Fig. 1

Schematic illustration showing how plasticity in behavior with age can occur on different levels. For simplicity, only linear plasticity between behavior and age are drawn here, but the same hierarchical structure applies to non-linear relationships. On the population level, (a) the age-specific mean behavior may be invariant with age, but (b) may also vary across ages. On the individual and genetic level, deviations from these age-specific means are considered. (c) All individuals show the same deviation from the average behavior at all ages, and there hence is no between-individual variation in plasticity over ages; no Individual – Age interaction (IxA). Alternatively, (d) individuals differ in their age-specific deviation from the age-specific means, thus showing variation in plasticity (IxA). Despite the presence of IxA, (e) Genotype – Age interaction (GxA) may be absent, or (f) GxA occurs without the ranking of genotypes changing across ages (reaction norms not crossing within the range of ages considered), or (g) GxA where the ranking of genotypes changes (reaction norms cross).