Hormonal plasticity to food restriction is heritable

Theoretical and empirical studies agree that populations harbor extensive among-individual variation in phenotypic plasticity, but the mechanisms generating and maintaining this variation are often unknown. Endocrine systems that exhibit plastic changes in response to environmental variation may be subject to natural selection, but their evolution requires heritable variation. It is currently unknown if endocrine plasticity to environmental challenges is heritable. We tested whether glucocorticoid responsiveness to food restriction is heritable in house sparrows, Passer domesticus, by subjecting individuals to a standardized dietary restriction and selecting individuals according to their hormonal responsiveness to the treatments: into high plastic, low plastic, and control groups and let them reproduce. Using a cross-foster design, we compared the parental and the F1 generation to partition the heritability of glucocorticoid responsiveness into genetic and environmental sources of variation. We found moderate heritability (h2>20%) of glucocorticoid plasticity in response to food availability in both restricted and adequate food conditions. Environmental variance played a larger role under restricted than adequate food conditions, whereas residual variance was much higher under adequate food conditions. Our findings provide empirical evidence for the existence of heritable individual variation in glucocorticoid plasticity that selection can act upon, especially in rapidly changing environments.