Hormonal regulation of the phenotype into environmentally appropriate pace‐of‐life syndromes

Abstract The risk of predation is an important driver that tailors life histories in various ways. Using an evolutionary model based on hormonal control, we study how different predation regimes affect adaptive risk‐taking and growth in fish populations. Growth, metabolism and foraging in the modelled fish are regulated by three simplified hormone functions: growth hormone, orexin, and thyroid hormone. A dynamic state‐dependent optimization model finds optimal hormone profiles for adaptive growth strategies in juvenile fish. We consider a gradient from species where behaviour and metabolic activity have large consequences for risk (typically benthic and camouflaged species), to the opposite endpoint where behaviour may modify predation risk to a smaller degree (as in the pelagic). Along this gradient, the model predicts changes in the pace of life from slow to fast, enacted by up‐regulation of the three hormone functions which in turn increase foraging and metabolism and change the priorities of energy reserves versus growth. Under all types of predation risk investigated, growth is faster when food availability is higher. Energy reserves are maintained primarily during periods of poor food availability and are used to accelerate growth during periods when food availability is high. The thyroid hormone function is up‐regulated predominantly when food availability is high and has an important role in trade‐offs balancing energetic gain and survival. At the individual time scale, the hormone system improves organismic flexibility and robustness. Over the phylogenetic time scale, hormone system adaptations have also restricted the phenotypic plasticity of individuals.