Extent of phenotypic flexibility during long-distance flight is determined by tissue-specific turnover rates: a new hypothesis

Phenotypic flexibility in organ size of migratory birds is typically explained in functional terms in accordance with the principal of economic design. However, proposed functional hypotheses do not adequately explain differences in phenotypic flexibility between organs during fasting and in-flight starvation. We show that the extent of phenotypic flexibility in organ mass in five species of migratory birds during actual migration or simulated in-flight starvation consistently ranked as follows from highest to lowest mass change: small intestine, liver, kidney, gizzard, heart, flight and leg muscle. This pattern of phenotypic flexibility in organ mass was not consistent with proposed functional hypotheses, and was almost completely explained by differences in tissue-specific turnover rate measured in vivo using nutrients differing in their isotopic values. Thus, the fundamental process of tissue-specific protein turnover determines extent of organ mass changes for birds during migration, this likely applies to other organisms during fasting, and no further functional explanation(s) for differences in the magnitude of phenotypic flexibility between organs is required.