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(A)
(B)
FIGURE 10.1 (A) Behavior of a fox from the tame population. (B) Behavior of a fox from the
aggressive population. (Photographs: Darya Shepeleva)
SELECTION OF FOXES FOR AGGRESSIVE BEHAVIOR
In parallel with selection for tameness, selective breeding of farm foxes for
aggressive behavior began in 1970s. Because there is deliberate selection on
commercial farms against animals that show excessively aggressive responses
to humans, selection of the aggressive strain at ICG was undertaken to pre-
serve this behavior for research. Fifty farm-bred silver foxes with the most
aggressive responses to humans were selected from several fox farms and
used as founders of the aggressive population. The critical distance between
experimenter and caged animal and the intensity of fox aggressive response
were the major criteria for measuring aggression in the fox population, and to
select animals for producing the next generation ( Table 10.1B ). Progress in
selection for aggressive behavior did not follow the same pattern as selection
for tame behavior, being slower in comparison ( Trut 1980a, b ). Foxes from
the aggressive population are, however, consistently and distinctly aggressive
towards humans and difficult to handle ( Figure 10.1B ).
BEHAVIORAL AND PHYSIOLOGICAL CHANGES ASSOCIATED
WITH SELECTION FOR TAME BEHAVIOR
The genetic basis of fox tame and aggressive phenotypes has been clearly
demonstrated in multiple experiments which included experimental cross-
breeding of tame and aggressive animals, cross-fostering of newborn pups,
and embryo transplantation ( Kukekova et al., 2008, 2011a; Trut, 1980a, b,
2001 ). Although selection of foxes for tame behavior was strictly limited to
defined behavioral criteria, further developmental and behavioral differences
emerged in tame foxes. Opening of the eyelids and the external auditory
canal was accelerated; the sensitive period for socialization persisted past
60 days of age (compared to less than 45 days in unselected foxes); and play
activity (normally only seen in infantile wild type foxes) extended into
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