LAUSR

Rationale The species‐specific relationship between phosphate (δ18OP values) and structural carbonate (δ18OC values) oxygen isotope ratios has been established for several modern and fossil animal species but until now it has not been investigated in European fallow deer (Dama dama dama). This study describes the relationship between phosphate and structural carbonate bioapatite in tooth enamel of extant fallow deer, which will help us further understand the species' unique environmental and cultural history. Methods The oxygen isotope composition of phosphate (δ18OP value) and structural carbonate (δ18OC value) of hydroxylapatite was determined in 51 modern fallow deer tooth enamel samples from across Europe and West Asia. The δ18OC values were measured on a GV IsoPrime dual‐inlet mass spectrometer and the δ18OP values on a temperature‐controlled elemental analyser (TC/EA) coupled to a DeltaPlus XL isotope ratio mass spectrometer via a ConFlo III interface. Results This study establishes a direct and linear relationship between the δ18OC and δ18OP values from fallow deer tooth enamel (δ18OC = +9.244(±0.216) + 0.958 * δ18OP (±0.013)). Despite the successful regression, the variation in δ18O values from samples collected in the same geographical area is greater than expected, although the results cluster in broad climatic groupings when Koppen‐Geiger classifications are taken into account for the individuals' locations. Conclusions This is the first comprehensive study of the relationship between ionic forms of oxygen (phosphate oxygen and structural carbonate) in fallow deer dental enamel. The new equation will allow direct comparison with other herbivore data. Variable δ18O values within populations of fallow deer broadly reflect the ecological zones they are found in which may explain this pattern of results in other euryphagic species.