Kangaroo metabolism does not cause the relationship between bone collagen δ 15 N and water availability

Summary1 A negative relationship between water availability and the abundance of 15 N relative to 14 N (expressed as δ 15 N) in the bone collagen of herbivores has been widely reported. However, the relative importance of dietary δ 15 N and animal metabolism in producing this effect remains unclear. 2 To evaluate the relative importance of these two factors, we examined variation in δ 15 N of both grass foliage and kangaroo ( Macropus spp.) bone collagen. We assessed whether the offset between grass and bone collagen δ 15 N was constant with respect to water availability. 3 An index of water availability (annual actual evapotranspiration/annual potential evapotranspiration) explained a considerable proportion of the variation in both grass δ 15 N ( R 2  = 0·40) and bone collagen δ 15 N ( R 2  = 0·57), and the slopes of these negative relationships were similar, with a near‐constant δ 15 N offset between grass foliage and bone collagen. 4 This finding suggests that dietary δ 15 N is the main cause of the negative relationship between kangaroo bone collagen δ 15 N and water availability, with metabolic factors having little discernible effect.