Yolk androgens as modulators of life history trade-offs in the spotless starling ("Sturnus unicolor")

Resumo

Maternal effects can potentially affect ecological and evolutionary processes, im-proving the adaptation of offspring to variable environments. In birds, mothers can buffer offspring from the impacts of environmental heterogeneity by adjustment of resources, such as hormones, that are transmitted to the developing offspring via the egg. Exposure to steroids during early development has been shown to affect a vari-ety of traits, leading to important fitness consequences. Although several experimental studies have shown how nestlings benefit from increased yolk androgens through increased growth and survival, high levels of yolk androgens also involve costs. Accelerated growth induced by yolk androgens may suppress immune responses because of the trade-off between body mass gain and immune function. Manipulation of the androgen environment of an embryo may al-so induce a wide range of life-history trade-offs. Most previous studies have focused on the effects of yolk androgens on early life stages. However, these maternal hor-mones can have long-term consequences on adult phenotype and fitness. In order to draw more accurate conclusions about the effect of yolk androgens as modulators of life-history trade-offs, one should consider that their consequences may vary depending on additional factors, such as embryo sex, developmental phase, type of androgen, dose and environmental context. Data for this thesis were obtained during the 2009-2014 breeding seasons in a nest-box population located in central Spain (Soto del Real, Madrid). We manipulated androgen levels in spotless starling eggs (Sturnus unicolor) and analyzed the balance of benefits and costs in early development, as well as their long term effects on adult phenotype and repro-ductive output. We found that the two main androgens found in avian yolks, testosterone (T) and androstenedione (A4), exert different effects on different traits of the developing nestling. However, their effects are not additive. Yolk androgens show complex dose-response effects during early development, including both linear and non-linear responses for different nestling traits, supporting the idea that the balance of cost and benefits is strongly determined by hormone levels (i.e. higher androgens levels are not always beneficial. In addition, these cost of androgen levels, often emerge as sex-specific immunosuppression, as high hormone levels negative impact some components of the innate and adaptive axes of the immune system (e.g. lyso-zyme activity of the plasma and cell-mediated immunoresponsiveness) in male nest-lings but not in females...