Hormonal plasticity to food restriction is heritable in the house sparrow, <i>Passer domesticus</i>

Theoretical and empirical studies agree that populations harbour extensive among-individual variation in phenotypic plasticity, but the mechanisms generating and maintaining this variation are often unknown. Endocrine systems, which can change plastically in response to environmental variation, may be shaped by natural selection, but their evolution requires heritable variation. It is currently unknown if endocrine plasticity in response to environmental challenges is heritable. We investigated this question in house sparrows, Passer domesticus, by testing glucocorticoid responsiveness to food restriction. We alternated restricted (70% of individual daily food intake) and adequate (110%) treatments twice, drawing blood samples at the end of each treatment. Based on glucocorticoid responsiveness, we classified individuals into high-plasticity, low-plasticity and medium (control) groups by selecting the 20 most responsive, least responsive and random individuals, respectively. We transferred these groups into separate aviaries and let them reproduce. In the next generation, we measured hormonal responsiveness using identical methods. Using a cross-foster design and quantitative genetic models, we partitioned the heritability of glucocorticoid responsiveness into genetic and environmental components. We found moderate heritability (h² > 30%) of glucocorticoid plasticity in response to food availability. The environmental and residual variances of glucocorticoid responsiveness were smaller than those for the intercept. Our findings provide empirical evidence for the existence of heritable individual variation in glucocorticoid plasticity, highlighting its potential to evolve under natural selection, particularly in dynamic and rapidly changing environments.