Foxes Bred for Friendliness: A Dog's Origin?

A long-term genetic experiment initiated by Russian scientist Dmitry Belyaev sought to replicate animal domestication by focusing solely on selecting for tameness in silver foxes. The experiment, which began with 30 males and 100 vixens from a fur farm, strictly bred only those foxes exhibiting calmness and friendliness toward humans. Initial generations showed a rapid shift, with foxes displaying behaviors like licking experimenters' hands and wagging their tails, mirroring canine traits.

Remarkably, unintended physical transformations began to appear by the eighth generation. These included depigmented coat patches, floppy ears, and curled tails, similar to those seen in domesticated dogs. These changes, along with others like shorter limbs and altered jaw structures, occurred without any direct selection for these traits, suggesting a biological link between tameness and physical development.

Researchers attribute these widespread changes to a reduction in the foxes' stress response system and alterations in brain chemistry. Corticosteroid levels decreased significantly, while serotonin, a neurotransmitter linked to reduced aggression, increased. These neurochemical shifts appear to influence embryonic development, leading to the emergence of common domesticated features.

The experiment also expanded the foxes' "socialization window," delaying the onset of fear responses in pups and allowing for more extended bonding with humans. By the sixth generation, a "domesticated elite" category was created for foxes exhibiting exceptionally high levels of human-oriented behavior, a trend that continued to increase over the decades of the study.