Coping styles in animals: current status in behavior and stress-physiology
Introduction
Psychosocial factors have long been recognized as important in health and disease both in man and in animals. It is not the physical characteristics of a certain aversive stimulus but rather the cognitive appraisal of that stimulus, which determines its aversive character and whether a state commonly described as stress is induced. The impact of aversive stimuli or stressors is determined by the ability of the organism to cope with the situation [1], [2]. Several definitions of coping can be given [3]. In the present paper, we prefer to use the term coping as the behavioral and physiological efforts to master the situation [3], [4]. Successful coping depends highly on the controllability and predictability of the stressor [5], [6]. A consistent finding across species is that whenever environmental stressors are too demanding and the individual cannot cope, its health is in danger. For this reason, it is important to understand the mechanisms and factors underlying the individual's capacity to cope with environmental challenges. A wide variety of medical, psychological and animal studies demonstrate that individuals may differ in their coping capacities. Factors that have been shown to affect the individual's coping capacity include genotype, development, early experience, social support, etc. Since many studies in humans indicate that coping mechanisms are important in health and disease [7], researchers have tried for a long time to determine the individual vulnerability to stress-related diseases using estimates of the individual coping capacity. One approach concerns attempts to classify coping responses into distinct coping styles. A coping style can be defined as a coherent set of behavioral and physiological stress responses which is consistent over time and which is characteristic to a certain group of individuals. It seems that coping styles have been shaped by evolution and form general adaptive response patterns in reaction to everyday challenges in the natural habitat. The concept of coping styles has been used in a wide variety of animal species (see Table 1). Despite the widespread use of the concept, it is not without debate [8]. This is due to several flaws in the studies using the concept. First, not all studies fulfill the criterion of coping style as a coherent set of behavioral and physiological characteristics because only one parameter has been studied. Second, the extent to which clearly distinct coping styles can be distinguished has been questioned [8], [9]. Special attention will be given here to the frequency distribution of coping styles in a population, the consistency over time and the one-dimensional character of the concept of coping styles. Finally, one may wonder to what extent the concept of coping styles is really related to the individual vulnerability to stress-mediated disease.
This review will discuss these major issues and it will be argued that the clustering of various behavioral characteristics may to some extent be causally related to differences in (re)activity of the neuroendocrine system.
Section snippets
Behavioral characteristics of coping styles
Much of our current thinking on coping styles is derived from the work of Jim Henry [10]. He suggested, on the basis of social stress research in animals and man, that two stress response patterns may be distinguished. The first type, the active response, was originally described by Cannon [11] as the fight-flight response. Behaviorally, the active response is characterized by territorial control and aggression. Engel and Schmale [12] originally described the second type of stress response as
Distinct coping styles: distribution and consistency over time
The concept of coping style and the way it is generally presented in the literature suggests that there are distinct phenotypes, which are more or less stable over time in their response to stressors. The early studies by Oortmerssen and colleagues, on a feral population of house mice suggest a bimodal distribution of male phenotypes as measured by the individual latency to attack a standard intruder into the home cage [18]. The idea of bimodal distributions has been strengthened by the fact
Neuroendocrine characteristics of coping styles
Differences in coping style have been observed in male rodents during both social and non-social stressful conditions (see Table 2). Coping styles are not only characterized by differences in behavior but also by differences in physiology and neuroendocrinology. As mentioned earlier, tests that measure aspects of initiative or proactivity seem to be most discriminative. The defensive burying tests in rodents is such a test, which allows the animal a choice between proactive and reactive coping.
Causal relationship between neuroendocrine and behavioral characteristics of coping
One may wonder to what extent the behavioral and physiological characteristics are causally related. Of course, it is highly unlikely that all differences in coping style can be reduced to one single causal factor. However, evidence is accumulating that a differential HPA axis reactivity may explain some of the behavioral differences. In different species, freezing behavior as part of the reactive coping response can be observed in response to an inescapable stressor or predator. In rats, a
Coping styles and differences in disease vulnerability
The concept of coping styles implies that animals have a differential way to adapt to various environmental conditions. Negative health consequences might arise if an animal cannot cope with the stressor or needs very demanding coping efforts. Sustained over-activation of various neuroendocrine systems may lead to specific types of pathology. Hence, in view of the differential neuroendocrine reactivity and neurobiological make-up, one may expect different types of stress-pathology to develop
Concluding comments
The concept of coping style has been frequently used in many studies and in an increasing number of species. However, only a few studies have a sufficiently broad approach to the individual behavioral and physiological characteristics and their consistency over time to be conclusive on the generality of the typology across species. Nevertheless, the available literature makes it tempting to consider the possibility that the distinctions between proactive and reactive coping styles represent
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