Dentine oxygen isotopes (δ18O) as a proxy for odontocete distributions and movements

Spatial variation in marine oxygen isotope ratios (δ¹⁸O) resulting from differential evaporation rates and precipitation inputs is potentially useful for characterizing marine mammal distributions and tracking movements across δ¹⁸O gradients. Dentine hydroxyapatite contains carbonate and phosphate that precipitate in oxygen isotopic equilibrium with body water, which in odontocetes closely tracks the isotopic composition of ambient water. To test whether dentine oxygen isotope composition reliably records that of ambient water and can therefore serve as a proxy for odontocete distribution and movement patterns, we measured δ¹⁸O values of dentine structural carbonate (δ¹⁸O_SC) and phosphate (δ¹⁸O_P) of seven odontocete species (n = 55 individuals) from regional marine water bodies spanning a surface water δ¹⁸O range of several per mil. Mean dentine δ¹⁸O_SC (range +21.2 to +25.5‰ VSMOW) and δ¹⁸O_P (+16.7 to +20.3‰) values were strongly correlated with marine surface water δ¹⁸O values, with lower dentine δ¹⁸O_SC and δ¹⁸O_P values in high‐latitude regions (Arctic and Eastern North Pacific) and higher values in the Gulf of California, Gulf of Mexico, and Mediterranean Sea. Correlations between dentine δ¹⁸O_SC and δ¹⁸O_P values with marine surface water δ¹⁸O values indicate that sequential δ¹⁸O measurements along dentine, which grows incrementally and archives intra‐ and interannual isotopic composition over the lifetime of the animal, would be useful for characterizing residency within and movements among water bodies with strong δ¹⁸O gradients, particularly between polar and lower latitudes, or between oceans and marginal basins.