Hormones as adaptive control systems in juvenile fish

Abstract

Growth is an important theme in many biological disciplines. Physiologists often relate growth rates to hormonal control of essential processes. Ecologists often study growth as function of gradients or combinations of environmental factors. Fewer studies have investigated the combined effects of environmental and hormonal control on growth. Here, we present an evolutionary optimization model of fish growth that combines internal regulation of growth by hormone levels with the external influence of food availability and predation risk. Hormones are represented by growth hormone, thyroid hormone and orexin functions. By studying a range from poor to rich environments, we find that the level of food availability in the environment results in different evolutionarily optimal strategies of hormone levels. With more food available, higher levels of hormones are optimal, resulting in higher food uptake and growth. By using this fitness-based approach we also find a consequence of evolutionary optimization of survival on optimal hormone use. Where foraging is risky, aerobic scope can be used strategically to increase the chance of escaping from predators. By comparing model results to empirical observations, many mechanisms can be recognized, for instance a change in pace-of-life due to resource availability, and reduced emphasis on reserves in more stable environments. Summary statement We combine physiological, environmental and evolutionary aspects of fish growth in a state-dependent model where the optimal regulation of growth and survival is achieved through hormonal regulation of behaviour.

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

“…Because part of our aim is to bridge the gap between proximate and ultimate explanations for growth, the modelled individuals face several tradeoffs between energetics and survival affected by their hormone strategy. We give a brief outline of the model below and refer to the Supplementary material Appendix 1 and Weidner et al (2020) for details. The extension in this model version replaces the constant food environment in Weidner et al (2020) with an environment where food availability varies stochastically through time.…”

“…We give a brief outline of the model below and refer to the Supplementary material Appendix 1 and Weidner et al (2020) for details. The extension in this model version replaces the constant food environment in Weidner et al (2020) with an environment where food availability varies stochastically through time.…”

“…To make the model tractable (Hilborn and Mangel 1997), we simplified the full complexity of the hormone system into three sets of hormone functions, which we refer to as such, to distinguish them from real molecules (Weidner et al 2020). First, the growth hormone function (GHF) controls growth, which we assume is isometric.…”

“…Even if dynamic optimisation cannot reveal the intermediate steps along the evolutionary trajectory, it can point to the optimal end-result. Here we use one such optimisation model designed to study how hormones and combinations of them can integrate bioenergetics and survival through their consequences for Darwinian fitness and adaptive traits (Weidner et al 2020).…”

“…Using the model of Weidner et al (2020), we have in this study moved the focus of the optimal policy from the endpoint (behaviour), not all the way to the starting point (genes), but to some midpoint (hormone system), where behaviour and thereafter growth and survival become consequences of evolutionary optimisation. We have investigated how hormones and combinations of them might prepare the phenotype for changes in a fluctuating environment.…”