AN ANALYSIS OF FEAR AND AGGRESSION DURING EARLY DEVELOPMENT OF BEHAVIOR IN SILVER FOXES (VULPES-VULPES)

The relationship between the development of behaviour in a novel situation and plasma cortisol level was analysed in 30-, 45- and 60-day-old domestic (D) foxes and foxes selected for enhanced aggressiveness (EA) towards man (Experiment 1). In the EA foxes, the increase in defensive responses (fear and aggression) was associated with a significant rise in plasma cortisol level at 45 and 60 days of age. The D foxes showed no defensive responses up to Day 60 and there were no associated changes in plasma cortisol level. In Experiment 2, chloditane, an inhibitor of adrenal cortex function, injected to the EA pups from Days 38-44 and 45-52 of life, attenuated fear and increased locomotor activity in the novel situation; there was an associated decline in plasma cortisol level. Treatment with chloditane, however, did not affect the number of foxes showing aggression in the novel situation and at the appearance of man both at the age of 60 days and in adulthood. Experiment 3 demonstrated a significant increase in locomotor activity in the novel situation and a decrease in the number of EA foxes exhibiting aggression after treatment with 1-tryptophan (a precursor of serotonin synthesis) after the age of 60 days. The EA foxes treated at an early age with 1-tryptophan showed a significant attenuation of aggression when adults. Taken together, the results demonstrate that plasma cortisol level and the fear response are related and also that enhanced aggressiveness affects greatly the development of behaviour in the EA foxes and contributes, together with the fear reaction, to limitation of the socialisation period.