Hormonal manipulations of growth rate and its influence on predator avoidance - foraging trade-offs

Theoretical investigations into the impact that predators exert on prey spe cies suggest that two parameters, growth rate and mortality rate, should be the most influential in determining when animals should risk exposure to a predator in order to achieve higher feeding rates. While these two paramet ers have usually been assumed to be environmentally determined, we used thy roid hormone (3,3',5-triiodo-L-thyronine (T-3)) to manipulate growth rates and examine the behavioural consequences associated with these manipulation s. In two experiments, we examined how the growth rate of fathead minnows ( Pimephales promelas) is affected by treatment with T-3, and used the result s from this experiment to make a priori predictions about their relative wi llingness to risk exposure to a predator in order to receive increased feed ing rates. The first experiment demonstrated that T-3 significantly reduced the growth rates of fathead minnows compared with an unmanipulated control . When groups were compared in their relative willingness to risk exposure to a predator, manipulated growth rates in the first experiment were an acc urate predictor of behaviour; groups with relatively high growth rates were more willing to risk exposure to a predator. These results are consistent with the theoretical expectation that growth rates should be an important f actor determining decisions that involve trade-offs.