Endocrine and Behavioural Plasticity in Vertebrates

Endocrine plasticity is the reversible change in endocrine traits in response to unpredicted changes in an animals’ physical and social environment. Inter-individual differences in how individuals’ endocrine traits vary can occur as a result of differences in how individuals’ hypothalamic-pituitary-adrenal/interrenal (HPA/I axis) respond to stressors. However, it remains unknown if and how the variation in individuals’ baseline stress hormones (measured by glucocorticoids) can predict stress-induced plasticity. In this thesis, I aim to gain a more in-depth understanding of variation in endocrine plasticity over a range of physical and social environmental changes across different vertebrate systems, and the potential factors (behavioural and morphological) driving it. First, I review the literature on between-individual variation in endocrine plasticity to better understand why and how plasticity occurs (Chapter 1) and provide an overview of my methods and study systems (Chapter 2). Second, I investigate the effects of familiarity and recent social context on cortisol responses (Chapter 3) and behavioural responses (Chapter 4) in three-spined stickleback fish (Gasterosteus aculeatus). Third I investigate endocrine plasticity in humans (Homo sapiens) and potential links to BIG 5 personality measures in a social task (Chapter 5). Fourth, I investigate long-term plasticity in Welsh mountain ewes (Ovis aries) in response to changing food availability (Chapter 6). Across these three different study systems I find i) endocrine repeatability, which can be considered equivalent to personality traits, ii) evidence for endocrine plasticity and between-individual differences in plasticity, and iii) links between endocrine repeatability and plasticity to between variations behavioural/morphological traits. In the final chapter (Chapter 7) I discuss how these findings advance our understanding of how individuals respond and adapt to environmental challenges.