Environmental unpredictability shapes glucocorticoid regulation across populations of tree swallows

Cedric Zimmer,L. Scott Johnson,Maren N. Vitousek,Conor C. Taff,Daniel R. Ardia,Alexandra P. Rose,David A. Aborn

doi:10.1038/s41598-020-70161-4

Abstract

The ability to respond appropriately to challenges is an important contributor to fitness. Variation in the regulation of glucocorticoid hormones, which mediate the phenotypic response to challenges, can therefore influence the ability to persist in a given environment. We compared stress responsiveness in four populations of tree swallows (Tachycineta bicolor) breeding under different environmental conditions to evaluate support for different selective pressures in driving the evolution of glucocorticoid regulation. In accordance with the environmental unpredictability hypothesis, stronger stress responses were seen in more unpredictable environments. Contrary to the reproductive value hypothesis, the stress response was not lower in populations engaging in more valuable reproductive attempts. Populations with stronger stress responses also had stronger negative feedback, which supports a “mitigating” rather than a “magnifying” effect of negative feedback on stress responses. These results suggest that combining a robust stress response with strong negative feedback may be important for persisting in unpredictable or rapidly changing environments.

Highlights

The ability to respond appropriately to challenges is an important contributor to fitness
Breeding season lengths in New York (NY) (74 days) and WY (70 days) were intermediate, but more similar to AK than TN. These patterns suggest that reproductive value was the lowest in TN and relatively similar across the other three populations (Fig. 1)
Our results suggest that the unpredictability of environmental challenges could affect how selection shapes glucocorticoid regulation

Summary

Introduction

The ability to respond appropriately to challenges is an important contributor to fitness. Variation in the regulation of glucocorticoid hormones, which mediate the phenotypic response to challenges, can influence the ability to persist in a given environment. When facing unpredictable challenges, circulating glucocorticoids increase dramatically, promoting a suite of processes that facilitate responding to and recovering from these c hallenges[14,15] When sustained, this stress response can trigger an emergency life-history stage in which breeding activities are usually reduced, and energy is redirected toward survival[15]. Breuner and c olleagues[24] compared males from three populations of white-crowned sparrows (Zonotrichia leucophrys) breeding at different latitudes from California to Alaska They found that males had similar stress-induced glucocorticoid levels, but differed in their corticosteroid-binding globulin and intracellular receptor affinity, suggesting that HPA axis regulation varied across the populations. While the stress response appears to vary with latitude, additional research is needed to determine which of the many potential selective pressures that covary with latitude are driving this variation

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Conclusion