5. Stressor intensity Apart from being qualitatively dened as an uncontrollable and/or unpredictable stimulus, a stressor has a quantitative dimension as well. An individuals interpretation of a situation and its reaction may vary from full control to only partial or complete loss of control. Moreover, a stressor may be mild in terms of its potential consequences or it may be life-threatening. Of course, a traumatic event that is life-threatening can be regarded as an unpredictable and uncontrollable situation. In theory, this leads to a three-dimensional constellation in which controllability and predictability form two dimensions and the third dimension is stressor intensity. However, because controllability and predictability are not fully independent, these two dimensions are combined in Fig. 6. Journal Identication = NBR Article Identication = 1416 Date: March 3, 2011

Time: 8:9 pm J.M. Koolhaas et al. / Neuroscience and Biobehavioral Reviews 35 (2011) 12911301 1297 This gure depicts a graded relationship between the degree of uncontrollability/unpredictability and the life-threatening nature of the situation. The lower part of the Y-axis indicates the safe condition where the organism is free from any environmental challenges, e.g. a clear and fully accepted position in the social hierarchy, presence of sufcient social support, freely available food, etc. As argued before, the term stress should be restricted to conditions depicted in the right top corner of the graph, for example an unstable social hierarchy, social outcast, limited access to food or resources (e.g. Bohus et al., 1991; Sapolsky, 1995; Bartolomucci, 2007).

This implies that species and individuals have a certain range of environmental conditions within which regulating processes operate adequately without requiring adaptive changes. We will call this range of environmental conditions the regulatory range. These environmental conditions may include a temperature range, a range of food availability or a range of social instability. The regulatory range should be distinguished from adaptive capacity. The distinction between regulatory range and adaptive capacity can be illustrated by considering the response of an organism to an environment of food shortage that gets gradually colder. The adaptive capacity includes behavioural responses such as migration in anticipation of a seasonal drop in temperature and food shortage, building a nest to reduce energy expenditure, food hoarding, etc. A physiological response may include changes in cardiovasc

At the level of the brain, some species such as groundhogs have the capacity to change the set-point of body temperature regulation and metabolism and go into torpor (Heldmaier et al., 2004). Thus, the full adaptive capacity includes mechanisms at the level of the brain, peripheral physiology and behaviour. In a fully t and healthy organism this set of mechanisms is optimized for a range of environmental conditions (regulatory range). However, there are many conditions that may reduce the actual adaptive capacity of an individual. For example, when an individual has been unable to build up sufcient adipose tissue due to food shortage, or has insufcient nest building material, its capacity to cope with a freezing temperature is reduced. In these situations, despite an individual being within his normal regulatory range, its adaptive capacity might be reduced and the stimulus will be perceived as a stressor, thus inducing a stress response, in agreemen