An explanatory framework for adaptive personality differences

Abstract

We develop a conceptual framework for the understanding of animal personalities in terms of adaptive evolution. We focus on two basic questions. First, why do behavioural types exhibit limited behavioural plasticity, that is, behavioural correlations both across contexts and over time? Second, how can multiple behavioural types coexist within a single population? We emphasize differences in 'state' among individuals in combination with state-dependent behaviour. Some states are inherently stable and individual differences in such states can explain stable differences in suites of behaviour if it is adaptive to make behaviour in various contexts dependent on such states. Behavioural stability and cross-context correlations in behaviour are more difficult to explain if individual states are potentially more variable. In such cases stable personalities can result from state-dependent behaviour if state and behaviour mutually reinforce each other by feedback mechanisms. We discuss various evolutionary mechanisms for the maintenance of variation (in states and/or behaviour), including frequency-dependent selection, spatial variation with incomplete matching between habitat and phenotype, bet-hedging in a temporally fluctuating environment, and non-equilibrium dynamics. Although state differences are important, we also discuss how social conventions and social signalling can give rise to adaptive personality differences in the absence of state differences.

Figure 1.

Explaining adaptive behavioural correlations. We consider three possible explanations for the evolution of behaviour that is consistent in time and correlated across contexts. The first is straightforward: when individual behaviour is dependent on states that are inherently stable (like sex or caste), the time consistency of state differences will be reflected in time-consistent behaviour; moreover cross-context correlations will result if the same state is of selective importance in different contexts. This explanation can be extended to potentially variable, labile states (like energy reserves, blood pressure or hormone levels) if positive feedbacks between state and behaviour induce time consistency in states. In the case of social interactions, consistent and correlated behaviour can evolve in the absence of state differences. Examples are social conventions (like winner–loser effects) or the coevolution of social responsiveness and behavioural consistency.

Figure 2.

Explaining the adaptive coexistence of behavioural types. Several eco-evolutionary mechanisms can explain the adaptive coexistence of multiple behavioural types within a population. Dependent on the mechanism, behavioural types may or may not obtain equal fitness and phenotypic variation may or may not be associated with genetic variation.