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Summary 1.Contests may involve injurious fighting, other types of direct physical aggression and communication. They occur over ownership access to mates and other resources that may increase an individual's attractiveness and its chance of survival. Traits that enhance resource holding potential may be the result of sexual selection, natural selection or a combination of both. 2. Agonistic behaviours are expected to be demanding to perform and costly in terms of changes in physiological state. The ability to meet the physiological costs may determine contest outcomes and constrain the intensity of agonistic activities. 3. The energetic costs have been investigated in a broad range of taxa using a variety of techniques. They include the mobilization of energy reserves, but a key cost in several taxa appears to be limited anaerobic capacity and subsequent accumulation of lactic acid. Androgens, stress hormones and neurohormones have also been shown to constrain fighting behaviour. However, due to key differences in the endocrine systems of vertebrates and invertebrates, the effects of hormones are far less consistent across taxa than in the case of metabolites. 4. Physiological constraints on fighting may vary according to their importance relative to circumstantial costs, the time-scale over which they exert their effects, their effects on different roles and their causal links with behaviour. Incorporating these factors into theoretical studies of contest behaviour may give further insights of how the costs of fighting influence agonistic behaviour.

Many phenotypic traits show plasticity but behaviour is often considered the 'most plastic' aspect of phenotype as it is likely to show the quickest response to temporal changes in conditions or 'situation'. However, it has also been noted that constraints on sensory acuity, cognitive structure and physiological capacities place limits on behavioural plasticity. Such limits to plasticity may generate consistent differences in behaviour between individuals from the same population. It has recently been suggested that these consistent differences in individual behaviour may be adaptive and the term 'animal personalities' has been used to describe them. In many cases, however, a degree of both behavioural plasticity and relative consistency is probable. To understand the possible functions of animal personalities, it is necessary to determine the relative strength of each tendency and this may be achieved by comparison of statistical effect sizes for tests of difference and concordance. Here, we describe a new statistical framework for making such comparisons and investigate cross-situational plasticity and consistency in the duration of startle responses in the European hermit crab Pagurus bernhardus, in the field and the laboratory. The effect sizes of tests for behavioural consistency were greater than for tests of behavioural plasticity, indicating for the first time the presence of animal personalities in a crustacean model.

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Variation in behaviour occurs at multiple levels, including between individuals (personality) and between situations ( plasticity). Behaviour also varies within individuals, and intra-individual variation (IIV) in behaviour describes within-individual residual variance in behaviour that remains after the effects of obvious external and internal influences on behaviour have been accounted for. IIV thus describes how predictable an individual's behaviour is. Differences in predictability, between individuals and between situations, might be biologically significant. For example, behaving unpredictably under predation threat might reduce the chance of capture. Here, we investigated the duration of startle responses in hermit crabs, in the presence and absence of a predator cue. Individuals differed in startle response duration ( personality) and while individuals also varied in their sensitivity to risk, mean response time was greater in the presence of a predator ( plasticity). Moreover, IIV was greater in the presence of a predator, providing some of the first evidence that the facultative injection of unpredictability into behaviour might represent a strategy for dealing with risk.