Meeting Abstract
Understanding the fitness relevance of hormonal phenotypes in free-living individuals has recently become a focus within behavioral ecology and evolutionary physiology. Hormones like glucocorticoids (GC) are key mediators of individual adjustments to environmental conditions, but their considerable plasticity has hampered progress in identifying hormonal phenotypes of individuals. Here, we use a reaction norm approach to characterize GC phenotypes and their relevance for fitness (reproductive success) in free-living great tits (Parus major). In early evening on cold winter nights, 24 great tits were captured from their roosts (nest boxes) and sampled for baseline corticosterone concentrations. Birds were then immediately returned to their roosts and nest box temperature was increased for 14 of the birds, while the rest served as control. Each trial lasted 3 hours, after which birds were recaptured and another blood sample was taken to assess the effect of the temperature manipulation on baseline GC. Birds were then kept in a cloth bag for 30 minutes to obtain a stress-induced GC sample and subsequently injected with dexamethasone. A final sample was taken 90 minutes later to assess negative feedback efficacy. Birds were then returned to the boxes. During the following breeding season, baseline and stress-induced GCs were obtained in 16 of the same birds, and their yearly number of fledglings was assessed. We will discuss relationships between baseline GC responses to environmental changes (temperature) and overall HPA-axis functioning as possible evidence for a GC phenotype. Additionally, we will discuss the implications of the responses to temperature in winter in relation to parental investment and reproductive success in spring for fitness.
