How does temperature affect behaviour? Multilevel analysis of plasticity, personality and predictability in hermit crabs

When animals are observed on multiple occasions, consistent between-individual differences in behaviour, often referred to as animal personality, may be observed. However, this does not mean that the behaviour of a given individual is readily predictable. While some individuals show low levels of variation around their behavioural mean, others show high levels of variation, and there may be significant between-individual differences in this intraindividual variation (‘IIV’) in behaviour. While it has been suggested that IIV might reduce susceptibility to predators, little is known about the functions or causation of IIV. We investigated the effects of temperature on the startle response duration of hermit crabs, Pagurus bernhardus. For poikilothermic animals, temperature has a direct and multiplicative influence on metabolic rate, which in turn is expected to influence behaviour because of its effect on energy requirements. At the level of mean startle response durations, the effect of temperature was dependent on treatment order; within treatment orders, individuals showed different reaction norms. In contrast, at the level of IIV, while the presence of significant between-individual differences was dependent on treatment order, there was a clear unidirectional effect in both treatment orders for individuals to be less predictable at higher temperature. Thus, predictability in behaviour appears to vary with a key environmental variable that is known to influence energy requirements and potentially the level of risk that individuals are willing to accept.