Experimental food restriction reveals individual differences in corticosterone reaction norms with no oxidative costs.

Ádám Z Lendvai,Mark F Haussmann,Ignacio T Moore,Vincent Fasanello,Frances Bonier,Laura A Schoenle,Jenny Q Ouyang,François Criscuolo

doi:10.1371/journal.pone.0110564

Abstract

Highly plastic endocrine traits are thought to play a central role in allowing organisms to respond rapidly to environmental change. Yet, not all individuals display the same degree of plasticity in these traits, and the costs of this individual variation in plasticity are unknown. We studied individual differences in corticosterone levels under varying conditions to test whether there are consistent individual differences in (1) baseline corticosterone levels; (2) plasticity in the hormonal response to an ecologically relevant stressor (food restriction); and (3) whether individual differences in plasticity are related to fitness costs, as estimated by oxidative stress levels. We took 25 wild-caught house sparrows into captivity and assigned them to repeated food restricted and control treatments (60% and 110% of their daily food intake), such that each individual experienced both food restricted and control diets twice. We found significant individual variation in baseline corticosterone levels and stress responsiveness, even after controlling for changes in body mass. However, these individual differences in hormonal responsiveness were not related to measures of oxidative stress. These results have implications for how corticosterone levels may evolve in natural populations and raise questions about what we can conclude from phenotypic correlations between hormone levels and fitness measures.

Highlights

A central goal in evolutionary ecology is to characterize patterns of selection on the optimal phenotype for a given environment
If natural selection strongly favors a single optimal hormonal response to the environment such that all individuals in the population have the same reaction norm, any phenotypic correlation between fitness and hormone levels must be due to the influence of the environment on both the optimal hormone levels and fitness, a scenario that is likely if individuals in a sample are confronted with heterogeneous environmental challenges (Fig. 1)
Changes in body mass were associated with changes in corticosterone levels: overall, during food restriction, corticosterone levels were doubled compared to the control periods (96%, 30.19 [16.15; 57.81] ng/mL increase, Fig. 2b)

Summary

Introduction

A central goal in evolutionary ecology is to characterize patterns of selection on the optimal phenotype for a given environment. The observed between-individual and fitness-trait correlations could be an artifact of biased sampling [10] or the result of unmeasured traits that are tightly correlated with both the focal trait and fitness, including developmental history [11] and environmental conditions [12] To illustrate this point, consider the case of hormone levels, which are often adjusted as a response to changes in the environment. Without knowing individual reaction norms, researchers may be tempted to interpret such phenotypic correlations as evidence of natural selection [13] This conclusion may be premature or unsubstantiated [14,15]; we need to measure the adaptive value of among individual variation in reaction norms of labile traits under different environmental conditions to understand how selection shapes plasticity of these traits

Methods

Results

Conclusion