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.