Sources of carbon isotope variation in kangaroo bone collagen and tooth enamel

The stable carbon isotopic composition (expressed as delta C-13) of herbivore remains is commonly used to reconstruct past changes in the relative abundance Of C-4 versus C-3 grass biomass (C-4 relative abundance). However, the strength of the relationship between herbivore delta C-13 and C-4 relative abundance in extant ecosystems has not been thoroughly examined. We determined sources of variation in delta C-13 of bone collagen and tooth enamel of kangaroos (Macropus spp.) collected throughout Australia by measuring delta C-13 of bone collagen (779 individuals) and tooth enamel (694 individuals). An index of seasonal water availability, i.e. the distribution of rainfall in the C-4 versus C-3 growing seasons, was used as a proxy for C-4 relative abundance, and this variable explained a large proportion of the variation in both collagen delta C-13 (68%) and enamel delta C-13 (68%). These figures increased to 78% and 77%, respectively, when differences between kangaroo species were accounted for. Vegetation characteristics, such as woodiness and the presence of an open forest canopy, had no effect on collagen or enamel delta C-13. While there was no relationship between collagen delta C-13 and kangaroo age at death, tooth enamel produced later in life, following weaning, was enriched in C-13 by 3.5& relative to enamel produced prior to weaning. From the observed relationships between seasonal water availability and collagen and enamel delta C-13, enrichment factors (epsilon*) for collagen-diet and enamel-diet (post-weaning) were estimated to be 5.2 parts per thousand +/- 0.5 (95% CI) and 11.7 parts per thousand +/- 0.6 (95% CI), respectively. The findings of this study confirm that at a continental scale, collagen and enamel delta C-13 of a group of large herbivores closely reflect C-4 relative abundance. This validates a fundamental assumption underpinning the use of isotopic analysis of herbivore remains to reconstruct changes in C-4 relative abundance. (c) 2007 Elsevier Ltd. All rights reserved.