Parental care modifies the role of early-life size and growth in shaping future physiology

Size and growth early in life are associated with physiological development and these traits influence fitness. Life history theory predicts that the relationship between traits reflect constraints involving allocation and acquisition of resources. Using longitudinal data from 113 wild nestling barn swallows ( Hirundo rustica erythrogaster ), we first characterized developmental changes in glucose metabolism, a physiological trait involved in energy mobilization and response to stress. Next, we used these data to test hypotheses from life history theory about allocation and acquisition of resources based on associations of nestling size and growth with glucose physiology and assessed whether these relationships are modified by parental care. We found that larger nestlings had higher baseline blood glucose and larger magnitude of change in glucose in response to a stressor. Further, this relationship was most pronounced among birds in nests that received the lowest amount of parental care. Given that glucose metabolism fuels activity and is critical in the vertebrate stress response, these results suggest that physiological constraints may contribute to the early-life disadvantage of being smaller, especially in the context of lower parental care. While these findings are inconsistent with a trade-off involving differential allocation of resources between life history traits, they align with the differential acquisition hypothesis.