The motivational nature of domestication-related behavior, as well as its formation and variation at the ontogenetic, population, and breed levels were studied. Food motivation participated in the environmental regulation of sheep defensive reactions to humans: an increase in motivation decreased the strength of defensive behavior, and vice versa. Defensive behavior became weaker with age, with the rate of decrease higher in sheep with high food motivation. Comparison of 11 breeds revealed that the polymorphism of domestication-related behavior was breed-specific. In low-specialized breeds, the "wild" class prevailed (77.0-94.7%); it was characterized by a pronounced defensive behavior. In commercial breeds, the range of variation was broader, with a higher proportion of "domestication" classes. In Ost-Friesian sheep, this proportion was 86.0%. It is suggested that the polymorphism of domestication-related behavior was formed in commercial breeds as a result of natural selection for adaptation to breeding-related stresses and artificial selection for productivity.