Hormonal adjustments to future expectations impact growth and survival in juvenile fish

Abstract

Evolutionary ecology often studies how environmental factors define optimal phenotypes without considering the bodily mechanisms involved in their regulation. Here we used a dynamic optimisation model to investigate optimally concerted hormonal control of the phenotype. We studied a semi‐realistic situation where hormonal control of appetite, metabolism and growth acts to prepare juvenile fish for an uncertain future with regard to food availability. We found a bottom–up effect in that hormone levels varied across environments and affected a range of phenotypic changes. We also describe a top–down effect as natural selection varied across environments, which affected evolutionary optimisation of hormone levels. These combined top–down and bottom–up effects produced a hormone‐regulated phenotype that adjusted its foraging intensity and risk‐taking in adaptive ways depending on the differences between current and expected long‐term environmental conditions. Hence, understanding the response of these fish to their current conditions also requires an understanding of their future expectations. We found that when food availability was low, it was optimal for the juvenile fish to have low growth hormone, thyroid hormone and orexin levels, contrary to when food availability was high when these levels were higher. Individual variation emerged from the individually experienced food availability trajectories: Those that on average experienced higher food availability grew faster and had higher short‐term mortality risk. They also had higher survival probability throughout the growth period. The opposite was true for individuals experiencing lower food availability. Hormonal mechanisms that often are overlooked by ecologists are thus important in the ultimate adaptive control of both behaviour and physiology, thereby impacting fitness through growth and survival.