Oxidative stress and hormetic responses in the early life of birds

Abstract

Understanding the physiological processes underlying the long-term effects of early life environmental conditions on Darwinian fitness is of central importance to evolutionary ecology, biomedical research, and conservation science. In particular, the extent to which early stress exposure is detrimental to fitness may depend on its severity, with mild stress exposure actually having a stimulatory and, possibly, beneficial effect through a hormetic response to the stressful stimulus. Oxidative stress is an aspect of stress physiology that has received comparatively less attention than hormones when considering the long-term effects of early life conditions. We therefore need to combine hormesis and oxidative stress in order to better understand how the early environment can help shape a phenotype adapted to the conditions it is most likely to experience in its adult environment. This chapter aims to discuss how hormones and nutrients can shape the redox machinery of the embryo and its future susceptibility to oxidative stress; how hormesis might provide a mechanistic framework for interpreting the long-term effects of early exposure to various stressor magnitudes; and how to reconcile discrepancies among studies. Examples from avian research are especially emphasised throughout the chapter, notably because avian models may have many advantages over mammalian models when addressing the (mal)adaptive effects of early life experiences on adult phenotype.