Variation in Embryonic Metabolic Reaction Norms and the Role of the Environment

AbstractEarly developmental environments can shape how organisms respond to later environments, but despite the potential for this phenomenon to alter the evolution of phenotypes and their underlying mechanisms in variable environments, details of this process are not understood. For example, both temperature and parental age can alter offspring metabolic plasticity and growth within species, yet the extent of such effects is unknown. We measured the reaction norms of embryonic heart rate in response to egg temperature and the change in egg mass over the incubation period in wild house sparrows. Using Bayesian linear mixed models, we estimated covariation in the intercepts and slopes of these reaction norms among clutches and eggs. We found that heart rate intercepts, not slopes, varied among clutches and that neither intercepts nor slopes varied among eggs within clutches. In contrast, egg mass intercepts and slopes varied among clutches and eggs. Ambient temperature did not explain variance in reaction norms. Instead, individuals from older mothers were more metabolically sensitive to egg temperature and lost less mass over the incubation period than individuals from younger mothers. Nevertheless, heart rate reaction norms and egg mass reaction norms did not covary. Our results suggest that early environments influenced by parents may contribute to variation in embryonic reaction norms. The structure of variation in embryonic reaction norms that exists both among clutches and among eggs demonstrates a complexity in plastic phenotypes that should be explored in future work. Furthermore, the potential for the embryonic environment to shape the reaction norms of other traits has implications for the evolution of plasticity more broadly.