The Silver Fox A Possible Model of Domestication
The process of behavioral and physical pae-domorphosis has been observed experimentally in the selective breeding of silver foxes carried out by the Russian geneticist D. K. Belyaev and his associates at the institute of Cytology and Genetics in Siberia (Trut, 1999). Belyaev (1979) speculated that the dog's early domestication proceeded "unconsciously" by selecting and breeding captive animals that exhibited a high tolerance for fear and a minimal tendency to behave aggressively toward humans. To test this hypothesis, Belyaev initiated a long-term genetics project in which foxes were selectively bred for tameness. The project has been ongoing for 40 years and has produced over 40,000 foxes. An important early finding was that ordinary farm-bred foxes exhibit a wide variability with regard to their response to human contact. He has estimated that approximately 30% of the farm-bred population is extremely aggressive, 20% fearful, and 40% aggressive-fearful, whereas the remaining 10% exhibit a quiet (neither fearful nor aggressive) exploratory behavior toward people. The foxes belonging to the quiet group are by no means tame or safe to handle, however.
The breeding program involved carefully selecting only those foxes that exhibited a prosocial "tame" response to human contact and handling. After fewer than 20 generations of selective breeding, tame foxes began to appear that exhibited striking physical and behavioral alterations in comparison to randomly bred counterparts (Fig. 1.6). Tame foxes are not only tolerant of human contact, they actively solicit and appear to enjoy social interaction with human handlers. Tame foxes engage in various doglike behaviors, including hand and face licking, solicitous jumping up, vigorous tail wagging, and excited vocalizations (e.g., barking)—all reminiscent of domestic dogs. The physical appearance of tame foxes has also undergone dramatic pae-domorphic and doglike changes that include
- Fig. 1.6. Tame foxes are affectionate and invite contact with human handlers. Among several physical characteristics that distinguish tame foxes from farm-bred counterparts is a piebald pelage. (Photos courtesy L. N. Trut, institute of Cytology and Genetics).
lop ears, a turned-up tail (a doglike characteristic not observed in wild foxes), and the development of piebald pelage. Such white spotting is commonly seen in a variety of domestic species and is highly correlated with tameness. Little (1920) has discussed the hereditary basis of piebald spotting in dogs, concluding that it may be a "mutational" change rather than a gradual one occurring as the result of selection pressures.
in addition to behavioral and morphological changes, Belyaev's tame foxes also underwent several concurrent physiological alterations. For instance, tame female foxes exhibit significant deviations from the norm in terms of their sexual readiness and behavior, becoming sexually receptive earlier in the year than is the custom among wild foxes. Endocrine studies have demonstrated that gonadal hormone activity in tame foxes is al tered, perhaps underlying and guiding the observed behavioral changes. As is commonly observed among most domestic dogs (but not wild ones), some tame foxes actually produce offspring twice a year. In spite of increased receptivity, however, as many as 30% to 40% of the females fail to reproduce successfully. Tame females either fail to actually produce offspring or display disturbances in maternal behavior, including a tendency to neglect their young or to kill and eat them (infan-tiphagia). Hediger (1955/1968) has noted similar degenerative effects in the maternal behavior of other domestic species. Another seasonal activity affected by domestication is molting. Tame foxes exhibit a protracted period of shedding—a destabilizing effect that may be genetically linked to the disruption of estrous cycles.
Several neurophysiological concomitants of domestication have been isolated in tame foxes. Belyaev's associates have found significant alterations of the relative reactivity of the hypothalamic-pituitary-adrenocortical (HPA) system of tame foxes in comparison to wild counterparts. By comparing the reactions of tame and wild foxes to emotionally provocative experiences, they have determined that the tame foxes are less reactive to stressful experiences than are wild ones. Also, interesting changes have been found in brain areas associated with the expression of emotion. Serotonin levels in the brain tissue of tame foxes are significantly higher than in wild counterparts. Popova and colleagues (1991) confirmed these early findings, having isolated significant alterations throughout the serotonergic system in the brains of domesticated foxes. Serotonin has been shown to be an important neuromodulator providing inhibitory regulation over stress-related behavior and aggression. popova and colleagues have speculated that many of the behavioral and physiological changes (e.g., polyestrous tendency and reduced HPA system reactivity) observed in tame foxes may be causally linked with alterations in these serotonergic systems.
Selection for tameness among silver foxes has also produced changes in catecholaminer-gic systems. For example, tame foxes exhibit an increase of norepinephrine and dopamine activity in critical brain centers associated with the expression of defensive behavior. Dygalo and Kalinina (1994) have demonstrated a significant increase of tyrosine hy-drolase activity in the brains of tame foxes in comparison to wild controls. Tyrosine hydro-lase is the rate-limiting factor determining the amount of dopamine and norepinephrine that can be produced by the brain. The authors conclude that variations observed in the production of this essential enzyme is caused by a genetic alteration of the catecholaminer-gic system itself—a direct result of selective breeding for tameness. Similar comparisons have not been made between dogs and wolves. This line of research is of great importance for a better understanding of the mechanisms controlling defensive behavior at the neural level and may ultimately lead to productive insights into the etiology and management of canine aggression and fear-related behavior problems.
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