Variation in the thermal plasticity of avian embryos is produced by the developmental environment, not genes

Limited evidence suggests that variation in phenotypic plasticity within populations may arise largely from environmental sources, thereby constraining its evolvability. This is of concern for temperature-sensitive metabolism in the face of climate change. We quantified the relative influence of the developmental environment versus genes on the metabolic plasticity of avian embryos to temperature. We partially cross-fostered 602 house sparrow eggs (Passer domesticus), measured the heart rate plasticity of these embryos to egg temperature, and partitioned variance in plasticity. We found that the foster (incubation) environment was the sole meaningful source of variance in embryonic plasticity (not genes, pre-laying effects, or ambient conditions). In contrast to heart rate plasticity, nestling growth was influenced by the foster environment, genes/pre-laying parental effects, and ambient conditions. Although embryonic plasticity to temperature varied in this population, these results suggest that it is unlikely to evolve quickly. Nevertheless, the expression of this plasticity may be able to shift between generations in response to changes in the developmental environment. Whether the multidimensional plasticity of heart rate to both current temperature and the developmental environment is itself an adaptive, evolved trait allowing avian embryos to optimize their metabolic plasticity to their current environment remains to be tested.

Methods
In this study, we cross-fostered the eggs of house sparrows in a free-living nest box population located at the University of Kentucky. Nests were paired into dyads, and two eggs were reciprocally swapped between the two nests within the dyad upon the completion of egg laying. We then repeatedly measured the heart rate, egg temperature, and egg mass for all individual eggs. Upon hatching, we repeatedly measured the tarsus length of nestlings until they fledged the nest. Data is presented as it was entered.