When organisms encounter environments that are heterogeneous in time, phenotypic plasticity is often favored by selection. The degree of such plasticity can vary during an organism's lifetime, but the factors promoting differential plastic responses at different ages or life stages remain poorly understood. Here we develop and analyze an evolutionary model to investigate how environmental information is optimally collected and translated into phenotypic adjustments at different ages. We demonstrate that plasticity must often be expected to vary with age in a non-monotonic fashion. Early in life it is generally optimal to delay phenotypic adjustments until sufficient information has been collected about the state of the environment to warrant a costly phenotypic adjustment. Towards the end of life, phenotypic adjustments are disfavored as well, because their beneficial effects can no longer fully be reaped before death.Our analysis clarifies how patterns of age-dependent plasticity are shaped by the interplay of environmental uncertainty, the accuracy of perceived information and the costs of phenotypic adjustments with life-history determinants such as the relative strengths of fecundity and viability selection experienced by the organism over its lifetime. We conclude by comparing our results with expectations for alternative mechanisms, including developmental constraints, that promote age-dependent plasticity.3

Maternal effects on offspring size can have a strong effect on fitness, as larger offspring often survive better under harsh environmental conditions. Selection should hence favour mothers that find an optimal solution to the offspring size versus number tradeoff. If environmental conditions are variable, there will not be a single optimal offspring size, as predicted in a constant environment, but plastic responses can be favoured. To be able to adjust offspring size in an adaptive manner, mothers have to use environmental cues to predict offspring environmental conditions. Cues can be unreliable, however, particularly in species where individuals occupy different niches at different life stages. Here we model the evolution of plasticity of offspring size when the environmental cues mothers use to predict the conditions experienced by their offspring are not perfectly reliable. Our results show that plastic strategies are likely to be superior to fixed strategies in a stochastically varying environment when the environmental cues are at least moderately reliable, with the threshold depending on plasticity costs and the difference of resources available to mothers. Plasticity is more likely to occur if resource availability is not too different between environments. For any given scenario, plasticity in offspring size is favoured if offspring survival varies greatly between environmental states. Whenever plastic strategies are optimal, the occurring switches performed by mothers between small and large offspring are predicted to be substantial, as small adjustments are unlikely to reap fitness benefits great enough to overcome the costs of plasticity.