Individual vertebrates are characterised by the flexible way they cope with stressors: mammals and birds provide good examples of coping behaviour. When stressed and an adequate solution is not available, vertebrates experience a state of uncertainty. Stress symptoms occur under conditions of lowered predictability and/or controllability of the environmental situation. Neuroendocrine changes associated with stress may reduce the immune and reproductive capability. Although the performance of disturbed behaviour has adaptive value, it simultaneously demonstrates an overtaxed and unhealthy state. Individual vertebrates appear to cope with stressors in a predominantly sympathetic or parasympathetic way. The severity of the stress experienced by an individual is also determined by the quality of its social contacts with conspecifics and men.