Plasticity of boldness: high perceived risk eliminates a relationship between boldness and body size in fathead minnows

Abstract

In the face of anthropogenic change, we require a better understanding of how adaptive behavioural changes emerge from the interaction between personality and phenotypic plasticity, to be able to predict population persistence. Predation is an important context where this interaction occurs. Sampling wild fish populations revealed that boldness is size dependent in habitats with low predation risk but not in high predation habitats. This is predicted to occur due to a trade-off between a prey's metabolic demand, as per the metabolic hypothesis (smaller fish have higher metabolic requirements and thus need to be bolder) and their risk of predation, as per the predation risk hypothesis (smaller fish are subject to higher predation risk and should thus be shyer). However, since selective predation on specific individuals, age effects or the extent of direct experience with predators were not controlled for in these studies, the role of body size in modulating boldness remains unclear. Here, we tested the relationship between body size, boldness and predation risk in a widespread North American prey fish, the fathead minnow, Pimephales promelas. From hatching onwards, we manipulated perceived risk by exposing minnows to either conspecific alarm cues (indicative of predation risk) or distilled water in a split-clutch design. When fish were 4 months old, we measured body size and determined boldness using an emergence assay. Different levels of perceived predation risk did not directly affect body size or general boldness in fathead minnows. However, as predicted by the metabolic and predation risk hypotheses, high perceived predation risk cancelled out the negative correlation between boldness and body size present in controls through phenotypic plasticity. This effect was independent of selective predation, direct experience with predators and age. By adjusting personality through phenotypic plasticity, perceived predation risk alone may thus be sufficient for individuals to maximize their fitness in high-risk habitats.