Adaptive dynamics of multiple behaviors along a shy-bold continuum

Background/Question/Methods

Consistent individual differences in behavior arrayed along a shy-bold continuum have been observed in a number of animal species. In at least some cases these behavioral differences appear to be heritable, which raises the question how such differences are maintained evolutionarily. Attempts to explain the maintenance of individual differences typically invokes trade-offs between competing inputs to fitness. For example, bold individuals may expose themselves more to predators while acquiring more resources, while shy individuals may avoid foraging in riskier environments but consume fewer resources. But such trade-offs alone cannot explain the co-existence of different behaviors; directional selection may simply yield a single intermediate behavior that strikes the best compromise between the competing inputs to fitness. To understand the co-existence of strategies along the shy-bold continuum we need to consider the frequency-dependence of fitness.

We modeled frequency-dependence in a shy-bold scenario using the approach of adaptive dynamics. Our model included a phenotypic trade-off between predation risk and energy that was mediated by metabolic rate. We solved for convergent stable equilibria along the shy-bold continuum and determined whether these equilibria could be invaded by rare strategies. We investigated how the invasibility of equilibria varied with frequency-dependence and the form of the trade-off.

Results/Conclusions

The evolution of multiple, co-evolutionarily stable behavioral strategies depended critically on whether fitness was frequency dependent. In the absence of frequency dependence only a single intermediate behavior evolved; the particular behavior depended on the form of trade-off, and the distribution of habitat riskiness.  In the presence of frequency-dependence the adaptive dynamics produced an equilibrium that was convergent stable, but subject to invasion by nearby behavioral strategies; in this case at least two co-evolutionarily stable strategies (shy and bold), and sometimes three (shy – intermediate – bold) could coexist. Our results suggest that consistent individual differences are more likely when the trade-offs that differentiate behaviors are accompanied by frequency-dependence in the fitness associated with those behaviors.