Using the reactive scope model to redefine the concept of social stress in fishes

The term 'social stress' has traditionally referred to physiological stress responses induced by the behaviour of conspecifics, particularly aggression or agonistic behaviours. Here, we review the physiological consequences of social status in fishes using the reactive scope model (RSM) to explain the divergent physiological phenotypes of dominant and subordinate fish. The RSM plots levels of different physiological mediators (e.g. behaviour, glucocorticoid hormones) over time, using them to define functional ranges that differ in their consequences for the animal. We discuss differences in growth, reproduction and tolerance of environmental challenges, all of which are suppressed in subordinate individuals, and focus on the underlying mechanisms that give rise to these phenotypes. Repeated and/or continual activation of the hypothalamic-pituitary-interrenal (HPI) axis in subordinate fish can lead to prolonged elevation of cortisol, a key physiological mediator. In turn, this increases physiological 'wear and tear' in these individuals, lowering their reactive scope (i.e. the physiological range of a healthy animal) and increasing their susceptibility to homeostatic overload. That is, they experience social stress and, ultimately, their capacity to cope with environmental challenges is limited. By contrast, reactive scope is maintained in dominant individuals, and hence they are better able to tolerate environmental challenges. Redefining social stress in terms of the RSM allows us to overcome the ambiguities and limitations associated with the concept of stress.