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.

Adaptive host responses to infection can resemble parasitic manipulation

“…Here we give a brief explanation of the main features of our model and refer to Weidner et al . (2020), Jensen et al . (2020a) for further details, including a list of parameters and variables.…”

“…We use an optimisation model of hormonal regulation of growth in fish (Jensen et al . 2020a, b; Weidner et al .…”

“…The fish respond to these fluctuations by adjusting their feeding behaviour, growth rate, and metabolism. When the conditions permit it, the fish may build energy reserves that they can draw from in times of scarcity (Jensen et al . 2020a).…”

Computational animal welfare: towards cognitive architecture models of animal sentience, emotion and wellbeing

“…The model in figure 1 is both an abstraction and simplification of the processes in the brains and bodies of animals during decision-making. Each neuronal response function [42,195] and hormonal modulation [69,128] represents the net aggregate of a range of processes, and these aggregates are therefore not in themselves observable. Relevant parameter values can be found by evolving populations of digital organisms [196,215,216] in environments that resemble the evolutionary history of the species [42,217,218].…”

“…Such predictive modelling is found even in insects [65,66]. Making predictions for the future is central to the adaptive functioning of the nervous [67,68] as well as hormonal system [69]. Thus, there is a growing realization that a stimulus-response paradigm is insufficient to account for complex behaviour [70,71].…”