Oxygen and carbon stable isotope ratios of kangaroo tooth enamel

Abstract

Stable isotopes are distributed in plants and organisms in an environment in different proportions and are often used to differentiate between environmental settings. Oxygen (δ18O) and carbon (δ13C) isotope ratios are known as recorders of climate and vegetation changes. δ18O ratios vary with precipitation, evaporation and transpiration processes while δ13C ratios differ between plant groups. Therefore, by analysing δ18O and δ13C ratios in herbivore tooth enamel, inferences about animals’ diet and ecology, as well as reconstructions of past climates can be made. δ18O ratios of herbivores often reflect precipitation δ18O values. However, previous studies of kangaroos and other arid-adapted species show this is not always true. Kangaroos are mainly non-obligate drinkers and ingest the majority of water from food, primarily grasses. A strong relationship between δ18O ratios of kangaroo tooth enamel and relative humidity is known, with leaf water suggested as a contributor. To test this theory, two isoscapes, one with precipitation δ18O ratios and one with modelled leaf water values, were constructed and compared to a meta-analysis of published kangaroo δ18O values. Significant correlations were found between δ18O ratios of tooth enamel and modelled leaf water (R2=0.40) compared to precipitation values (R2=0.020). Kangaroo δ18O ratios also showed significant correlation with relative humidity (R2=0.46) and aridity (R2=0.37). No significant interspecies differences in slopes were found when compared to the group regression, which indicate that kangaroos all reflect similar proportions of modelled leaf water. Tooth enamel δ13C ratios reflect relative distributions of C3 and C4 grasses but species differences also relate to diet preferences and ecological traits. These analyses show that δ18O and δ13C ratios of kangaroo tooth enamel reflect modelled leaf water values and vegetation, and can be used to make inferences about past climate relative humidity to more accurately reconstruct past environments.