Ecological studies in feral populations of mice, fish and birds start to recognize the functional significance of phenotypes that differ individually in their behavioral and neuroendocrine response to environmental challenge. Within a species, the capacity to cope with environmental challenges largely determines the individual survival in the natural habitat. Recent studies indicate that the individual variation within a species may buffer the species for strong fluctuations in the natural habitat. A conceptual framework will be presented that is based on the view that the individual variation in aggressive behavior can be considered more generally as a variation in actively coping with environmental challenges. Highly aggressive individuals adopt a proactive coping style, whereas low levels of aggression indicate a passive or reactive style of coping. Similar coping styles have now been identified in a range of species including cattle, pigs and horses. They can be considered as trait characteristics that are stable over time and across situations. Evidence will be presented that a proactive coping style is best in a stable environment; these animals heavily rely on predictions. Reactive coping is more suited to variable environmental conditions because it is characterized by a continuous use of environmental input. Because the 2 extreme coping styles are adapted to different environmental conditions, there is differential stress vulnerability. Serious health problems may develop when coping fails. Social stress studies show that proactive individuals are resilient under stable environmental conditions but vulnerable when outcome expectancies are violated. Reactive individuals are in fact rather flexible and seem to adapt more easily to a changing environment. The health consequences of this interaction between individual coping style and social environment will be illustrated with examples from the cardiovascular system and the immune system. It will be argued that understanding animal welfare and the individual vulnerability to stress related disease requires a fundamental understanding of the functional individual variation as it occurs in nature and the underlying neurobiology and neuroendocrinology.