GILMOUR, K.M.; Carleton University: Physiological causes and consequences of social status in rainbow trout

When rainbow trout (Oncorhynchus mykiss) are held in small groups, they form linear dominance hierarchies. The most dominant fish holds the best position in the environment, gains the largest share of food and exhibits aggression towards fish lower in the hierarchy. At the other end of the hierarchy, subordinate fish exhibit behavioural inhibition, including reduced activity and feeding, and may be subjected to aggressive attacks from more dominant fish. These behavioural characteristics associated with social status are thought to be due, at least in part, to changes in brain monoamines resulting from social interactions. In addition to these behavioural characteristics, differences in physiological factors are associated with social status. Dominant fish exhibit higher growth rates and better condition than subordinate fish. Low social status appears to be a chronic stress, as indicated by chronic elevation of circulating cortisol concentrations in subordinate fish. High cortisol levels may, in turn, be responsible for many of the deleterious physiological consequences of low social status, including lower growth rates and condition, immunosuppression and increased mortality. Our recent work suggests that cortisol levels may also be a factor in determining the outcome of social interactions in pairs of rainbow trout, and hence in determining social status. Rainbow trout treated with cortisol were significantly more likely to become subordinate in paired encounters with a conspecific. Interactions between circulating cortisol levels and brain monoamines may provide a mechanism through which cortisol influences social status.