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Body size is a key determinant of mortality risk. In natural populations, a broad range of relationships are observed between body size and mortality, including positive and negative correlations. Previous evolutionary modeling has shown that negatively size-dependent mortality can result in life-history bistability, with early maturation at small size and late maturation at large size representing alternative fitness optima. Here we present a general analysis of conditions under which such life-history bistabilities can occur, reporting the following findings. First, alternative fitness optima can be found for any arbitrarily chosen forms of mortality functions, including functions according to which mortality smoothly declines with size. Second, while bistabilities occur more readily under negatively size-dependent mortality, our analysis reveals that they can also emerge under positively size-dependent mortality, a feature missed in earlier work. Third, any sharp drop of mortality with size facilitates bistability. Fourth, if the mortality regime involves more than one such sharp drop, multistable life histories can occur, with alternative fitness optima straddling each of the drops. Paradoxically, our findings imply that, fifth, a species-poor predator community capable of creating a rugged mortality regime is conducive to evolutionary multistability, which could act as a stepping stone toward prey life-history diversification, whereas a species-rich predator community that results in a smoothly varying mortality regime may prevent diversification through this pathway. [ABSTRACT FROM AUTHOR]

Social animals can greatly benefit from well-developed social skills. Because the frequency and diversity of social interactions often increase with the size of social groups, the benefits of advanced social skills can be expected to increase with group size. Variation in social skills often arises during ontogeny, depending on early social experience. Whether variation of social-group sizes affects development of social skills and related changes in brain structures remains unexplored. We investigated whether, in a cooperatively breeding cichlid, early group size (1) shapes social behavior and social skills and (2) induces lasting plastic changes in gross brain structures and (3) whether the development of social skills is confined to a sensitive ontogenetic period. Rearing-group size and the time juveniles spent in these groups interactively influenced the development of social skills and the relative sizes of four main brain regions. We did not detect a sensitive developmental period for the shaping of social behavior within the 2-month experience phase. Instead, our results suggest continuous plastic behavioral changes over time. We discuss how developmental effects on social behavior and brain architecture may adaptively tune phenotypes to their current or future environments. [ABSTRACT FROM AUTHOR]

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 nonmonotonic 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. Toward the end of life, phenotypic adjustments are disfavored as well because their beneficial effects can no longer be fully 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. [ABSTRACT FROM AUTHOR]

When environmental conditions vary stochastically, individuals accrue fitness benefits by exhibiting phenotypic plasticity. Here we analyze a general dynamic-programming model describing an individual's optimal energy allocation in a stochastic environment. After maturation, individuals repeatedly decide how to allocate incoming energy between reproduction and maintenance. We analyze the optimal fraction of energy invested in reproduction and the resultant degree of plasticity in dependence on environmental variability and predictability. Our analyses reveal unexpected patterns of optimal energy allocation. When energy availability is low, all energy is allocated to reproduction, although this implies that individuals will not survive after reproduction. Above a certain threshold of energy availability, the optimal reproductive investment decreases to a minimum and even vanishes entirely in highly variable environments. With further improving energy availability, optimal reproductive investment gradually increases again. Costs of plasticity affect this allocation pattern only quantitatively. Our results show that optimal reproductive investment does not increase monotonically with growing energy availability and that small changes in energy availability can lead to major variations in optimal energy allocation. Our results help to unify two apparently opposing predictions from lifehistory theory, that organisms should increase reproductive investment both with improved environmental conditions and when conditions deteriorate ("terminal investment"). [ABSTRACT FROM AUTHOR]