Boldness is a personality trait of animals that is widespread across many different species. The boldness-shyness continuum represents a fundamental axis of behavioral variation and has been in the center of attention of behavioral ecologists. When relating boldness to a predation context, differences in boldness between individuals can lead to a range of responses from predator ignorance to complete predator avoidance. While it has been known that boldness has a genetic component, it has also evolved to be markedly influenced by the environment, which makes boldness a plastic trait. This is because for example, it is not always advantageous to be bold in a predator-rich environment due to the risk of being eaten. Likewise, being shy in a predator-free environment does not offer many advantages due to the risk of losing when competing over food or mates with bolder individuals. At the same time, the ideal level of boldness also depends on body size. Small animals are growing quickly, have a small stomach and few fat reserves; thus they have a metabolic need to constantly search for food – this is not the case for larger individuals that have ample reserves. At the same time, smaller individuals have a higher risk of getting eaten – many different predators will readily feed on small animals. This is not the case for large animals, which may have only few predators, if any. Consequently, there is a relationship between body size and boldness, which has been theoretically predicted and observed in previous studies on natural fish populations. In predator-free rivers and lakes, boldness is size-dependent; smaller fish are bolder than larger individuals due to their increased need of finding food. However, in predator-rich ecosystems, the higher predation risk for smaller fish balances out this size-dependency, making small and large fish equally bold.
As previous studies on this topic have only been performed in natural habitats, they could not control for various confounding factors that may influence this result. First, predators may have eaten all the fish of a specific personality or a specific body size beforehand so that these fish could not be tested at the time of the experiments. Second, direct experience with predators, such as attacks that followed an inappropriate boldness response and were just narrowly escaped, may be responsible for shifts in personality. Third, as fish grow throughout their whole life, older fish are also larger and the observed “size-dependent” effects may be an effect of age instead. Older fish in natural predator-rich habitats simply have more experience with dealing with predators and are the only large fish that were actually able to survive until the time of the experiments.
To investigate whether the theoretically predicted and previously observed size-dependent boldness response is consistent even in the absence of these potentially confounding factors, I conducted a laboratory study. Here, I raised fathead minnows, Pimephales promelas under either simulated presence or absence of predators. To simulate predation risk, I applied alarm cues, which are released by injured fish and signal the presence of predators to conspecifics. From hatching onwards, I raised siblings under either continuous exposure to alarm cues or a control treatment. At 4 months age, I then assessed the boldness of individual fish in both treatments. I found that boldness was size-dependent only in fish from the control treatment. In alarm cue-exposed fish, this pattern was not present. These results are consistent with theoretical predictions and the previous studies in natural ecosystems and confirm that perceived predation risk alone can cause a plastic shift in boldness.
This study is the first published one that I have conducted during my abroad research fellowship at the University of Saskatchewan in Canada. The complete study is available in the journal Animal Behaviour. I hope you enjoyed the read!