Oxygen and hydrogen isotopes in rodent tissues: Impact of diet, water and ontogeny

Stable oxygen (δ18O) and hydrogen (δD) isotope compositions in heterotroph tissues are known to reflect the seasonal variation of meteoric water intake and dietary inputs. However, the magnitude of the seasonal signal recorded in different tissues of ecological and geological significance has not been fully established. In addition, the effects of organism age and body size on tissue δ18O and δD values remain unexplored. This paper examines carbonate and phosphate oxygen (δ18O) from tooth enamel, and the oxygen (δ18O) and hydrogen (δD) values from the bone collagen, subcutaneous fat, and hair of rats raised on a monotonous diet approaching constant hydrogen and oxygen isotopic composition and local drinking water. The rats were of two age classes, post-weaning (40 g) and mature (375 g), and from two different hydrological regions (Houston, Texas, USA and Germantown, New York, USA) having dissimilar tap water isotopic values but a single food source with nearly identical hydrogen and oxygen values. Significant differences in the tissue δD and δ18O of post-weaning and mature rats were found; however these ontogenetic differences did not completely obscure the contribution of drinking water δ18O and δD. Animals raised under different hydrological regimes had distinct tissue δD and δ18O values. Apatite(carbonate), apatite(phosphate), hair, collagen and subcutaneous fat δ18O reflect varying proportions of the difference in drinking water δ18O values. Phosphate oxygen in enamel apatite appears most sensitive to drinking water δ18O differences, while subcutaneous fat of adult rats appears to best reflect the differences in the δD of the drinking water sources. An examination of the statistical significance associated with the isotopic differences in δD and δ18O between the Texas and New York rat tissues suggests that differences in the isotopic composition of local water of less than 2.5‰ in δ18O and 10–20‰ in δD may be invisible in tissue values of archeological, palaeontological, or ecological interest unless isotopically distinct food resources contribute to regional isotopic separations.