The influence of metabolic rate on the contribution of stable-hydrogen and oxygen isotopes in drinking water to quail blood plasma and feathers

The stable-hydrogen and oxygen isotopic composition of animal tissues are important tools for estimating the geographic origin of migrating animals. Estimating the origin of animals using H and O isotopes requires knowledge of how closely and consistently tissues reflect precipitation isotopes at the location in which the tissues were grown. One hypothesis to explain variation in isotope discrimination values in tissues from individuals residing at the same location is metabolic rate. Variation in metabolic rate between individuals could alter the relative proportions of drinking water and food atoms that are integrated into a tissue. Individuals with high metabolic rates ingest greater amounts of water compared with those with lower metabolic rates in order to balance metabolic water losses. Increased water consumption may increase the rate of hydrogen and oxygen atom turnover within the blood plasma, thereby limiting the extent to which tissue precursors isotopically exchange with their surroundings. A first test of this hypothesis was carried out by experimentally elevating the metabolic rate of Japanese quail (Coturnix japonica) over a 64-day period. We spiked drinking water with 99.9% 2H2O and 10% 18O to trace the incorporation of drinking water to feather tissue while holding the isotopic value of the food constant. We observed a 15% increase in metabolic rate was associated with a 6% decrease in the relative contribution of drinking water hydrogen to feather keratin between treatments. In contrast, no differences in the relative contribution of drinking water oxygen to plasma or feathers were detected because of the small isotopic separation (e.g., only 9 parts per thousand) between initial and final drinking water d18O values. Results suggest a difference in metabolic rates between individuals residing in the same geographic location have the potential to partially obscure the relationship between precipitation and feather d2H values.