FEATURE ARTICLE
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
Lyudmila Trut
Physical Changes
Physically, the foxes differ markedly from their wild relatives.
Some of the differences have obvious links to the changes in their
social behavior. In dogs, for example, it is well known that the
first weeks of life are crucial for forming primary social bonds
with human beings. The "window" of bonding opens when a
puppy becomes able to sense and explore its surroundings, and it
closes when the pup starts to fear unknown stimuli. According to our
studies, nondomesticated fox pups start responding to auditory
stimuli on day 16 after birth, and their eyes are completely open by
day 18 or 19. On average, our domesticated fox pups respond to
sounds two days earlier and open their eyes a day earlier than their
nondomesticated cousins. Nondomesticated foxes first show the fear
response at 6 weeks of age; domesticated ones show it after 9 weeks
or even later. (Dogs show it at 8 to 12 weeks, depending on the
breed.) As a result, domesticated pups have more time to become
incorporated into a human social environment.
Moreover, we have found that
the delayed development of the fear response is linked to changes in
plasma levels of corticosteroids, hormones concerned with an
animal's adaptation to stress. In foxes, the level of
corticosteroids rises sharply between the ages of 2 to 4 months and
reach adult levels by the age of 8 months. One of our studies found
that the more advanced an animal's selection for domesticated
behavior was, the later it showed the fear response and the later
came the surge in its plasma corticosteroids. Thus, selection for
domestication gives rises to changes in the timing of the postnatal
development of certain physiological and hormonal mechanisms
underlying the formation of social behavior.
Other physical changes mirror those in dogs and other domesticated
animals. In our foxes, novel traits began to appear in the eighth to
tenth selected generations. The first ones we noted were changes in
the foxes' coat color, chiefly a loss of pigment in certain areas of
the body, leading in some cases to a star-shaped pattern on the face
similar to that seen in some breeds of dog. Next came traits such as
floppy ears and rolled tails similar to those in some breeds of dog.
After 15 to 20 generations we noted the appearance of foxes with
shorter tails and legs and with underbites or overbites. The novel
traits are still fairly rare. Most of them show up in no more than a
few animals per 100 to a few per 10,000. Some have been seen in
commercial populations, though at levels at least a magnitude lower
than we recorded in our domesticated foxes.
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