his experiment; this species is related to the dog, but it is not domesticated. The initial foxes in Belyaev’s experiment were not trained in any way, but simply tested for tameness at an early age. Starting at age one month, a human researcher would try to feed and pet the foxes, either alone or in the company of other foxes. The animals’ responses varied from aggressive behaviors (such as biting), to indifference, to seeking interaction with the person more than with the other foxes. The tamest foxes were then selected for breeding the next generation, although fresh genes were supplied through continual outbreeding.
Belyaev and his colleagues did indeed create a population of foxes that differed in temperament and behavior from their wild cousins. The foxes changed physically as well, with alterations in coat color appearing as early as the eighth generation—typically a loss of pigment resulting in white patches. The foxes also developed floppy ears and curved tails, mirroring traits seen in dogs as well as other domesticated species.
One of Belyaev’s hypotheses was therefore satisfied: Selecting for one trait (behavior) also changed other traits (here, aspects of the foxes’ physical form). A common thread in many of the observed changes across the generations in this experiment was that the timing of key developmental steps had been altered. Belyaev predicted that hormonal and neurochemical differences would be evident, and that such changes would be regulatory in nature and would control early development in a top-down fashion. In particular, two developmental milestones were different in the tamer foxes: their eyes opened several days earlier, and their fear response kicked in about three weeks later than the norm for wild foxes. These two events might have worked together to increase the openness of young foxes to interacting with humans and doing so without fear. At the same time, Belyaev found reduced levels of the stress hormone corticosterone in the domesticated foxes. Even the changes in coat color were linked to changes in the timing of development.