Differentiating aquatic mammal habitat and foraging ecology with stable isotopes in tooth enamel

We analyzed the carbon and oxygen isotope composition of tooth enamel from mammals inhabiting marine and terrestrial ecosystems to determine whether these stable isotopes were robust indicators of foraging and habitat preferences. Consumers were separated into six habitats (offshore, nearshore, kelp beds, estuarine, freshwater, terrestrial). Consumer δ¹³C values were correlated with the δ¹³C values of primary producers within each habitat, suggesting that δ¹³C values of tooth enamel are a viable proxy for foraging zones. Offshore and terrestrial consumer δ¹³C values were not significantly different, however, indicating that carbon isotope analysis alone is not sufficient to distinguish foraging within these two ecosystems. We propose that oxygen isotopes can be used along with δ¹³C values to further clarify habitat use. Oxygen isotopes were assessed as an indicator of habitat use. Consumers were grouped into four categories: aquatic-marine, aquatic-estuarine, aquatic-freshwater, and terrestrial. Populations of aquatic taxa had significantly lower standard deviations for δ¹⁸O values than those of terrestrial taxa. Mean δ¹⁸O values of aquatic taxa were significantly different among groups, but surprisingly, the mean values for freshwater taxa were higher than those for marine taxa. We conclude that variation in δ¹⁸O values of mammalian populations is a valid indicator of aquatic habits, but that mean δ¹⁸O values should be utilized with caution when trying to discriminate between marine and freshwater habitat use. Together, δ¹³C and δ¹⁸O values serve as valuable tools for identifying foraging and habitat preferences in modern marine and terrestrial ecosystems, and may provide similar information on ancient ecosystems.