How the cascading effects of a single behavioral trait can generate personality

Why are some personalities less plastic?

Individuals from the same population typically show consistent differences in behavioural traits that are frequently associated with differences in contextual plasticity. Yet such a correlation might arise either because some individuals are better able than others to detect environmental changes or because the benefits of being plastic are condition-dependent. To discriminate between these two competing hypotheses, I developed an individual-based model that simulates a population in which individuals of varying fighting ability compete by pairwise interactions using either the fixed hawk (aggressive) or dove (peaceful) strategies or a conditional assessment strategy. As anticipated, the model predicts that only individuals with low (and/or intermediate) fighting ability should use the assessment strategy, giving rise to a negative (or dome-shaped) relationship between aggressiveness and plasticity. The proportion of plastic individuals, however, should be affected not only by the environmental conditions in which individuals live but also by the mechanism that would maintain variation in the traits that determine the benefits of plasticity. In particular, if individual differences in fighting ability may be eroded by natural selection, it predicts that ecological conditions that cause assortative interactions (e.g. high predation risks) would contribute in maintaining variation among individuals in their fighting ability, thereby favouring greater plasticity.

Individual variation in functional response parameters is explained by body size but not by behavioural types in a poeciliid fish

Functional responses are per-capita feeding rate models whose parameters often scale with individual body size but the parameters may also be further influenced by behavioural traits consistently differing among individuals, i.e. behavioural types or animal personalities. Behavioural types may intrinsically lead to lower feeding rates when consistently shy, inactive and easily stressed individuals cannot identify or respond to risk-free environments or need less food due to lower metabolic rates linked to behaviour. To test how much variation in functional response parameters is explained by body size and how much by behavioural types, we estimated attack rate and handling time individually for differently sized female least killifish (Heterandria formosa) and repeatedly measured behavioural traits for each individual. We found that individual fish varied substantially in their attack rate and in their handling time. Behavioural traits were stable over time and varied consistently among individuals along two distinct personality axes. The individual variation in functional responses was explained solely by body size, and contrary to our expectations, not additionally by the existing behavioural types in exploration activity and coping style. While behavioural trait-dependent functional responses may offer a route to the understanding of the food web level consequences of behavioural types, our study is so far only the second one on this topic. Importantly, our results indicate in contrast to that previous study that behavioural types do not per se affect individual functional responses assessed in the absence of external biotic stressors.